shquote(3) - NetBSD Manual Pages

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SHQUOTE(3)              NetBSD Library Functions Manual             SHQUOTE(3)


NAME

     shquote, shquotev -- quote argument strings for use with the shell


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <stdlib.h>

     size_t
     shquote(const char *arg, char *buf, size_t bufsize);

     size_t
     shquotev(int argc, char * const *argv, char *buf, size_t bufsize);


DESCRIPTION

     The shquote() and shquotev() functions copy strings and transform the
     copies by adding shell escape and quoting characters.  They are used to
     encapsulate arguments to be included in command strings passed to the
     system() and popen() functions, so that the arguments will have the cor-
     rect values after being evaluated by the shell.

     The exact method of quoting and escaping may vary, and is intended to
     match the conventions of the shell used by system() and popen().  It may
     not match the conventions used by other shells.  In this implementation,
     the following transformation is applied to each input string:

     ·       it is surrounded by single quotes ('),

     ·       any single quotes in the input are escaped by replacing them with
             the four-character sequence: '\'', and

     ·       extraneous pairs of single quotes (caused by multiple adjacent
             single quotes in the input string, or by single quotes at the
             beginning or end of the input string) are elided.

     The shquote() function transforms the string specified by its arg argu-
     ment, and places the result into the memory pointed to by buf.

     The shquotev() function transforms each of the argc strings specified by
     the array argv independently.  The transformed strings are placed in the
     memory pointed to by buf, separated by spaces.  It does not modify the
     pointer array specified by argv or the strings pointed to by the pointers
     in the array.

     Both functions write up to bufsize - 1 characters of output into the
     buffer pointed to by buf, then add a NUL character to terminate the out-
     put string.  If bufsize is given as zero, the buf parameter is ignored
     and no output is written.


RETURN VALUES

     The shquote() and shquotev() functions return the number of characters
     necessary to hold the result from operating on their input strings, not
     including the terminating NUL.  That is, they return the length of the
     string that would have been written to the output buffer, if it were
     large enough.  If an error occurs during processing, the value
     ((size_t)-1) is returned and errno is set appropriately.


EXAMPLES

     The following code fragment demonstrates how you might use shquotev() to
     construct a command string to be used with system().  The command uses an
     environment variable (which will be expanded by the shell) to determine
     the actual program to run.  Note that the environment variable may be
     expanded by the shell into multiple words.  The first word of the expan-
     sion will be used by the shell as the name of the program to run, and the
     rest will be passed as arguments to the program.

           char **argv, c, *cmd;
           size_t cmdlen, len, qlen;
           int argc;

           ...

           /*
            * Size buffer to hold the command string, and allocate it.
            * Buffer of length one given to snprintf() for portability.
            */
           cmdlen = snprintf(&c, 1, "${PROG-%s} ", PROG_DEFAULT);
           qlen = shquotev(argc, argv, NULL, 0);
           if (qlen == (size_t)-1) {
                   ...
           }
           cmdlen += qlen + 1;
           cmd = malloc(cmdlen);
           if (cmd == NULL) {
                   ...
           }

           /* Create the command string. */
           len = snprintf(cmd, cmdlen, "${PROG-%s} ", PROG_DEFAULT);
           qlen = shquotev(argc, argv, cmd + len, cmdlen - len);
           if (qlen == (size_t)-1) {
                   /* Should not ever happen. */
                   ...
           }
           len += qlen;

           /* "cmd" can now be passed to system(). */

     The following example shows how you would implement the same functional-
     ity using the shquote() function directly.

           char **argv, c, *cmd;
           size_t cmdlen, len, qlen;
           int argc, i;

           ...

           /*
            * Size buffer to hold the command string, and allocate it.
            * Buffer of length one given to snprintf() for portability.
            */
           cmdlen = snprintf(&c, 1, "${PROG-%s} ", PROG_DEFAULT);
           for (i = 0; i < argc; i++) {
                   qlen = shquote(argv[i], NULL, 0);
                   if (qlen == (size_t)-1) {
                           ...
                   }
                   cmdlen += qlen + 1;
           }
           cmd = malloc(cmdlen);
           if (cmd == NULL) {
                   ...
           }

           /* Start the command string with the env var reference. */
           len = snprintf(cmd, cmdlen, "${PROG-%s} ", PROG_DEFAULT);

           /* Quote all of the arguments when copying them. */
           for (i = 0; i < argc; i++) {
                   qlen = shquote(argv[i], cmd + len, cmdlen - len);
                   if (qlen == (size_t)-1) {
                           /* Should not ever happen. */
                           ...
                   }
                   len += qlen;
                   cmd[len++] = ' ';
           }
           cmd[--len] = '\0';

           /* "cmd" can now be passed to system(). */


SEE ALSO

     sh(1), popen(3), system(3)


BUGS

     This implementation does not currently handle strings containing multi-
     byte characters properly.  To address this issue, /bin/sh (the shell used
     by system() and popen()) must first be fixed to handle multibyte charac-
     ters.  When that has been done, these functions can have multibyte char-
     acter support enabled.

NetBSD 6.1                     September 7, 2008                    NetBSD 6.1