WORDEXP(3)                 POSIX Programmer's Manual                WORDEXP(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       wordexp, wordfree -- perform word expansions

SYNOPSIS
       #include <wordexp.h>

       int wordexp(const char *restrict words, wordexp_t *restrict pwordexp,
           int flags);
       void wordfree(wordexp_t *pwordexp);

DESCRIPTION
       The wordexp() function shall perform word expansions as described in
       the Shell and Utilities volume of POSIX.1-2017, Section 2.6, Word
       Expansions, subject to quoting as described in the Shell and Utilities
       volume of POSIX.1-2017, Section 2.2, Quoting, and place the list of
       expanded words into the structure pointed to by pwordexp.

       The words argument is a pointer to a string containing one or more
       words to be expanded. The expansions shall be the same as would be
       performed by the command line interpreter if words were the part of a
       command line representing the arguments to a utility. Therefore, the
       application shall ensure that words does not contain an unquoted
       <newline> character or any of the unquoted shell special characters
       ''||'', ''&&'', '';;'', ''<<'', ''>>'' except in the context of command substitution
       as specified in the Shell and Utilities volume of POSIX.1-2017, Section
       2.6.3, Command Substitution.  It also shall not contain unquoted
       parentheses or braces, except in the context of command or variable
       substitution. The application shall ensure that every member of words
       which it expects to have expanded by wordexp() does not contain an
       unquoted initial comment character. The application shall also ensure
       that any words which it intends to be ignored (because they begin or
       continue a comment) are deleted from words.  If the argument words
       contains an unquoted comment character (<number-sign>) that is the
       beginning of a token, wordexp() shall either treat the comment
       character as a regular character, or interpret it as a comment
       indicator and ignore the remainder of words.

       The structure type wordexp_t is defined in the <wordexp.h> header and
       includes at least the following members:

         +--------------+--------------+------------------------------------+
         |Member Type   | Member Name  |            Description             |
         +--------------+--------------+------------------------------------+
         |size_t        |we__wordc      | Count of words matched by words.   |
         |char ****       |we__wordv      | Pointer to list of expanded words. |
         |size_t        |we__offs       | Slots to reserve at the beginning  |
         |              |              | of pwordexp->we__wordv.             |
         +--------------+--------------+------------------------------------+
       The wordexp() function shall store the number of generated words into
       pwordexp->we__wordc and a pointer to a list of pointers to words in
       pwordexp->we__wordv. Each individual field created during field
       splitting (see the Shell and Utilities volume of POSIX.1-2017, Section
       2.6.5, Field Splitting) or pathname expansion (see the Shell and
       Utilities volume of POSIX.1-2017, Section 2.6.6, Pathname Expansion)
       shall be a separate word in the pwordexp->we__wordv list. The words
       shall be in order as described in the Shell and Utilities volume of
       POSIX.1-2017, Section 2.6, Word Expansions.  The first pointer after
       the last word pointer shall be a null pointer.  The expansion of
       special parameters described in the Shell and Utilities volume of
       POSIX.1-2017, Section 2.5.2, Special Parameters is unspecified.

       It is the caller's responsibility to allocate the storage pointed to by
       pwordexp.  The wordexp() function shall allocate other space as needed,
       including memory pointed to by pwordexp->we__wordv. The wordfree()
       function frees any memory associated with pwordexp from a previous call
       to wordexp().

       The flags argument is used to control the behavior of wordexp().  The
       value of flags is the bitwise-inclusive OR of zero or more of the
       following constants, which are defined in <wordexp.h>:

       WRDE_APPEND   Append words generated to the ones from a previous call
                     to wordexp().

       WRDE_DOOFFS   Make use of pwordexp->we__offs. If this flag is set,
                     pwordexp->we__offs is used to specify how many null
                     pointers to add to the beginning of pwordexp->we__wordv.
                     In other words, pwordexp->we__wordv shall point to
                     pwordexp->we__offs null pointers, followed by
                     pwordexp->we__wordc word pointers, followed by a null
                     pointer.

       WRDE_NOCMD    If the implementation supports the utilities defined in
                     the Shell and Utilities volume of POSIX.1-2017, fail if
                     command substitution, as specified in the Shell and
                     Utilities volume of POSIX.1-2017, Section 2.6.3, Command
                     Substitution, is requested.

       WRDE_REUSE    The pwordexp argument was passed to a previous successful
                     call to wordexp(), and has not been passed to wordfree().
                     The result shall be the same as if the application had
                     called wordfree() and then called wordexp() without
                     WRDE_REUSE.

       WRDE_SHOWERR  Do not redirect stderr to /dev/null.

       WRDE_UNDEF    Report error on an attempt to expand an undefined shell
                     variable.

       The WRDE_APPEND flag can be used to append a new set of words to those
       generated by a previous call to wordexp().  The following rules apply
       to applications when two or more calls to wordexp() are made with the
       same value of pwordexp and without intervening calls to wordfree():

        1. The first such call shall not set WRDE_APPEND. All subsequent calls
           shall set it.

        2. All of the calls shall set WRDE_DOOFFS, or all shall not set it.

        3. After the second and each subsequent call, pwordexp->we__wordv shall
           point to a list containing the following:

            a. Zero or more null pointers, as specified by WRDE_DOOFFS and
               pwordexp->we__offs

            b. Pointers to the words that were in the pwordexp->we__wordv list
               before the call, in the same order as before

            c. Pointers to the new words generated by the latest call, in the
               specified order

        4. The count returned in pwordexp->we__wordc shall be the total number
           of words from all of the calls.

        5. The application can change any of the fields after a call to
           wordexp(), but if it does it shall reset them to the original value
           before a subsequent call, using the same pwordexp value, to
           wordfree() or wordexp() with the WRDE_APPEND or WRDE_REUSE flag.

       If the implementation supports the utilities defined in the Shell and
       Utilities volume of POSIX.1-2017, and words contains an unquoted
       character--<newline>, ''||'', ''&&'', '';;'', ''<<'', ''>>'', ''(('', ''))'', ''{{'', ''}}''--in
       an inappropriate context, wordexp() shall fail, and the number of
       expanded words shall be 0.

       Unless WRDE_SHOWERR is set in flags, wordexp() shall redirect stderr to
       /dev/null for any utilities executed as a result of command
       substitution while expanding words.  If WRDE_SHOWERR is set, wordexp()
       may write messages to stderr if syntax errors are detected while
       expanding words, unless the stderr stream has wide orientation in which
       case the behavior is undefined.  It is unspecified whether any write
       errors encountered while outputting such messages will affect the
       stderr error indicator or the value of errno.

       The application shall ensure that if WRDE_DOOFFS is set, then
       pwordexp->we__offs has the same value for each wordexp() call and
       wordfree() call using a given pwordexp.

       The results are unspecified if WRDE_APPEND and WRDE_REUSE are both
       specified.

       The following constants are defined as error return values:

       WRDE_BADCHAR  One of the unquoted characters--<newline>, ''||'', ''&&'', '';;'',
                     ''<<'', ''>>'', ''(('', ''))'', ''{{'', ''}}''--appears in words in an
                     inappropriate context.

       WRDE_BADVAL   Reference to undefined shell variable when WRDE_UNDEF is
                     set in flags.

       WRDE_CMDSUB   Command substitution requested when WRDE_NOCMD was set in
                     flags.

       WRDE_NOSPACE  Attempt to allocate memory failed.

       WRDE_SYNTAX   Shell syntax error, such as unbalanced parentheses or
                     unterminated string.

RETURN VALUE
       Upon successful completion, wordexp() shall return 0. Otherwise, a non-
       zero value, as described in <wordexp.h>, shall be returned to indicate
       an error. If wordexp() returns the value WRDE_NOSPACE, then
       pwordexp->we__wordc and pwordexp->we__wordv shall be updated to reflect
       any words that were successfully expanded. In other error cases, if the
       WRDE_APPEND flag was specified, pwordexp->we__wordc and
       pwordexp->we__wordv shall not be modified.

       The wordfree() function shall not return a value.

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       The wordexp() function is intended to be used by an application that
       wants to do all of the shell's expansions on a word or words obtained
       from a user. For example, if the application prompts for a pathname (or
       list of pathnames) and then uses wordexp() to process the input, the
       user could respond with anything that would be valid as input to the
       shell.

       The WRDE_NOCMD flag is provided for applications that, for security or
       other reasons, want to prevent a user from executing shell commands.
       Disallowing unquoted shell special characters also prevents unwanted
       side-effects, such as executing a command or writing a file.

       POSIX.1-2008 does not require the wordexp() function to be thread-safe
       if passed an expression referencing an environment variable while any
       other thread is concurrently modifying any environment variable; see
       exec.

       Even though the WRDE_SHOWERR flag allows the implementation to write
       messages to stderr during command substitution or syntax errors, this
       standard does not provide any way to detect write failures during the
       output of such messages.

       Applications which use wide-character output functions with stderr
       should ensure that any calls to wordexp() do not write to stderr, by
       avoiding use of the WRDE_SHOWERR flag.

RATIONALE
       This function was included as an alternative to glob().  There had been
       continuing controversy over exactly what features should be included in
       glob().  It is hoped that by providing wordexp() (which provides all of
       the shell word expansions, but which may be slow to execute) and glob()
       (which is faster, but which only performs pathname expansion, without
       tilde or parameter expansion) this will satisfy the majority of
       applications.

       While wordexp() could be implemented entirely as a library routine, it
       is expected that most implementations run a shell in a subprocess to do
       the expansion.

       Two different approaches have been proposed for how the required
       information might be presented to the shell and the results returned.
       They are presented here as examples.

       One proposal is to extend the echo utility by adding a -q option. This
       option would cause echo to add a <backslash> before each <backslash>
       and <blank> that occurs within an argument. The wordexp() function
       could then invoke the shell as follows:


           (void) strcpy(buffer, "echo -q");
           (void) strcat(buffer, words);
           if ((flags & WRDE_SHOWERR) == 0)
               (void) strcat(buffer, "2>/dev/null");
           f = popen(buffer, "r");

       The wordexp() function would read the resulting output, remove unquoted
       <backslash> characters, and break into words at unquoted <blank>
       characters. If the WRDE_NOCMD flag was set, wordexp() would have to
       scan words before starting the subshell to make sure that there would
       be no command substitution. In any case, it would have to scan words
       for unquoted special characters.

       Another proposal is to add the following options to sh:

       -w wordlist
             This option provides a wordlist expansion service to
             applications. The words in wordlist shall be expanded and the
             following written to standard output:

              1. The count of the number of words after expansion, in decimal,
                 followed by a null byte

              2. The number of bytes needed to represent the expanded words
                 (not including null separators), in decimal, followed by a
                 null byte

              3. The expanded words, each terminated by a null byte

             If an error is encountered during word expansion, sh exits with a
             non-zero status after writing the former to report any words
             successfully expanded

       -P    Run in ``protected'' mode. If specified with the -w option, no
             command substitution shall be performed.

       With these options, wordexp() could be implemented fairly simply by
       creating a subprocess using fork() and executing sh using the line:


           execl(<shell path>, "sh", "-P", "-w", words, (char *)0);

       after directing standard error to /dev/null.

       It seemed objectionable for a library routine to write messages to
       standard error, unless explicitly requested, so wordexp() is required
       to redirect standard error to /dev/null to ensure that no messages are
       generated, even for commands executed for command substitution. The
       WRDE_SHOWERR flag can be specified to request that error messages be
       written.

       The WRDE_REUSE flag allows the implementation to avoid the expense of
       freeing and reallocating memory, if that is possible. A minimal
       implementation can call wordfree() when WRDE_REUSE is set.

FUTURE DIRECTIONS
       None.

SEE ALSO
       exec, fnmatch(), glob()

       The Base Definitions volume of POSIX.1-2017, <wordexp.h>

       The Shell and Utilities volume of POSIX.1-2017, Chapter _2, Shell
       Command Language

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           WORDEXP(3)
