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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(3POSIX)