sh(1)
- NetBSD Manual Pages
SH(1) NetBSD General Commands Manual SH(1)
NAME
sh -- command interpreter (shell)
SYNOPSIS
sh [-abCEeFfhIiLmnpquVvx] [+abCEeFfhIiLmnpquVvx] [-o option_name]
[+o option_name] [command_file [argument ...]]
sh -c [-s] [-abCEeFfhIiLmnpquVvx] [+abCEeFfhIiLmnpquVvx] [-o option_name]
[+o option_name] command_string [command_name [argument ...]]
sh -s [-abCEeFfhIiLmnpquVvx] [+abCEeFfhIiLmnpquVvx] [-o option_name]
[+o option_name] [argument ...]
DESCRIPTION
sh is the standard command interpreter for the system. The current ver-
sion of sh is in the process of being changed to conform more closely to
the POSIX 1003.2 and 1003.2a specifications for the shell. This version
has many features which make it appear similar in some respects to the
Korn shell, but it is not a Korn shell clone (see ksh(1)). This man page
is not intended to be a tutorial or a complete specification of the
shell.
Overview
The shell is a command that reads lines from either a file or the termi-
nal, interprets them, and generally executes other commands. A shell is
the program that is running when a user logs into the system. (Users can
select which shell is executed for them at login with the chsh(1) com-
mand). The shell implements a language that has flow control constructs,
a macro facility that provides a variety of features in addition to data
storage, along with built in history and line editing capabilities. It
incorporates many features to aid interactive use and has the advantage
that the interpretative language is common to both interactive and non-
interactive use (shell scripts). That is, commands can be typed directly
to the running shell or can be put into a file and the file can be exe-
cuted directly by the shell.
Invocation
If no arguments are present and if the standard input, and output, of the
shell are connected to a terminal (or if the -i flag is set), and the -c
option is not present, the shell is considered an interactive shell. An
interactive shell generally prompts before each command and handles pro-
gramming and command errors differently (as described below). When first
starting, the shell inspects argument 0, and if it begins with a dash
`-', the shell is also considered a login shell. This is normally done
automatically by the system when the user first logs in. A login shell
first reads commands from the files /etc/profile and .profile if they
exist. If the environment variable ENV is set on entry to a shell, or is
set in the .profile of a login shell, and either the shell is interac-
tive, or the posix option is not set, the shell next reads commands from
the file named in ENV. Therefore, a user should place commands that are
to be executed only at login time in the .profile file, and commands that
are executed for every shell inside the ENV file. To set the ENV vari-
able to some file, place the following line in your .profile of your home
directory
ENV=$HOME/.shinit; export ENV
substituting for ``.shinit'' any filename you wish. Since the ENV file
can be read for every invocation of the shell, including shell scripts
and non-interactive shells, the following paradigm is useful for
restricting commands in the ENV file to interactive invocations. Place
commands within the ``case'' and ``esac'' below (these commands are
described later):
case $- in *i*)
# commands for interactive use only
...
esac
If command line arguments besides the options have been specified, and
neither -c nor -s was given, then the shell treats the first argument as
the name of a file from which to read commands (a shell script). This
also becomes $0 and the remaining arguments are set as the positional
parameters of the shell ($1, $2, etc). Otherwise, if -c was given, then
the first argument, which must exist, is taken to be a string of sh com-
mands to execute. Then if any additional arguments follow the command
string, those arguments become $0, $1, ... Otherwise, if additional
arguments were given (which implies that -s was set) those arguments
become $1, $2, ... If $0 has not been set by the preceding processing,
it will be set to argv[0] as passed to the shell, which will usually be
the name of the shell itself. If -s was given, or if neither -c nor any
additional (non-option) arguments were present, the shell reads commands
from its standard input.
Argument List Processing
Currently, all of the single letter options that can meaningfully be set
using the set built-in, have a corresponding name that can be used as an
argument to the -o option. The set -o name is provided next to the sin-
gle letter option in the description below. Some options have only a
long name, they are described after the flag options, they are used with
-o or +o only, either on the command line, or with the set built-in com-
mand. Other options described are for the command line only. Specifying
a dash ``-'' turns the option on, while using a plus ``+'' disables the
option. The following options can be set from the command line and,
unless otherwise stated, with the set built-in (described later).
-a allexport Automatically export any variable to which a
value is assigned while this flag is set.
-b notify Enable asynchronous notification of background
job completion. (Not implemented.)
-C noclobber Don't overwrite existing files with ``>''.
-c Read commands from the command_string operand
instead of, or in addition to, from the stan-
dard input. Special parameter 0 will be set
from the command_name operand if given, and the
positional parameters ($1, $2, etc.) set from
the remaining argument operands, if any. -c is
only available at invocation, it cannot be set,
and there is no form using ``+''.
-E emacs Enable the built-in emacs style command line
editor (disables -V if it has been set). (See
the Command Line Editing section below.)
-e errexit If not interactive, exit immediately if any
untested command fails. If interactive, and an
untested command fails, cease all processing of
the current command and return to prompt for a
new command. The exit status of a command is
considered to be explicitly tested if the com-
mand is used to control an if, elif, while, or
until, or if the command is the left hand oper-
and of an ``&&'' or ``||'' operator, or if it
is a pipeline (or simple command) preceded by
the ``!'' operator. With pipelines, only the
status of the entire pipeline (indicated by the
last command it contains) is tested when -e is
set to determine if the shell should exit.
-F fork Cause the shell to always use fork(2) instead
of attempting vfork(2) when it needs to create
a new process. This should normally have no
visible effect, but can slow execution. The sh
can be compiled to always use fork(2) in which
case altering the -F flag has no effect.
-f noglob Disable pathname expansion.
-h trackall Functions defined while this option is set will
have paths bound to commands to be executed by
the function at the time of the definition.
When off when a function is defined, the file
system is searched for commands each time the
function is invoked. (Not implemented.)
-I ignoreeof Ignore EOFs from input when interactive.
(After a large number of consecutive EOFs the
shell will exit anyway.)
-i interactive Force the shell to behave interactively.
-L local_lineno When set, before a function is defined, causes
the variable LINENO when used within the func-
tion, to refer to the line number defined such
that first line of the function is line 1.
When reset, LINENO in a function refers to the
line number within the file within which the
definition of the function occurs. This option
defaults to ``on'' in this shell. For more
details see the section LINENO below.
-m monitor Turn on job control (set automatically when
interactive).
-n noexec Read and parse commands, but do not execute
them. This is useful for checking the syntax
of shell scripts. If -n becomes set in an
interactive shell, it will automatically be
cleared just before the next time the command
line prompt (PS1) is written.
-p nopriv Do not attempt to reset effective UID if it
does not match UID. This is not set by default
to help avoid incorrect usage by setuid root
programs via system(3) or popen(3).
-q quietprofile If the -v or -x options have been set, do not
apply them when reading initialization files,
these being /etc/profile, .profile, and the
file specified by the ENV environment variable.
-s stdin Read commands from standard input (set automat-
ically if neither -c nor file arguments are
present). If after procesing a command_string
with the -c option, the shell has not exited,
and the -s option is set, it will continue
reading more commands from standard input.
This option has no effect when set or reset
after the shell has already started reading
from the command_file, or from standard input.
Note that the -s flag being set does not cause
the shell to be interactive.
-u nounset Write a message to standard error when attempt-
ing to obtain a value from a variable that is
not set, and if the shell is not interactive,
exit immediately. For interactive shells,
instead return immediately to the command
prompt and read the next command. Note that
expansions (described later, see Word
Expansions below) using the `+', `-', `=', or
`?' operators test if the variable is set,
before attempting to obtain its value, and
hence are unaffected by -u.
-V vi Enable the built-in vi(1) command line editor
(disables -E if it has been set). (See the
Command Line Editing section below.)
-v verbose The shell writes its input to standard error as
it is read. Useful for debugging.
-x xtrace Write each command to standard error (preceded
by the expanded value of ``$PS4'') before it is
executed. Useful for debugging.
cdprint Make an interactive shell always print the new
directory name when changed by the cd command.
In a non-interactive shell this option has no
effect.
nolog Prevent the entry of function definitions into
the command history (see fc in the Built-ins
section.) (Not implemented.)
posix Enables closer adherence to the POSIX shell
standard. This option will default set at
shell startup if the environment variable
POSIXLY_CORRECT is present. That can be over-
ridden (set or reset) by the -o option on the
command line. Currently this option controls
whether (!posix) or not (posix) the file given
by the ENV variable is read at startup by a
non-interactive shell. It also controls
whether file descriptors greater than 2 opened
using the exec built-in command are passed on
to utilities executed (``yes'' in posix mode),
whether a colon (:) terminates the user name in
tilde (~) expansions other than in assignment
statements (``no'' in posix mode), and whether
the shell treats an empty brace-list compound
statement as a syntax error (expected by POSIX)
or permits it. Such statements ``{ }'' can be
useful when defining dummy functions. Lastly,
in posix mode, only one ``!'' is permitted
before a pipeline.
promptcmds Allows command substitutions (as well as param-
eter and arithmetic expansions, which are
always performed) upon the prompt strings PS1,
PS2, and PS4 each time, before they are output.
This option should not be set until after the
prompts have been set (or verified) to avoid
accidentally importing unwanted command substi-
tutions from the environment.
tabcomplete Enables filename completion in the command line
editor. Typing a tab character will extend the
current input word to match a filename. If
more than one filename matches it is only
extended to be the common prefix. Typing a
second tab character will list all the matching
names. One of the editing modes, either -E or
-V, must be enabled for this to work.
Lexical Structure
The shell reads input in terms of lines from a file and breaks it up into
words at whitespace (blanks and tabs), and at certain sequences of char-
acters that are special to the shell called ``operators''. There are two
types of operators: control operators and redirection operators (their
meaning is discussed later). The following is a list of operators:
Control operators:
& && ( ) ; ;; ;& | || <newline>
Redirection operators:
< > >| << >> <& >& <<- <>
Quoting
Quoting is used to remove the special meaning of certain characters or
words to the shell, such as operators, whitespace, or keywords. There
are three types of quoting: matched single quotes, matched double quotes,
and backslash.
Backslash
An unquoted backslash preserves the literal meaning of the following
character, with the exception of <newline>. An unquoted backslash pre-
ceding a <newline> is treated as a line continuation, the two characters
are simply removed.
Single Quotes
Enclosing characters in single quotes preserves the literal meaning of
all the characters (except single quotes, making it impossible to put
single quotes in a single-quoted string).
Double Quotes
Enclosing characters within double quotes preserves the literal meaning
of all characters except dollar sign ($), backquote (`), and backslash
(\). The backslash inside double quotes is historically weird, and
serves to quote only the following characters (and these not in all con-
texts):
$ ` " \ <newline>,
where a backslash newline is a line continuation as above. Otherwise it
remains literal.
Reserved Words
Reserved words are words that have special meaning to the shell and are
recognized at the beginning of a line and after a control operator. The
following are reserved words:
! { } case
do done elif else
esac fi for if
in then until while
Their meanings are discussed later.
Aliases
An alias is a name and corresponding value set using the alias built-in
command. Whenever a reserved word (see above) may occur, and after
checking for reserved words, the shell checks the word to see if it
matches an alias. If it does, it replaces it in the input stream with
its value. For example, if there is an alias called ``lf'' with the
value ``ls -F'', then the input:
lf foobar <return>
would become
ls -F foobar <return>
Aliases provide a convenient way for naive users to create shorthands for
commands without having to learn how to create functions with arguments.
They can also be used to create lexically obscure code. This use is
strongly discouraged.
Commands
The shell interprets the words it reads according to a language, the
specification of which is outside the scope of this man page (refer to
the BNF in the POSIX 1003.2 document). Essentially though, a line is
read and if the first word of the line (or after a control operator) is
not a reserved word, then the shell has recognized a simple command.
Otherwise, a complex command or some other special construct may have
been recognized.
Simple Commands
If a simple command has been recognized, the shell performs the following
actions:
1. Leading words of the form ``name=value'' are stripped off, the
value is expanded, as described below, and the results are
assigned to the environment of the simple command. Redirect-
ion operators and their arguments (as described below) are
stripped off and saved for processing in step 3 below.
2. The remaining words are expanded as described in the Word
Expansions section below. The first remaining word is consid-
ered the command name and the command is located. Any remain-
ing words are considered the arguments of the command. If no
command name resulted, then the ``name=value'' variable
assignments recognized in item 1 affect the current shell.
3. Redirections are performed, from first to last, in the order
given, as described in the next section.
Redirections
Redirections are used to change where a command reads its input or sends
its output. In general, redirections open, close, or duplicate an exist-
ing reference to a file. The overall format used for redirection is:
[n] redir-op file
where redir-op is one of the redirection operators mentioned previously.
The following is a list of the possible redirections. The [n] is an
optional number, as in `3' (not `[3]'), that refers to a file descriptor.
If present it must occur immediately before the redirection operator,
with no intervening white space, and becomes a part of that operator.
[n]> file Redirect standard output (or n) to file.
[n]>| file The same, but override the -C option.
[n]>> file Append standard output (or n) to file.
[n]< file Redirect standard input (or n) from file.
[n1]<&n2 Duplicate standard input (or n1) from file descriptor
n2. n2 is expanded if not a digit string, the result
must be a number.
[n]<&- Close standard input (or n).
[n1]>&n2 Duplicate standard output (or n1) to n2.
[n]>&- Close standard output (or n).
[n]<> file Open file for reading and writing on standard input (or
n).
The following redirection is often called a ``here-document''.
[n]<< delimiter
here-doc-text ...
delimiter
The ``here-doc-text'' starts immediately after the next unquoted newline
character following the here-doc redirection operator. If there is more
than one here-document redirection on the same line, then the text for
the first (from left to right) is read first, and subsequent here-doc-
text for later here-doc redirections follows immediately after, until all
such redirections have been processed.
All the text on successive lines up to the delimiter, which must appear
on a line by itself, with nothing other than an immediately following
newline, is saved away and made available to the command on standard
input, or file descriptor n if it is specified. If the delimiter as
specified on the initial line is quoted, then the here-doc-text is
treated literally; otherwise, the text is treated much like a double
quoted string, except that `"' characters have no special meaning, and
are not escaped by `\', and is subjected to parameter expansion, command
substitution, and arithmetic expansion as described in the Word
Expansions section below. If the operator is ``<<-'' instead of ``<<'',
then leading tabs in all lines in the here-doc-text, including before the
end delimiter, are stripped. If the delimiter is not quoted, lines in
here-doc-text that end with an unquoted \ are joined to the following
line, the \ and following newline are simply removed while reading the
here-doc, which thus guarantees that neither of those lines can be the
end delimiter.
It is a syntax error for the end of the input file (or string) to be
reached before the delimiter is encountered.
Search and Execution
There are three types of commands: shell functions, built-in commands,
and normal programs -- and the command is searched for (by name) in that
order. A command that contains a slash `/' in its name is always a nor-
mal program. They each are executed in a different way.
When a shell function is executed, all of the shell positional parameters
(except $0, which remains unchanged) are set to the arguments of the
shell function. The variables which are explicitly placed in the envi-
ronment of the command (by placing assignments to them before the func-
tion name) are made local to the function and are set to the values
given, and exported for the benefit of programs executed with the func-
tion. Then the command given in the function definition is executed.
The positional parameters, and local variables, are restored to their
original values when the command completes. This all occurs within the
current shell, and the function can alter variables, or other settings,
of the shell.
Shell built-ins are executed internally to the shell, without spawning a
new process.
Otherwise, if the command name doesn't match a function or built-in, the
command is searched for as a normal program in the file system (as
described in the next section). When a normal program is executed, the
shell runs the program, passing the arguments and the environment to the
program. If the program is not a normal executable file, and if it does
not begin with the "magic number" whose ASCII representation is "#!", so
execve(2) returns ENOEXEC then) the shell will interpret the program in a
sub-shell. The child shell will reinitialize itself in this case, so
that the effect will be as if a new shell had been invoked to handle the
ad-hoc shell script, except that the location of hashed commands located
in the parent shell will be remembered by the child.
Note that previous versions of this document and the source code itself
misleadingly and sporadically refer to a shell script without a magic
number as a "shell procedure".
Path Search
When locating a command, the shell first looks to see if it has a shell
function by that name. Then it looks for a built-in command by that
name. If a built-in command is not found, one of two things happen:
1. Command names containing a slash are simply executed without per-
forming any searches.
2. Otherwise, the shell searches each entry in PATH in turn for the
command. The value of the PATH variable should be a series of
entries separated by colons. Each entry consists of a directory
name. The current directory may be indicated implicitly by an empty
directory name, or explicitly by a single period. If a directory
searched contains an executable file with the same name as the com-
mand given, the search terminates, and that program is executed.
Command Exit Status
Each command has an exit status that can influence the behavior of other
shell commands. The paradigm is that a command exits with zero in normal
cases, or to indicate success, and non-zero for failure, error, or a
false indication. The man page for each command should indicate the var-
ious exit codes and what they mean. Additionally, the built-in commands
return exit codes, as does an executed shell function.
If a command consists entirely of variable assignments then the exit sta-
tus of the command is that of the last command substitution if any, oth-
erwise 0.
If redirections are present, and any fail to be correctly performed, any
command present is not executed, and an exit status of 2 is returned.
Complex Commands
Complex commands are combinations of simple commands with control opera-
tors or reserved words, together creating a larger complex command. More
generally, a command is one of the following:
· simple command
· pipeline
· list or compound-list
· compound command
· function definition
Unless otherwise stated, the exit status of a command is that of the last
simple command executed by the command.
Pipelines
A pipeline is a sequence of one or more commands separated by the control
operator `|', and optionally preceded by the ``!'' reserved word. The
standard output of all but the last command is connected to the standard
input of the next command. The standard output of the last command is
inherited from the shell, as usual.
The format for a pipeline is:
[!] command1 [| command2 ...]
The standard output of command1 is connected to the standard input of
command2. The standard input, standard output, or both of a command is
considered to be assigned by the pipeline before any redirection speci-
fied by redirection operators that are part of the command are performed.
If the pipeline is not in the background (discussed later), the shell
waits for all commands to complete.
If the reserved word ! does not precede the pipeline, the exit status is
the exit status of the last command specified in the pipeline. Other-
wise, the exit status is the logical NOT of the exit status of the last
command. That is, if the last command returns zero, the exit status is
1; if the last command returns greater than zero, the exit status is
zero.
Because pipeline assignment of standard input or standard output or both
takes place before redirection, it can be modified by redirection. For
example:
$ command1 2>&1 | command2
sends both the standard output and standard error of command1 to the
standard input of command2.
A ; or <newline> terminator causes the preceding AND-OR-list (described
next) to be executed sequentially; a & causes asynchronous execution of
the preceding AND-OR-list. The exit status of an asynchronous AND-OR-
list is zero. The actual status of the commands, after they have com-
pleted, can be obtained using the wait built-in command described later.
Note that unlike some other shells, each process in the pipeline is a
child of the invoking shell (unless it is a shell built-in, in which case
it executes in the current shell -- but any effect it has on the environ-
ment is wiped).
Background Commands -- &
If a command is terminated by the control operator ampersand (&), the
shell executes the command asynchronously -- that is, the shell does not
wait for the command to finish before executing the next command.
The format for running a command in background is:
command1 & [command2 & ...]
If the shell is not interactive, the standard input of an asynchronous
command is set to /dev/null. The process identifier of the most recent
command started in the background can be obtained from the value of the
special parameter ``!'' (see Special Parameters).
Lists -- Generally Speaking
A list is a sequence of one or more commands separated by newlines, semi-
colons, or ampersands, and optionally terminated by one of these three
characters. The commands in a list are executed in the order they are
written. If command is followed by an ampersand, the shell starts the
command and immediately proceeds to the next command; otherwise it waits
for the command to terminate before proceeding to the next one. A new-
line is equivalent to a `;' when no other operator is present, and the
command being input could syntactically correctly be terminated at the
point where the newline is encountered, otherwise it is just whitespace.
Short-Circuit List Operators
``&&'' and ``||'' are AND-OR list operators. ``&&'' executes the first
command, and then executes the second command if and only if the exit
status of the first command is zero. ``||'' is similar, but executes the
second command if and only if the exit status of the first command is
nonzero. ``&&'' and ``||'' both have the same priority. Note that these
operators are left-associative, so ``true || echo bar && echo baz''
writes ``baz'' and nothing else. This is not the way it works in C.
Flow-Control Constructs -- if, while, for, case
The syntax of the if command is
if list
then list
[ elif list
then list ] ...
[ else list ]
fi
The first list is executed, and if the exit status of that list is zero,
the list following the then is executed. Otherwise the list after an
elif (if any) is executed and the process repeats. When no more elif
reserved words, and accompanying lists, appear, the list after the else
reserved word, if any, is executed.
The syntax of the while command is
while list
do list
done
The two lists are executed repeatedly while the exit status of the first
list is zero. The until command is similar, but has the word until in
place of while, which causes it to repeat until the exit status of the
first list is zero.
The syntax of the for command is
for variable [ in word ... ]
do list
done
The words are expanded, or "$@" if no words are given, and then the list
is executed repeatedly with the variable set to each word in turn. do
and done may be replaced with `{' and `}', but doing so is non-standard
and not recommended.
The syntax of the break and continue commands is
break [ num ]
continue [ num ]
break terminates the num innermost for, while, or until loops. continue
breaks execution of the num-1 innermost for, while, or until loops, and
then continues with the next iteration of the enclosing loop. These are
implemented as built-in commands. The parameter num, if given, must be
an unsigned positive integer (greater than zero). If not given, 1 is
used.
The syntax of the case command is
case word in
[(] pattern ) list ;&
[(] pattern ) list ;;
...
esac
The pattern can actually be one or more patterns (see Shell Patterns
described later), separated by ``|'' characters.
Word is expanded and matched against each pattern in turn, from first to
last, with each pattern being expanded just before the match is
attempted. When a match is found, pattern comparisons cease, and the
associated ``list'' (which may be empty) is evaluated. If the list is
terminated with ``;&'' execution then falls through to the following
list, if any, without evaluating its pattern, or attempting a match.
When a list terminated with ``;;'' has been executed, or when esac is
reached execution of the case statement is complete. The exit status is
that of the last command executed from the last list evaluated, if any,
or zero otherwise.
Grouping Commands Together
Commands may be grouped by writing either
(list)
or
{ list; }
Note that while parentheses are operators, and do not require any extra
syntax, braces are reserved words, so the opening brace must be followed
by white space (or some other operator), and the closing brace must occur
in a position where a new command word might otherwise appear.
The first of these executes the commands in a sub-shell. Built-in com-
mands grouped into a (list) will not affect the current shell. The sec-
ond form does not fork another shell so is slightly more efficient, and
allows for commands which do affect the current shell. Grouping commands
together this way allows you to redirect their output as though they were
one program:
{ echo -n "hello " ; echo "world" ; } > greeting
Note that ``}'' must follow a control operator (here, ``;'') so that it
is recognized as a reserved word and not as another command argument.
Functions
The syntax of a function definition is
name () command [redirect...]
A function definition is an executable statement; when executed it
installs a function named name and returns an exit status of zero. The
command is normally a list enclosed between ``{'' and ``}''. The stan-
dard syntax also allows the command to be any of the other compound com-
mands, including a sub-shell, all of which are supported. As an exten-
sion, this shell also allows a simple command (or even another function
definition) to be used, though users should be aware this is non-standard
syntax. This means that
l() ls $@
works to make ``l'' an alternative name for the ls command.
If the optional redirect, (see Redirections), which may be of any of the
normal forms, is given, it is applied each time the function is called.
This means that a simple ``Hello World'' function might be written (in
the extended syntax) as:
hello() cat <<EOF
Hello World!
EOF
To be correctly standards conforming this should be re-written as:
hello() { cat; } <<EOF
Hello World!
EOF
Note the distinction between those forms, and
hello() { cat <<EOF
Hello World!
EOF
}
which reads and processes the here document each time the shell executes
the function, and which applies that input only to the cat command, not
to any other commands that might appear in the function.
Variables may be declared to be local to a function by using the local
command. This should usually appear as the first statement of a func-
tion, though is an executable command which can be used anywhere in a
function. See Built-ins below for its definition.
The function completes after having executed command with exit status set
to the status returned by command. If command is a compound-command it
can use the return command (see Built-ins below) to finish before com-
pleting all of command.
Variables and Parameters
The shell maintains a set of parameters. A parameter denoted by a name
is called a variable. When starting up, the shell turns all the environ-
ment variables into shell variables, and exports them. New variables can
be set using the form
name=value
Variables set by the user must have a name consisting solely of alphabet-
ics, numerics, and underscores - the first of which must not be numeric.
A parameter can also be denoted by a number or a special character as
explained below.
Positional Parameters
A positional parameter is a parameter denoted by a number (n > 0). The
shell sets these initially to the values of its command line arguments
that follow the name of the shell script. The set built-in can also be
used to set or reset them, and shift can be used to manipulate the list.
To refer to the 10th (and later) positional parameters, the form ${n}
must be used. Without the braces, a digit following ``$'' can only refer
to one of the first 9 positional parameters, or the special parameter
``0''. The word ``$10'' is treated identically to ``${1}0''.
Special Parameters
A special parameter is a parameter denoted by one of the following spe-
cial characters. The value of the parameter is listed next to its char-
acter.
* Expands to the positional parameters, starting from one.
When the expansion occurs within a double-quoted string it
expands to a single field with the value of each parameter
separated by the first character of the IFS variable, or by
a <space> if IFS is unset.
@ Expands to the positional parameters, starting from one.
When the expansion occurs within double quotes, each posi-
tional parameter expands as a separate argument. If there
are no positional parameters, the expansion of @ generates
zero arguments, even when @ is double-quoted. What this
basically means, for example, is if $1 is ``abc'' and $2 is
``def ghi'', then "$@" expands to the two arguments:
"abc" "def ghi"
# Expands to the number of positional parameters.
? Expands to the exit status of the most recent pipeline.
- (Hyphen, or minus.)
Expands to the current option flags (the single-letter
option names concatenated into a string) as specified on
invocation, by the set built-in command, or implicitly by
the shell.
$ Expands to the process ID of the invoked shell. A sub-shell
retains the same value of $ as its parent.
! Expands to the process ID of the most recent background com-
mand executed from the current shell. For a pipeline, the
process ID is that of the last command in the pipeline. If
no background commands have yet been started by the shell,
then ``!'' will be unset. Once set, the value of ``!'' will
be retained until another background command is started.
0 (Zero.) Expands to the name of the shell or shell script.
Word Expansions
This section describes the various expansions that are performed on
words. Not all expansions are performed on every word, as explained
later.
Tilde expansions, parameter expansions, command substitutions, arithmetic
expansions, and quote removals that occur within a single word expand to
a single field. It is only field splitting or pathname expansion that
can create multiple fields from a single word. The single exception to
this rule is the expansion of the special parameter @ within double
quotes, as was described above.
The order of word expansion is:
1. Tilde Expansion, Parameter Expansion, Command Substitution, Arith-
metic Expansion (these all occur at the same time).
2. Field Splitting is performed on fields generated by step (1) unless
the IFS variable is null.
3. Pathname Expansion (unless set -f is in effect).
4. Quote Removal.
The $ character is used to introduce parameter expansion, command substi-
tution, or arithmetic evaluation.
Tilde Expansion (substituting a user's home directory)
A word beginning with an unquoted tilde character (~) is subjected to
tilde expansion. Provided all of the subsequent characters in the word
are unquoted up to an unquoted slash (/) or when in an assignment or not
in posix mode, an unquoted colon (:), or if neither of those appear, the
end of the word, they are treated as a user name and are replaced with
the pathname of the named user's home directory. If the user name is
missing (as in ~/foobar), the tilde is replaced with the value of the
HOME variable (the current user's home directory).
In variable assignments, an unquoted tilde immediately after the assign-
ment operator (=), and each unquoted tilde immediately after an unquoted
colon in the value to be assigned is also subject to tilde expansion as
just stated.
Parameter Expansion
The format for parameter expansion is as follows:
${expression}
where expression consists of all characters until the matching ``}''.
Any ``}'' escaped by a backslash or within a quoted string, and charac-
ters in embedded arithmetic expansions, command substitutions, and vari-
able expansions, are not examined in determining the matching ``}''.
The simplest form for parameter expansion is:
${parameter}
The value, if any, of parameter is substituted.
The parameter name or symbol can be enclosed in braces, which are
optional in this simple case, except for positional parameters with more
than one digit or when parameter is followed by a character that could be
interpreted as part of the name. If a parameter expansion occurs inside
double quotes:
1. Pathname expansion is not performed on the results of the expansion.
2. Field splitting is not performed on the results of the expansion,
with the exception of the special rules for @.
In addition, a parameter expansion where braces are used, can be modified
by using one of the following formats. If the ``:'' is omitted in the
following modifiers, then the test in the expansion applies only to unset
parameters, not null ones.
${parameter:-word} Use Default Values. If parameter is unset or null,
the expansion of word is substituted; otherwise,
the value of parameter is substituted.
${parameter:=word} Assign Default Values. If parameter is unset or
null, the expansion of word is assigned to parame-
ter. In all cases, the final value of parameter is
substituted. Only variables, not positional param-
eters or special parameters, can be assigned in
this way.
${parameter:?[word]} Indicate Error if Null or Unset. If parameter is
unset or null, the expansion of word (or a message
indicating it is unset if word is omitted) is writ-
ten to standard error and a non-interactive shell
exits with a nonzero exit status. An interactive
shell will not exit, but any associated command(s)
will not be executed. If the parameter is set, its
value is substituted.
${parameter:+word} Use Alternative Value. If parameter is unset or
null, null is substituted; otherwise, the expansion
of word is substituted. The value of parameter is
not used in this expansion.
${#parameter} String Length. The length in characters of the
value of parameter.
The following four varieties of parameter expansion provide for substring
processing. In each case, pattern matching notation (see Shell
Patterns), rather than regular expression notation, is used to evaluate
the patterns. If parameter is * or @, the result of the expansion is
unspecified. Enclosing the full parameter expansion string in double
quotes does not cause the following four varieties of pattern characters
to be quoted, whereas quoting characters within the braces has this
effect.
${parameter%word} Remove Smallest Suffix Pattern. The word is
expanded to produce a pattern. The parameter
expansion then results in parameter, with the
smallest portion of the suffix matched by the pat-
tern deleted. If the word is to start with a `%'
character, it must be quoted.
${parameter%%word} Remove Largest Suffix Pattern. The word is
expanded to produce a pattern. The parameter
expansion then results in parameter, with the
largest portion of the suffix matched by the pat-
tern deleted. The ``%%'' pattern operator only
produces different results from the ``%'' operator
when the pattern contains at least one unquoted
`*'.
${parameter#word} Remove Smallest Prefix Pattern. The word is
expanded to produce a pattern. The parameter
expansion then results in parameter, with the
smallest portion of the prefix matched by the pat-
tern deleted. If the word is to start with a `#'
character, it must be quoted.
${parameter##word} Remove Largest Prefix Pattern. The word is
expanded to produce a pattern. The parameter
expansion then results in parameter, with the
largest portion of the prefix matched by the pat-
tern deleted. This has the same relationship with
the `#' pattern operator as ``%%'' has with ``%''.
Command Substitution
Command substitution allows the output of a command to be substituted in
place of the command (and surrounding syntax). Command substitution
occurs when the command is enclosed as follows:
$(command)
or (``backquoted'' version):
`command`
The shell expands the command substitution by executing command in a sub-
shell environment and replacing the command substitution with the stan-
dard output of the command, removing sequences of one or more <newline>s
at the end of the substitution. (Embedded <newline>s before the end of
the output are not removed; however, during field splitting, they may be
translated into <space>s, depending on the value of IFS and any quoting
that is in effect.)
Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value. The format for arithmetic expan-
sion is as follows:
$((expression))
The expression in an arithmetic expansion is treated as if it were in
double quotes, except that a double quote character inside the expression
is just a normal character (it quotes nothing.) The shell expands all
tokens in the expression for parameter expansion, command substitution,
and quote removal (the only quoting character is the backslash `\', and
only when followed by another `\', a dollar sign `$', a backquote ``' or
a newline.)
Next, the shell evaluates the expanded result as an arithmetic expression
and substitutes the calculated value of that expression.
Arithmetic expressions use a syntax similar to that of the C language,
and are evaluated using the `intmax_t' data type (this is an extension to
POSIX, which requires only `long' arithmetic.) Shell variables may be
referenced by name inside an arithmetic expression, without needing a
``$'' sign. Variables that are not set, or which have an empty (null
string) value, used this way evaluate as zero (that is, ``x'' in arith-
metic, as an R-Value, is evaluated as ``${x:-0}'') unless the sh -u flag
is set, in which case a reference to an unset variable is an error. Note
that unset variables used in the ${var} form expand to a null string,
which might result in syntax errors. Referencing the value of a variable
which is not numeric is an error.
All of the C expression operators applicable to integers are supported,
and operate as they would in a C expression, except the unary ``++'' and
``--'' operators (in both prefix and postfix forms) and the ``,'' (comma)
operator, which are currently not supported.
It should not be necessary to state that the C operators which operate
on, or produce, pointer types, are not supported. Those include unary
``*'' and ``&'' and the struct and array referencing binary operators:
``.'', ``->'' and ``[''.
White Space Splitting (Field Splitting)
After parameter expansion, command substitution, and arithmetic expansion
the shell scans the results of expansions and substitutions that did not
occur in double quotes, and ``$@'' even if it did, for field splitting
and multiple fields can result.
The shell treats each character of the IFS as a delimiter and uses the
delimiters to split the results of parameter expansion and command sub-
stitution into fields.
Non-whitespace characters in IFS are treated strictly as parameter sepa-
rators. So adjacent non-whitespace IFS characters will produce empty
parameters. On the other hand, any sequence of whitespace characters
that occur in IFS (known as IFS whitespace) can occur, leading and trail-
ing IFS whitespace, and any IFS whitespace surrounding a non whitespace
IFS delimiter, is removed. Any sequence of IFS whitespace characters
without a non-whitespace IFS delimiter acts as a single field separator.
If IFS is unset it is assumed to contain space, tab, and newline, all of
which are IFS whitespace characters. If IFS is set to a null string,
there are no delimiters, and no field splitting occurs.
Pathname Expansion (File Name Generation)
Unless the -f flag is set, file name generation is performed after word
splitting is complete. Each word is viewed as a series of patterns, sep-
arated by slashes. The process of expansion replaces the word with the
names of all existing files whose names can be formed by replacing each
pattern with a string that matches the specified pattern. There are two
restrictions on this: first, a pattern cannot match a string containing a
slash, and second, a pattern cannot match a string starting with a period
unless the first character of the pattern is a period. The next section
describes the patterns used for both Pathname Expansion and the case com-
mand.
Shell Patterns
A pattern consists of normal characters, which match themselves, and
meta-characters. The meta-characters are ``!'', ``*'', ``?'', and ``[''.
These characters lose their special meanings if they are quoted. When
command or variable substitution is performed and the dollar sign or
backquotes are not double-quoted, the value of the variable or the output
of the command is scanned for these characters and they are turned into
meta-characters.
An asterisk (``*'') matches any string of characters. A question mark
(``?'') matches any single character. A left bracket (``['') introduces
a character class. The end of the character class is indicated by a
right bracket (``]''); if this ``]'' is missing then the ``['' matches a
``['' rather than introducing a character class. A character class
matches any of the characters between the square brackets. A named class
of characters (see wctype(3)) may be specified by surrounding the name
with (``[:'') and (``:]''). For example, (``[[:alpha:]]'') is a shell
pattern that matches a single letter. A range of characters may be spec-
ified using a minus sign (``-''). The character class may be comple-
mented by making an exclamation mark (``!'') the first character of the
character class.
To include a ``]'' in a character class, make it the first character
listed (after the ``!'', if any). To include a ``-'', make it the first
(after !) or last character listed. If both ``]'' and ``-'' are to be
included, the ``]'' must be first (after !) and the ``-'' last, in the
character class.
Built-ins
This section lists the built-in commands which are built-in because they
need to perform some operation that can't be performed by a separate
process. Or just because they traditionally are. In addition to these,
there are several other commands that may be built in for efficiency
(e.g. printf(1), echo(1), test(1), etc).
: [arg ...]
A null command that returns a 0 (true) exit value. Any arguments
or redirects are evaluated, then ignored.
. file
The dot command reads and executes the commands from the specified
file in the current shell environment. The file does not need to
be executable and is looked up from the directories listed in the
PATH variable if its name does not contain a directory separator
(`/'). The return command (see Built-ins below) can be used for a
premature return from the sourced file.
The POSIX standard has been unclear on how loop control keywords
(break and continue) behave across a dot command boundary. This
implementation allows them to control loops surrounding the dot
command, but obviously such behavior should not be relied on. It
is now permitted by the standard, but not required.
alias [name[=string ...]]
If name=string is specified, the shell defines the alias name with
value string. If just name is specified, the value of the alias
name is printed. With no arguments, the alias built-in prints the
names and values of all defined aliases (see unalias).
bg [job] ...
Continue the specified jobs (or the current job if no jobs are
given) in the background.
command [-p] [-v] [-V] command [arg ...]
Execute the specified command but ignore shell functions when
searching for it. (This is useful when you have a shell function
with the same name as a command.)
-p search for command using a PATH that guarantees to find all
the standard utilities.
-V Do not execute the command but search for the command and
print the resolution of the command search. This is the
same as the type built-in.
-v Do not execute the command but search for the command and
print the absolute pathname of utilities, the name for
built-ins or the expansion of aliases.
cd [-P] [directory [replace]]
Switch to the specified directory (default $HOME). If replace is
specified, then the new directory name is generated by replacing
the first occurrence of the string directory in the current direc-
tory name with replace. Otherwise if directory is `-', then the
current working directory is changed to the previous current work-
ing directory as set in OLDPWD. Otherwise if an entry for CDPATH
appears in the environment of the cd command or the shell variable
CDPATH is set and the directory name does not begin with a slash,
and its first (or only) component isn't dot or dot dot, then the
directories listed in CDPATH will be searched for the specified
directory. The format of CDPATH is the same as that of PATH.
The -P option instructs the shell to update PWD with the specified
physical directory path and change to that directory. This is the
default.
When the directory changes, the variable OLDPWD is set to the
working directory before the change.
Some shells also support a -L option, which instructs the shell to
update PWD with the logical path and to change the current direc-
tory accordingly. This is not supported.
In an interactive shell, the cd command will print out the name of
the directory that it actually switched to if this is different
from the name that the user gave, or always if the cdprint option
is set. The destination may be different either because the
CDPATH mechanism was used or if the replace argument was used.
eval string ...
Concatenate all the arguments with spaces. Then re-parse and exe-
cute the command.
exec [command arg ...]
Unless command is omitted, the shell process is replaced with the
specified program (which must be a real program, not a shell
built-in or function). Any redirections on the exec command are
marked as permanent, so that they are not undone when the exec
command finishes. When the posix option is not set, file descrip-
tors created via such redirections are marked close-on-exec (see
open(2) O_CLOEXEC or fcntl(2) F_SETFD / FD_CLOEXEC), unless the
descriptors they point to refer to the standard input, output, or
error (file descriptors 0, 1, 2). Traditionally Bourne-like
shells (except ksh(1)), made those file descriptors available to
exec'ed processes. This behavior is required by the POSIX stan-
dard, so when the posix option is set, this shell also acts that
way. To be assured the close-on-exec setting is off, redirect the
descriptor to (or from) itself, either when invoking a command for
which the descriptor is wanted open, or by using exec (perhaps the
same exec as opened it, after the open) to leave the descriptor
open in the shell and pass it to all commands invoked subse-
quently. Alternatively, see the fdflags command below, which can
set, or clear, this, and other, file descriptor flags.
exit [exitstatus]
Terminate the shell process. If exitstatus is given it is used as
the exit status of the shell; otherwise the exit status of the
preceding command (the current value of $?) is used.
export [-npx] name ...
export -p [-x]
With no options, but one or more names, the specified names are
exported so that they will appear in the environment of subsequent
commands. With -n the specified names are un-exported. Variables
can also be un-exported using the unset built in command. With -x
(exclude) the specified names are marked not to be exported, and
any that had been exported, will be un-exported. Later attempts
to export the variable will be refused. Note this does not pre-
vent explicitly exporting a variable to a single command, script
or function by preceding that command invocation by a variable
assignment to that variable, provided the variable is not also
readonly. That is
export -x FOO; # FOO will now not be exported by default
FOO=some_value my_command
still passes the value (FOO=some_value) to my_command through the
environment.
The shell allows the value of a variable to be set at the same
time it is exported by writing
export name=value
With no arguments the export command lists the names of all
exported variables, or if -x was given, all variables marked not
for export. With the -p option specified the output will be for-
matted suitably for non-interactive use.
The export built-in exits with status 0, unless an invalid option,
or option combination, is given, or unless an attempt is made to
export a variable which has been marked as unavailable for export,
in which cases it exits with status 1.
Note that there is no restriction upon exporting, or un-exporting,
readonly variables. The no-export flag can be reset by unsetting
the variable and creating it again - provided it is not also read-
only.
fc [-e editor] [first [last]]
fc -l [-nr] [first [last]]
fc -s [old=new] [first]
The fc built-in lists, or edits and re-executes, commands previ-
ously entered to an interactive shell.
-e editor
Use the editor named by editor to edit the commands. The
editor string is a command name, subject to search via the
PATH variable. The value in the FCEDIT variable is used as
a default when -e is not specified. If FCEDIT is null or
unset, the value of the EDITOR variable is used. If EDITOR
is null or unset, ed(1) is used as the editor.
-l (ell)
List the commands rather than invoking an editor on them.
The commands are written in the sequence indicated by the
first and last operands, as affected by -r, with each com-
mand preceded by the command number.
-n Suppress command numbers when listing with -l.
-r Reverse the order of the commands listed (with -l) or
edited (with neither -l nor -s).
-s Re-execute the command without invoking an editor.
first
last Select the commands to list or edit. The number of previ-
ous commands that can be accessed are determined by the
value of the HISTSIZE variable. The value of first or last
or both are one of the following:
[+]number
A positive number representing a command number;
command numbers can be displayed with the -l option.
-number
A negative decimal number representing the command
that was executed number of commands previously.
For example, -1 is the immediately previous command.
string
A string indicating the most recently entered command that
begins with that string. If the old=new operand is not
also specified with -s, the string form of the first oper-
and cannot contain an embedded equal sign.
The following environment variables affect the execution of fc:
FCEDIT Name of the editor to use.
HISTSIZE The number of previous commands that are accessible.
fg [job]
Move the specified job or the current job to the foreground. A
foreground job can interact with the user via standard input, and
receive signals from the terminal.
fdflags [-v] [fd ...]
fdflags [-v] -s flags fd [...]
Get or set file descriptor flags. The -v argument enables verbose
printing, printing flags that are also off, and the flags of the
file descriptor being set after setting. The -s flag interprets
the flags argument as a comma separated list of file descriptor
flags, each preceded with a ``+'' or a ``-'' indicating to set or
clear the respective flag. Valid flags are: append, async, sync,
nonblock, fsync, dsync, rsync, direct, nosigpipe, and cloexec.
Unique abbreviations of these names, of at least 2 characters, may
be used on input. See fcntl(2) and open(2) for more information.
getopts optstring var
The POSIX getopts command, not to be confused with the Bell Labs
-derived getopt(1).
The first argument should be a series of letters, each of which
may be optionally followed by a colon to indicate that the option
requires an argument. The variable specified is set to the parsed
option.
The getopts command deprecates the older getopt(1) utility due to
its handling of arguments containing whitespace.
The getopts built-in may be used to obtain options and their argu-
ments from a list of parameters. When invoked, getopts places the
value of the next option from the option string in the list in the
shell variable specified by var and its index in the shell vari-
able OPTIND. When the shell is invoked, OPTIND is initialized to
1. For each option that requires an argument, the getopts built-
in will place it in the shell variable OPTARG. If an option is
not allowed for in the optstring, then OPTARG will be unset.
optstring is a string of recognized option letters (see
getopt(3)). If a letter is followed by a colon, the option is
expected to have an argument which may or may not be separated
from it by whitespace. If an option character is not found where
expected, getopts will set the variable var to a ``?''; getopts
will then unset OPTARG and write output to standard error. By
specifying a colon as the first character of optstring all errors
will be ignored.
A nonzero value is returned when the last option is reached. If
there are no remaining arguments, getopts will set var to the spe-
cial option, ``--'', otherwise, it will set var to ``?''.
The following code fragment shows how one might process the argu-
ments for a command that can take the options [a] and [b], and the
option [c], which requires an argument.
while getopts abc: f
do
case $f in
a | b) flag=$f;;
c) carg=$OPTARG;;
\?) echo $USAGE; exit 1;;
esac
done
shift $((OPTIND - 1))
This code will accept any of the following as equivalent:
cmd -acarg file file
cmd -a -c arg file file
cmd -carg -a file file
cmd -a -carg -- file file
hash -rv command ...
The shell maintains a hash table which remembers the locations of
commands. With no arguments whatsoever, the hash command prints
out the contents of this table. Entries which have not been
looked at since the last cd command are marked with an asterisk;
it is possible for these entries to be invalid.
With arguments, the hash command removes the specified commands
from the hash table (unless they are functions) and then locates
them. With the -v option, hash prints the locations of the com-
mands as it finds them. The -r option causes the hash command to
delete all the entries in the hash table except for functions.
inputrc file
Read the file to set key bindings as defined by editrc(5).
jobid [job]
Print the process id's of the processes in the job. If the job
argument is omitted, the current job is used.
jobs This command lists out all the background processes which are
children of the current shell process.
local [-INx] [variable | -] ...
Define local variables for a function. Local variables have their
attributes, and values, as they were before the local declaration,
restored when the function terminates.
With the -N flag, variables made local, are unset initially inside
the function. Unless the -x flag is also given, such variables
are also unexported. The -I flag, which is the default in this
shell, causes the initial value and exported attribute of local
variables to be inherited from the variable with the same name in
the surrounding scope, if there is one. If there is not, the
variable is initially unset, and not exported. The -N and -I
flags are mutually exclusive, if both are given, the last speci-
fied applies. The read-only and unexportable attributes are
always inherited, if a variable with the same name already exists.
The -x flag (lower case) causes the local variable to be exported,
while the function runs, unless it has the unexportable attribute.
This can also be accomplished by using the export command, giving
the same variable names, after the local command.
Making an existing read-only variable local is possible, but
pointless. If an attempt is made to assign an initial value to
such a variable, the local command fails, as does any later
attempted assignment. If the readonly command is applied to a
variable that has been declared local, the variable cannot be
(further) modified within the function, or any other functions it
calls, however when the function returns, the previous status (and
value) of the variable is returned.
Values may be given to local variables on the local command line
in a similar fashion as used for export and readonly. These val-
ues are assigned immediately after the initialization described
above. Note that any variable references on the command line will
have been expanded before local is executed, so expressions like
local -N X=${X}
are well defined, first $X is expanded, and then the command run
is
local -N X=old-value-of-X
After arranging to preserve the old value and attributes, of X
(``old-value-of X'') local unsets X, unexports it, and then
assigns the ``old-value-of-X'' to X.
The shell uses dynamic scoping, so that if you make the variable x
local to function f, which then calls function g, references to
the variable x made inside g will refer to the variable x declared
inside f, not to the global variable named x.
Another way to view this, is as if the shell just has one flat,
global, namespace, in which all variables exist. The local com-
mand conceptually copies the variable(s) named to unnamed tempo-
rary variables, and when the function ends, copies them back
again. All references to the variables reference the same global
variables, but while the function is active, after the local com-
mand has run, the values and attributes of the variables might be
altered, and later, when the function completes, be restored.
Note that the parameters $1, $2, ... (see Positional Parameters),
and $#, $* and $@ (see Special Parameters), are always made local
in all functions, and are reset inside the function to represent
the options and arguments passed to the function. Note that $0
however retains the value it had outside the function, as do all
the other special parameters.
The only other special parameter that can be made local is ``-''.
Making ``-'' local causes any shell options that are changed via
the set command inside the function to be restored to their origi-
nal values when the function returns. If any of the shell's magic
variables (those which return a value which may vary without the
variable being explicitly altered, e.g.: SECONDS or HOSTNAME) are
made local in a function, they will lose their special properties
when set within the function, including by the local command
itself (if not to be set in the function, there is little point in
making a variable local) but those properties will be restored
when the function returns.
It is an error to use local outside the scope of a function defi-
nition. When used inside a function, it exits with status 0,
unless an undefined option is used, or at attempt is made to
assign a value to a read-only variable.
Note that either -I or -N should always be used, or variables made
local should always be given a value, or explicitly unset, as the
default behavior (inheriting the earlier value, or starting unset
after local) differs amongst shell implementations. Using ``local
-'' is an extension not implemented by most shells.
See the section LINENO below for details of the effects of making
the variable LINENO local.
pwd [-LP]
Print the current directory. If -L is specified the cached value
(initially set from PWD) is checked to see if it refers to the
current directory; if it does the value is printed. Otherwise the
current directory name is found using getcwd(3). The environment
variable PWD is set to the printed value.
The default is pwd -L, but note that the built-in cd command
doesn't support the -L option and will cache (almost) the absolute
path. If cd is changed (as unlikely as that is), pwd may be
changed to default to pwd -P.
If the current directory is renamed and replaced by a symlink to
the same directory, or the initial PWD value followed a symbolic
link, then the cached value may not be the absolute path.
The built-in command may differ from the program of the same name
because the program will use PWD and the built-in uses a sepa-
rately cached value.
read [-p prompt] [-r] variable [...]
The prompt is printed if the -p option is specified and the stan-
dard input is a terminal. Then a line is read from the standard
input. The trailing newline is deleted from the line and the line
is split as described in the field splitting section of the Word
Expansions section above, and the pieces are assigned to the vari-
ables in order. If there are more pieces than variables, the
remaining pieces (along with the characters in IFS that separated
them) are assigned to the last variable. If there are more vari-
ables than pieces, the remaining variables are assigned the null
string. The read built-in will indicate success unless EOF is
encountered on input, in which case failure is returned.
By default, unless the -r option is specified, the backslash ``\''
acts as an escape character, causing the following character to be
treated literally. If a backslash is followed by a newline, the
backslash and the newline will be deleted.
readonly name ...
readonly [-p]
The specified names are marked as read only, so that they cannot
be subsequently modified or unset. The shell allows the value of
a variable to be set at the same time it is marked read only by
writing
readonly name=value
With no arguments the readonly command lists the names of all read
only variables. With the -p option specified the output will be
formatted suitably for non-interactive use.
return [n]
Stop executing the current function or a dot command with return
value of n or the value of the last executed command, if not spec-
ified. For portability, n should be in the range from 0 to 255.
The POSIX standard says that the results of `return' outside a
function or a dot command are unspecified. This implementation
treats such a return as a no-op with a return value of 0 (success,
true). Use the exit command instead, if you want to return from a
script or exit your shell.
set [{ -options | +options | -- }] arg ...
The set command performs four different functions.
With no arguments, it lists the values of all shell variables.
With a single option of either `-o' or `+o' set outputs the cur-
rent values of the options. In the -o form, all options are
listed, with their current values. In the +o form, the shell out-
puts a string that can later be used as a command to reset all
options to their current values.
If options are given, it sets the specified option flags, or
clears them as described in the Argument List Processing section.
In addition to the options listed there, when the ``option name''
given to set -o is default all of the options are reset to the
values they had immediately after sh initialization, before any
startup scripts, or other input, had been processed. While this
may be of use to users or scripts, its primary purpose is for use
in the output of ``set +o'', to avoid that command needing to list
every available option. There is no +o default.
The fourth use of the set command is to set the values of the
shell's positional parameters to the specified arguments. To
change the positional parameters without changing any options, use
``--'' as the first argument to set. If no following arguments
are present, the set command will clear all the positional parame-
ters (equivalent to executing ``shift $#''.) Otherwise the fol-
lowing arguments become ``$1'', ``$2'', ..., and ``$#'' is set to
the number of arguments present.
setvar variable value
Assigns value to variable. (In general it is better to write
variable=value rather than using setvar. setvar is intended to be
used in functions that assign values to variables whose names are
passed as parameters.)
shift [n]
Shift the positional parameters n times. If n is omitted, 1 is
assumed. Each shift sets the value of $1 to the previous value of
$2, the value of $2 to the previous value of $3, and so on,
decreasing the value of $# by one. The shift count must be less
than or equal to the number of positional parameters ( ``$#'')
before the shift.
trap action signal ...
trap -
trap [-l]
trap [-p] signal ...
trap N signal ...
Cause the shell to parse and execute action when any of the speci-
fied signals are received. The signals are specified by signal
number or as the name of the signal. If signal is 0 or its equiv-
alent, EXIT, the action is executed when the shell exits. The
action may be a null (empty) string, which causes the specified
signals to be ignored. With action set to `-' the specified sig-
nals are set to their default actions. If the first signal is
specified in its numeric form, then action can be omitted to
achieve the same effect. This archaic, but still standard, form
should not be relied upon, use the explicit `-' action. If no
signals are specified with an action of `-', all signals are
reset.
When the shell forks off a sub-shell, it resets trapped (but not
ignored) signals to the default action. On non-interactive
shells, the trap command has no effect on signals that were
ignored on entry to the shell. On interactive shells, the trap
command will catch or reset signals ignored on entry.
Issuing trap with option -l will print a list of valid signal
names. trap without any arguments causes it to write a list of
signals and their associated non-default actions to the standard
output in a format that is suitable as an input to the shell that
achieves the same trapping results. With the -p flag, trap prints
the same information for the signals specified, or if none are
given, for all signals, including those where the action is the
default.
Examples:
trap
List trapped signals and their corresponding actions.
trap -l
Print a list of valid signals.
trap '' INT QUIT tstp 30
Ignore signals INT QUIT TSTP USR1.
trap date INT
Run the ``date'' command (print the date) upon receiving signal
INT.
trap HUP INT
Run the ``HUP'' command, or function, upon receiving signal INT.
trap 1 2
Reset the actions for signals 1 (HUP) and 2 (INT) to their
defaults.
traps=$(trap -p)
# more commands ...
trap 'action' SIG
# more commands ...
eval "$traps"
Save the trap status, execute commands, changing some traps, and
then reset all traps to their values at the start of the sequence.
The -p option is required in the first command here, or any sig-
nals that were previously untrapped (in their default states) and
which were altered during the intermediate code, would not be
reset by the final ``eval''.
type [name ...]
Interpret each name as a command and print the resolution of the
command search. Possible resolutions are: shell keyword, alias,
shell built-in, command, tracked alias and not found. For aliases
the alias expansion is printed; for commands and tracked aliases
the complete pathname of the command is printed.
ulimit [-H | -S] [-a | -btfdscmlrpnv [value]]
Inquire about or set the hard or soft limits on processes or set
new limits. The choice between hard limit (which no process is
allowed to violate, and which may not be raised once it has been
lowered) and soft limit (which causes processes to be signaled but
not necessarily killed, and which may be raised) is made with
these flags:
-H set or inquire about hard limits
-S set or inquire about soft limits. If neither -H nor
-S is specified, the soft limit is displayed or both
limits are set. If both are specified, the last one
wins.
The limit to be interrogated or set, then, is chosen by specifying
any one of these flags:
-a show all the current limits
-b show or set the limit on the socket buffer size of a
process (in bytes)
-c show or set the limit on the largest core dump size
that can be produced (in 512-byte blocks)
-d show or set the limit on the data segment size of a
process (in kilobytes)
-f show or set the limit on the largest file that can be
created (in 512-byte blocks)
-l show or set the limit on how much memory a process can
lock with mlock(2) (in kilobytes)
-m show or set the limit on the total physical memory
that can be in use by a process (in kilobytes)
-n show or set the limit on the number of files a process
can have open at once
-p show or set the limit on the number of processes this
user can have at one time
-r show or set the limit on the number of threads this
user can have at one time
-s show or set the limit on the stack size of a process
(in kilobytes)
-t show or set the limit on CPU time (in seconds)
-v show or set the limit on how large a process address
space can be
If none of these is specified, it is the limit on file size that
is shown or set. If value is specified, the limit is set to that
number; otherwise the current limit is displayed.
Limits of an arbitrary process can be displayed or set using the
sysctl(8) utility.
umask [-S] [mask]
Set the value of umask (see umask(2)) to the specified octal
value. If the argument is omitted, the umask value is printed.
With -S a symbolic form is used instead of an octal number.
unalias [-a] [name]
If name is specified, the shell removes that alias. If -a is
specified, all aliases are removed.
unset [-efvx] name ...
If -v is specified, the specified variables are unset and unex-
ported. Readonly variables cannot be unset. If -f is specified,
the specified functions are undefined. If -e is given, the speci-
fied variables are unexported, but otherwise unchanged, alterna-
tively, if -x is given, the exported status of the variable will
be retained, even after it is unset.
If no flags are provided -v is assumed. If -f is given with one
of the other flags, then the named variables will be unset, or
unexported, and functions of the same names will be undefined.
The -e and -x flags both imply -v. If -e is given, the -x flag is
ignored.
The exit status is 0, unless an attempt was made to unset a read-
only variable, in which case the exit status is 1. It is not an
error to unset (or undefine) a variable (or function) that is not
currently set (or defined.)
wait [job]
Wait for the specified job to complete and return the exit status
of the last process in the job, or 127 if the job is not a current
child of the shell. If the argument is omitted, wait for all jobs
to complete and then return an exit status of zero. If the wait
is interrupted by a signal, its exit status will be greater than
128.
Once waited upon, by specific process number or job-id, or by a
wait with no arguments, knowledge of the child is removed from the
system, and it cannot be waited upon again.
Command Line Editing
When sh is being used interactively from a terminal, the current command
and the command history (see fc in the Built-ins section) can be edited
using emacs-mode or vi-mode command-line editing. The command `set -o
emacs' enables emacs-mode editing. The command `set -o vi' enables vi-
mode editing and places the current shell process into vi insert mode.
(See the Argument List Processing section above.)
The vi mode uses commands similar to a subset of those described in the
vi(1) man page. With vi-mode enabled, sh can be switched between insert
mode and command mode. It's similar to vi(1): pressing the <ESC> key
will throw you into command VI command mode. Pressing the <return> key
while in command mode will pass the line to the shell.
The emacs mode uses commands similar to a subset available in the
emacs(1) editor. With emacs-mode enabled, special keys can be used to
modify the text in the buffer using the control key.
sh uses the editline(3) library. See editline(7) for a list of the pos-
sible command bindings, and the default settings in emacs and vi modes.
Also see editrc(5) for the commands that can be given to configure
editline(7) in the file named by the EDITRC parameter, or a file used
with the inputrc built-in command, or using editline(7)'s configuration
command line.
When command line editing is enabled, the editline(7) functions control
printing of the PS1 and PS2 prompts when required. As, in this mode, the
command line editor needs to keep track of what characters are in what
position on the command line, care needs to be taken when setting the
prompts. Normal printing characters are handled automatically, however
mode setting sequences, which do not actually display on the terminal,
need to be identified to editline(7). This is done, when needed, by
choosing a character that is not needed anywhere in the prompt, including
in the mode setting sequences, any single character is acceptable, and
assigning it to the shell parameter PSlit. Then that character should be
used, in pairs, in the prompt string. Between each pair of PSlit charac-
ters are mode setting sequences, which affect the printing attributes of
the following (normal) characters of the prompt, but do not themselves
appear visibly, nor change the terminal's cursor position.
Each such sequence, that is PSlit character, mode setting character
sequence, and another PSlit character, must currently be followed by at
least one following normal prompt character, or it will be ignored. That
is, a PSlit character cannot be the final character of PS1 or PS2, nor
may two PSlit delimited sequences appear adjacent to each other. Each
sequence can contain as many mode altering sequences as are required how-
ever. Only the first character from PSlit will be used. When set PSlit
should usually be set to a string containing just one character, then it
can simply be embedded in PS1 (or PS2) as in
PS1="${PSlit}mset${PSlit}XYZ${PSlit}mclr${PSlit}ABC"
The prompt visible will be ``XYZABC'' with the ``XYZ'' part shown accord-
ing as defined by the mode setting characters ``mset'', and then cleared
again by ``mclr''. See tput(1) for one method to generate appropriate
mode sequences. Note that both parts, XYZ and ABC, must each contain at
least one character.
If PSlit is unset, which is its initial state, or set to a null string,
no literal character will be defined, and all characters of the prompt
strings will be assumed to be visible characters (which includes spaces
etc.) To allow smooth use of prompts, without needing redefinition, when
editline(7) is disabled, the character chosen should be one which will be
ignored by the terminal if received, as when edlitline(7) is not in use,
the prompt strings are simply written to the terminal. For example, set-
ting:
PSlit="$(printf '\1')"
PS1="${PSlit}$(tput bold blink)${PSlit}\$${PSlit}$(tput sgr0)${PSlit} "
will arrange for the primary prompt to be a bold blinking dollar sign, if
supported by the current terminal, followed by an (ordinary) space, and,
as the SOH (Control-A) character ('\1') will not normally affect a termi-
nal, this same prompt will usually work with editline(7) enabled or dis-
abled.
ENVIRONMENT
CDPATH The search path used with the cd built-in.
EDITRC Gives the name of the file containing commands for
editline(7). See editrc(5) for possible content and format.
The file is processed, when in interactive mode with command
line editing enabled, whenever EDITRC is set (even with no
actual value change,) and if command line editing changes from
disabled to enabled, or the editor style used is changed.
(See the -E and -V options of the set builtin command,
described in Built-ins above, which are documented further
above in Argument List Processing.) If unset
``$HOME/.editrc'' is used.
ENV Names the file sourced at startup by the shell. Unused by
this shell after initialization, but is usually passed through
the environment to descendant shells.
EUSER Set to the login name of the effective user id running the
shell, as returned by
getpwuid(geteuid())->pw_name
(See getpwuid(3) and geteuid(2) for more details.) This is
obtained each time EUSER is expanded, so changes to the
shell's execution identity cause updates without further
action. If unset, it returns nothing. If set it loses its
special properties, and is simply a variable.
HISTSIZE The number of lines in the history buffer for the shell.
HOME Set automatically by login(1) from the user's login directory
in the password file (passwd(5)). This environment variable
also functions as the default argument for the cd built-in.
HOSTNAME Set to the current hostname of the system, as returned by
gethostname(3). This is obtained each time HOSTNAME is
expanded, so changes to the system's name are reflected with-
out further action. If unset, it returns nothing. If set it
loses its special properties, and is simply a variable.
IFS Input Field Separators. This is normally set to <space>,
<tab>, and <newline>. See the White Space Splitting section
for more details.
LANG The string used to specify localization information that
allows users to work with different culture-specific and lan-
guage conventions. See nls(7).
LINENO The current line number in the script or function. See the
section LINENO below for more details.
MAIL The name of a mail file, that will be checked for the arrival
of new mail. Overridden by MAILPATH. The check occurs just
before PS1 is written, immediately after reporting jobs which
have changed status, in interactive shells only. New mail is
considered to have arrived if the monitored file has increased
in size since the last check.
MAILPATH A colon ``:'' separated list of file names, for the shell to
check for incoming mail. This environment setting overrides
the MAIL setting. There is a maximum of 10 mailboxes that can
be monitored at once.
PATH The default search path for executables. See the Path Search
section above.
POSIXLY_CORRECT
If set in the environment upon initialization of the shell,
then the shell option posix will be set. (See the description
of the set command in the Built-ins section.) After initial-
ization it is unused by the shell, but is usually passed
through the environment to descendant processes, including
other instances of the shell, which may interpret it in a sim-
ilar way.
PPID The process identified of the parent process of the current
shell. This value is set at shell startup, ignoring any value
in the environment, and then made readonly.
PS1 The primary prompt string, which defaults to ``$ '', unless
you are the superuser, in which case it defaults to ``# ''.
This string is subject to parameter, arithmetic, and if
enabled by setting the promptcmds option, command substitution
before being output. During execution of commands used by
command substitution, execution tracing, the xtrace (set -x)
option is temporarily disabled. If promptcmds is not set and
the prompt string uses command substitution, the prompt used
will be an appropriate error string. For other expansion
errors, a message will be output, and the unexpanded string
will then be used as the prompt.
PS2 The secondary prompt string, which defaults to ``> ''. After
expansion (as for PS1) it is written whenever more input is
required to complete the current command.
PS4 Output, after expansion like PS1, before each line when execu-
tion trace (set -x) is enabled. PS4 defaults to ``+ ''.
PSc Initialised by the shell, ignoring any value from the environ-
ment, to a single character string, either `#' or `$', depend-
ing upon whether the current user is the superuser or not.
This is intended for use when building a custom PS1.
PSlit Defines the character which may be embedded in pairs, in PS1
or PS2 to indicate to editline(7) that the characters between
each pair of occurrences of the PSlit character will not
appear in the visible prompt, and will not cause the termi-
nal's cursor to change position, but rather set terminal
attributes for the following prompt character(s) at least one
of which must be present. See Command Line Editing above for
more information.
RANDOM Returns a different pseudo-random integer, in the range
[0,32767] each time it is accessed. RANDOM can be assigned an
integer value to seed the PRNG. If the value assigned is a
constant, then the sequence of values produces on subsequent
references of RANDOM will repeat after the next time the same
constant is assigned. Note, this is not guaranteed to remain
constant from one version of the shell to another - the PRNG
algorithm, or seeding method is subject to change. If RANDOM
is assigned an empty value (null string) then the next time
RANDOM is accessed, it will be seeded from a more genuinely
random source. The sequence of pseudo-random numbers gener-
ated will not be able to be generated again (except by luck,
whether good or bad, depends!) This is also how the initial
seed is generated, if none has been assigned before RANDOM is
first accessed after shell initialization.
SECONDS Returns the number of seconds since the current shell was
started. If unset, it remains unset, and returns nothing,
unless set again. If set, it loses its special properties,
and becomes a normal variable.
START_TIME
Initialised by the shell to the number of seconds since the
Epoch (see localtime(3)) when the shell was started. The
value of
$((START_TIME + SECONDS))
represents the current time, if START_TIME has not been modi-
fied, and SECONDS has not been set or unset.
TERM The default terminal setting for the shell. This is inherited
by children of the shell, and is used in the history editing
modes.
ToD When referenced, uses the value of ToD_FORMAT (or ``%T'' if
ToD_FORMAT is unset) as the format argument to strftime(3) to
encode the current time of day, in the time zone defined by TZ
if set, or current local time if not, and returns the result.
If unset ToD returns nothing. If set, it loses its special
properties, and becomes a normal variable.
ToD_FORMAT
Can be set to the strftime(3) format string to be used when
expanding ToD. Initially unset.
TZ If set, gives the time zone (see localtime(3), environ(7)) to
use when formatting ToD and if exported, other utilities that
deal with times. If unset, the system's local wall clock time
zone is used.
NETBSD_SHELL
Unlike the variables previously mentioned, this variable is
somewhat strange, in that it cannot be set, inherited from the
environment, modified, or exported from the shell. If set, by
the shell, it indicates that the shell is the sh defined by
this manual page, and gives its version information. It can
also give information in additional space separated words,
after the version string. If the shell was built as part of a
reproducible build, the relevant date that was used for that
build will be included. Finally, any non-standard compilation
options, which may affect features available, that were used
when building the shell will be listed. NETBSD_SHELL behaves
like any other variable that has the read-only and un-
exportable attributes set.
LINENO
LINENO is in many respects a normal shell variable, containing an integer
value. and can be expanded using any of the forms mentioned above which
can be used for any other variable.
LINENO can be exported, made readonly (which prevents attempts to assign
to it, and to unset it, but which does not change the value, that is the
current line number, from being obtained when LINENO is referenced,) and
can be unset. All of those act as they would with any other variable.
However, LINENO should normally not ever be set or unset. In this shell
setting LINENO reverses the effect of an earlier unset, but does not oth-
erwise affect the value obtained. If unset, LINENO should not normally
be set again, doing so is not portable. If LINENO is set or unset, dif-
ferent shells act differently. The value of LINENO is never imported
from the environment when the shell is started, though if present there,
as with any other variable, LINENO will be exported by this shell.
LINENO is set automatically by the shell to be the number of the source
line on which it occurs. When exported, LINENO is exported with its
value set to the line number it would have had had it been referenced on
the command line of the command to which it is exported. Line numbers
are counted from 1, which is the first line the shell reads from any par-
ticular file. For this shell, standard input, including in an interac-
tive shell, the user's terminal, is just another file and lines are
counted there as well. However note that not all shells count interac-
tive lines this way, it is not wise to rely upon LINENO having a useful
value, except in a script, or a function.
The role of LINENO in functions is less clear. In some shells, LINENO
continues to refer to the line number in the script which defines the
function, in others lines count from one within the function, always (and
resume counting normally once the function definition is complete) and
others count in functions from one if the function is defined interac-
tively, but otherwise just reference the line number in the script in
which the function is defined. This shell gives the user the option to
choose. If the -L flag (the local_lineno option, see Argument List
Processing) is set, when the function is defined, then the function
defaults to counting lines with one being the first line of the function.
When the -L flag is not set, the shell counts lines in a function defini-
tion in the same continuous sequence as the lines that surround the func-
tion definition. Further, if LINENO is made local (see Built-ins above)
inside the function, the function can decide which behavior it prefers.
If LINENO is made local and inherited, and not given a value, as in
local -I LINENO
then from that point in the function, LINENO will give the line number as
if lines are counted in sequence with the lines that surround the func-
tion definition (and any other function definitions in which this is
nested.) If LINENO is made local, and in that same command, given a
value, as
local [-I|-N] LINENO=value
then LINENO will give the line number as if lines are counted from one
from the beginning of the function. The value nominally assigned in this
case is irrelevant, and ignored. For completeness, if lineno is made
local and unset, as in
local -N LINENO
then LINENO is simply unset inside the function, and gives no value at
all.
Now for some technical details. The line on which LINENO occurs in a
parameter expansion, is the line that contains the `$' that begins the
expansion of LINENO. In the case of nested expansions, that `$' is the
one that actually has LINENO as its parameter. In an arithmetic expan-
sion, where no `$' is used to evaluate LINENO but LINENO is simply refer-
enced as a variable, then the value is the line number of the line that
contains the `L' of LINENO. For functions line one of the function defi-
nition (when relevant) is the line that contains the first character of
the function name in the definition. When exported, the line number of
the command is the line number where the first character of the word
which becomes the command name occurs.
When the shell opens a new file, for any reason, it counts lines from one
in that file, and then resumes its original counting once it resumes
reading the previous input stream. When handling a string passed to eval
the line number starts at the line on which the string starts, and then
if the string contains internal newline characters, those characters
increase the line number. This means that references to LINENO in such a
case can produce values larger than would be produced by a reference on
the line after the eval.
FILES
$HOME/.profile
/etc/profile
EXIT STATUS
Errors that are detected by the shell, such as a syntax error, will cause
the shell to exit with a non-zero exit status. If the shell is not an
interactive shell, the execution of the shell file will be aborted. Oth-
erwise the shell will return the exit status of the last command exe-
cuted, or if the exit built-in is used with a numeric argument, it will
return the argument.
SEE ALSO
csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
editline(3), getopt(3), editrc(5), passwd(5), editline(7), environ(7),
nls(7), sysctl(8)
HISTORY
A sh command appeared in Version 1 AT&T UNIX. It was replaced in
Version 7 AT&T UNIX with a version that introduced the basis of the cur-
rent syntax. That was, however, unmaintainable so we wrote this one.
BUGS
Setuid shell scripts should be avoided at all costs, as they are a sig-
nificant security risk.
The characters generated by filename completion should probably be quoted
to ensure that the filename is still valid after the input line has been
processed.
The trap command cannot usefully be used, yet, within a command substitu-
tion, to obtain the current trap values, as all command substitutions are
currently executed within a sub-shell environment, and in sub-shells all
non-ignored, non-default, traps are reset. As a workaround, it is possi-
ble to redirect output from ``trap'' or ``trap -p'' to a file, and then
read the file later using the ``.'' command.
Many, many, more. (But less than there were...)
NetBSD 8.1 October 6, 2017 NetBSD 8.1
Powered by man-cgi (2024-03-20).
Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.