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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.1‐2008, Section 2.6, Word
Expansions, subject to quoting as described in the Shell and
Utilities volume of POSIX.1‐2008, 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.1‐2008,
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.1‐2008,
Section 2.6.5, Field Splitting) or pathname expansion (see the Shell
and Utilities volume of POSIX.1‐2008, 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.1‐2008, 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.1‐2008, 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.1‐2008, fail if
command substitution, as specified in the Shell and
Utilities volume of POSIX.1‐2008, 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.1‐2008, 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; however, 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 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 cases, they 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.
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.1‐2008, wordexp.h(0p)
The Shell and Utilities volume of POSIX.1‐2008, Chapter 2, Shell
Command Language
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 WORDEXP(3P)