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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
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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 .