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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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WORDEXP(3P) POSIX Programmer's Manual WORDEXP(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
wordexp, wordfree — perform word expansions
#include <wordexp.h>
int wordexp(const char *restrict words, wordexp_t *restrict pwordexp,
int flags);
void wordfree(wordexp_t *pwordexp);
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.
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.
No errors are defined.
The following sections are informative.
None.
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(1p).
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.
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.
None.
exec(1p), fnmatch(3p), glob(3p)
The Base Definitions volume of POSIX.1‐2017, wordexp.h(0p)
The Shell and Utilities volume of POSIX.1‐2017, Chapter 2, Shell
Command Language
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(3P)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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