file(1)
- NetBSD Manual Pages
FILE(1) NetBSD General Commands Manual FILE(1)
NAME
file -- determine file type
SYNOPSIS
file [-bchikLnNprsvz] [-f namefile] [-F separator] [-m magicfiles] file
file -C [-m magicfile]
DESCRIPTION
This manual page documents version 4.21 of the file command.
file tests each argument in an attempt to classify it. There are three
sets of tests, performed in this order: filesystem tests, magic number
tests, and language tests. The first test that succeeds causes the file
type to be printed.
The type printed will usually contain one of the words text (the file
contains only printing characters and a few common control characters and
is probably safe to read on an ASCII terminal), executable (the file con-
tains the result of compiling a program in a form understandable to some
UNIX kernel or another), or data meaning anything else (data is usually
`binary' or non-printable). Exceptions are well-known file formats (core
files, tar archives) that are known to contain binary data. When modify-
ing the file /usr/share/misc/magic or the program itself, make sure to
preserve these keywords. People depend on knowing that all the readable
files in a directory have the word ``text'' printed. Don't do as Berke-
ley did and change ``shell commands text'' to ``shell script''. Note
that the file /usr/share/misc/magic is built mechanically from a large
number of small files in the subdirectory Magdir in the source distribu-
tion of this program.
The filesystem tests are based on examining the return from a stat(2)
system call. The program checks to see if the file is empty, or if it's
some sort of special file. Any known file types appropriate to the sys-
tem you are running on (sockets, symbolic links, or named pipes (FIFOs)
on those systems that implement them) are intuited if they are defined in
the system header file <sys/stat.h>.
The magic number tests are used to check for files with data in particu-
lar fixed formats. The canonical example of this is a binary executable
(compiled program) a.out file, whose format is defined in <elf.h>,
<a.out.h> and possibly <exec.h> in the standard include directory. These
files have a `magic number' stored in a particular place near the begin-
ning of the file that tells the UNIX operating system that the file is a
binary executable, and which of several types thereof. The concept of a
`magic number' has been applied by extension to data files. Any file
with some invariant identifier at a small fixed offset into the file can
usually be described in this way. The information identifying these
files is read from the compiled magic file /usr/share/misc/magic.mgc, or
/usr/share/misc/magic if the compile file does not exist. In addition
file will look in $HOME/.magic.mgc, or $HOME/.magic for magic entries.
If a file does not match any of the entries in the magic file, it is
examined to see if it seems to be a text file. ASCII, ISO-8859-x, non-
ISO 8-bit extended-ASCII character sets (such as those used on Macintosh
and IBM PC systems), UTF-8-encoded Unicode, UTF-16-encoded Unicode, and
EBCDIC character sets can be distinguished by the different ranges and
sequences of bytes that constitute printable text in each set. If a file
passes any of these tests, its character set is reported. ASCII,
ISO-8859-x, UTF-8, and extended-ASCII files are identified as ``text''
because they will be mostly readable on nearly any terminal; UTF-16 and
EBCDIC are only ``character data'' because, while they contain text, it
is text that will require translation before it can be read. In addi-
tion, file will attempt to determine other characteristics of text-type
files. If the lines of a file are terminated by CR, CRLF, or NEL,
instead of the Unix-standard LF, this will be reported. Files that con-
tain embedded escape sequences or overstriking will also be identified.
Once file has determined the character set used in a text-type file, it
will attempt to determine in what language the file is written. The lan-
guage tests look for particular strings (cf <names.h> that can appear
anywhere in the first few blocks of a file. For example, the keyword .br
indicates that the file is most likely a troff(1) input file, just as the
keyword struct indicates a C program. These tests are less reliable than
the previous two groups, so they are performed last. The language test
routines also test for some miscellany (such as tar(1) archives).
Any file that cannot be identified as having been written in any of the
character sets listed above is simply said to be ``data''.
OPTIONS
-b, --brief
Do not prepend filenames to output lines (brief mode).
-c, --checking-printout
Cause a checking printout of the parsed form of the magic file.
This is usually used in conjunction with the -m flag to debug a
new magic file before installing it.
-C, --compile
Write a magic.mgc output file that contains a pre-parsed version
of the magic file.
-e, --exclude testname
Exclude the test named in testname from the list of tests made to
determine the file type. Valid test names are:
apptype
Check for EMX application type (only on EMX).
ascii
Check for various types of ascii files.
compress
Don't look for, or inside compressed files.
elf
Don't print elf details.
fortran
Don't look for fortran sequences inside ascii files.
soft
Don't consult magic files.
tar
Don't examine tar files.
token
Don't look for known tokens inside ascii files.
troff
Don't look for troff sequences inside ascii files.
-f, --files-from namefile
Read the names of the files to be examined from namefile (one per
line) before the argument list. Either namefile or at least one
filename argument must be present; to test the standard input,
use `-' as a filename argument.
-F, --separator separator
Use the specified string as the separator between the filename
and the file result returned. Defaults to `:'.
-h, --no-dereference
option causes symlinks not to be followed (on systems that sup-
port symbolic links). This is the default if the environment
variable POSIXLY_CORRECT is not defined.
-i, --mime
Causes the file command to output mime type strings rather than
the more traditional human readable ones. Thus it may say
``text/plain; charset=us-ascii'' rather than ``ASCII text''. In
order for this option to work, file changes the way it handles
files recognized by the command itself (such as many of the text
file types, directories etc), and makes use of an alternative
``magic'' file. (See ``FILES'' section, below).
-k, --keep-going
Don't stop at the first match, keep going.
-L, --dereference
option causes symlinks to be followed, as the like-named option
in ls(1) (on systems that support symbolic links). This is the
default if the environment variable POSIXLY_CORRECT is defined.
-m, --magic-file list
Specify an alternate list of files containing magic numbers.
This can be a single file, or a colon-separated list of files.
If a compiled magic file is found alongside, it will be used
instead. With the -i -or -mime option, the program adds
``.mime'' to each file name.
-n, --no-buffer
Force stdout to be flushed after checking each file. This is
only useful if checking a list of files. It is intended to be
used by programs that want filetype output from a pipe.
-N, --no-pad
Don't pad filenames so that they align in the output.
-p, --preserve-date
On systems that support utime(2) or utimes(2), attempt to pre-
serve the access time of files analyzed, to pretend that file
never read them.
-r, --raw
Don't translate unprintable characters to \ooo. Normally file
translates unprintable characters to their octal representation.
-s, --special-files
Normally, file only attempts to read and determine the type of
argument files which stat(2) reports are ordinary files. This
prevents problems, because reading special files may have pecu-
liar consequences. Specifying the -s option causes file to also
read argument files which are block or character special files.
This is useful for determining the filesystem types of the data
in raw disk partitions, which are block special files. This
option also causes file to disregard the file size as reported by
stat(2) since on some systems it reports a zero size for raw disk
partitions.
-v, --version
Print the version of the program and exit.
-z, --uncompress
Try to look inside compressed files.
-0, --print0
Output a null character `\0' after the end of the filename. Nice
to cut(1) the output. This does not affect the separator which is
still printed.
--help Print a help message and exit.
FILES
/usr/share/misc/magic.mgc Default compiled list of magic numbers
/usr/share/misc/magic Default list of magic numbers
/usr/share/misc/magic.mime.mgc Default compiled list of magic numbers,
used to output mime types when the -i
option is specified.
/usr/share/misc/magic.mime Default list of magic numbers, used to
output mime types when the -i option is
specified.
ENVIRONMENT
The environment variable MAGIC can be used to set the default magic num-
ber file name. If that variable is set, then file will not attempt to
open $HOME/.magic. file adds ``.mime'' and/or ``.mgc'' to the value of
this variable as appropriate. The environment variable POSIXLY_CORRECT
controls (on systems that support symbolic links), if file will attempt
to follow symlinks or not. If set, then file follows symlink, otherwise
it does not. This is also controlled by the -L and -h options.
SEE ALSO
magic(5), strings(1), od(1), hexdump(1)
STANDARDS CONFORMANCE
This program is believed to exceed the System V Interface Definition of
FILE(CMD), as near as one can determine from the vague language contained
therein. Its behavior is mostly compatible with the System V program of
the same name. This version knows more magic, however, so it will pro-
duce different (albeit more accurate) output in many cases.
The one significant difference between this version and System V is that
this version treats any white space as a delimiter, so that spaces in
pattern strings must be escaped. For example,
>10 string language impress (imPRESS data)
in an existing magic file would have to be changed to
>10 string language\ impress (imPRESS data)
In addition, in this version, if a pattern string contains a backslash,
it must be escaped. For example
0 string \begindata Andrew Toolkit document
in an existing magic file would have to be changed to
0 string \\begindata Andrew Toolkit document
SunOS releases 3.2 and later from Sun Microsystems include a file command
derived from the System V one, but with some extensions. My version dif-
fers from Sun's only in minor ways. It includes the extension of the `&'
operator, used as, for example,
>16 long&0x7fffffff >0 not stripped
MAGIC DIRECTORY
The magic file entries have been collected from various sources, mainly
USENET, and contributed by various authors. Christos Zoulas (address
below) will collect additional or corrected magic file entries. A con-
solidation of magic file entries will be distributed periodically.
The order of entries in the magic file is significant. Depending on what
system you are using, the order that they are put together may be incor-
rect. If your old file command uses a magic file, keep the old magic
file around for comparison purposes (rename it to
/usr/share/misc/magic.orig ).
EXAMPLES
$ file file.c obj/file /dev/wd0a
file.c: C program text
obj/file: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV),
for NetBSD 4.99.12, dynamically linked (uses shared libs),
not stripped
/dev/wd0a: block special (0/0)
# file -s /dev/rwd0[abe]
/dev/rwd0a: x86 boot sector, BSD disklabel
/dev/rwd0b: data
/dev/rwd0e: Unix Fast File system (little-endian), last mounted on
/usr, last written at Mon Feb 10 13:22:40 2003, clean flag 2, num-
ber of blocks 28754208, number of data blocks 27812712, number of
cylinder groups 3566, block size 8192, fragment size 1024, minimum
percentage of free blocks 5, rotational delay 0ms, disk rotational
speed 60rps, TIME optimization
HISTORY
There has been a file command in every UNIX since at least Research
Version 4 (man page dated November, 1973). The System V version intro-
duced one significant major change: the external list of magic number
types. This slowed the program down slightly but made it a lot more
flexible.
This program, based on the System V version, was written by Ian Darwin
<ian@darwinsys.com> without looking at anybody else's source code.
John Gilmore revised the code extensively, making it better than the
first version. Geoff Collyer found several inadequacies and provided
some magic file entries. Contributions by the `&' operator by Rob McMa-
hon, cudcv@warwick.ac.uk, 1989.
Guy Harris, guy@netapp.com, made many changes from 1993 to the present.
Primary development and maintenance from 1990 to the present by Christos
Zoulas (christos@astron.com).
Altered by Chris Lowth, chris@lowth.com, 2000: Handle the -i option to
output mime type strings and using an alternative magic file and internal
logic.
Altered by Eric Fischer (enf@pobox.com), July, 2000, to identify charac-
ter codes and attempt to identify the languages of non-ASCII files.
The list of contributors to the "Magdir" directory (source for the
/usr/share/misc/magic file) is too long to include here. You know who
you are; thank you.
LEGAL NOTICE
Copyright (c) Ian F. Darwin, Toronto, Canada, 1986-1999. Covered by the
standard Berkeley Software Distribution copyright; see the file
LEGAL.NOTICE in the source distribution.
The files tar.h and is_tar.c were written by John Gilmore from his pub-
lic-domain tar(1) program, and are not covered by the above license.
BUGS
There must be a better way to automate the construction of the Magic file
from all the glop in Magdir. What is it?
file uses several algorithms that favor speed over accuracy, thus it can
be misled about the contents of text files.
The support for text files (primarily for programming languages) is sim-
plistic, inefficient and requires recompilation to update. Their use of
ASCII TAB as a field delimiter is ugly and makes it hard to edit the
files, but is entrenched.
It might be advisable to allow upper-case letters in keywords for e.g.,
troff(1) commands vs man page macros. Regular expression support would
make this easy.
The program doesn't grok FORTRAN. It should be able to figure FORTRAN by
seeing some keywords which appear indented at the start of line. Regular
expression support would make this easy.
The list of keywords in ascmagic probably belongs in the Magic file.
This could be done by using some keyword like `*' for the offset value.
Complain about conflicts in the magic file entries. Make a rule that the
magic entries sort based on file offset rather than position within the
magic file?
The program should provide a way to give an estimate of ``how good'' a
guess is. We end up removing guesses (e.g. ``From '' as first 5 chars
of file) because they are not as good as other guesses (e.g.
``Newsgroups:'' versus ``Return-Path:'' ). Still, if the others don't
pan out, it should be possible to use the first guess.
This program is slower than some vendors' file commands. The new support
for multiple character codes makes it even slower.
This manual page, and particularly this section, is too long.
AVAILABILITY
You can obtain the original author's latest version by anonymous FTP on
ftp.astron.com in the directory /pub/file/file-X.YZ.tar.gz
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