WORDEXP(P) POSIX Programmer's Manual WORDEXP(P)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 IEEE Std 1003.1-2001, Section 2.6,
Word Expansions, subject to quoting as in the Shell and Utilities vol‐
ume of IEEE Std 1003.1-2001, 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 per‐
formed by the command line interpreter if words were the part of a com‐
mand line representing the arguments to a utility. Therefore, the
application shall ensure that words does not contain an unquoted <new‐
line> or any of the unquoted shell special characters '|' , '&' , ';' ,
'<' , '>' except in the context of command substitution as specified in
the Shell and Utilities volume of IEEE Std 1003.1-2001, 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 ini‐
tial 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 split‐
ting (see the Shell and Utilities volume of IEEE Std 1003.1-2001, Sec‐
tion 2.6.5, Field Splitting) or pathname expansion (see the Shell and
Utilities volume of IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001, Section 2.6, Word Expansions. The first
pointer after the last word pointer shall be a null pointer. The expan‐
sion of special parameters described in the Shell and Utilities volume
of IEEE Std 1003.1-2001, Section 2.5.2, Special Parameters is unspeci‐
fied.
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 fol‐
lowing constants, which are defined in <wordexp.h>:
WRDE_APPEND
Append words generated to the ones from a previous call to word‐
exp().
WRDE_DOOFFS
Make use of pwordexp->we_offs. If this flag is set, pword‐
exp->we_offs is used to specify how many null pointers to add to
the beginning of pwordexp->we_wordv. In other words, pword‐
exp->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 IEEE Std 1003.1-2001, fail if com‐
mand substitution, as specified in the Shell and Utilities vol‐
ume of IEEE Std 1003.1-2001, Section 2.6.3, Command Substitu‐
tion, 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 vari‐
able.
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 word‐
exp(), but if it does it shall reset them to the original value
before a subsequent call, using the same pwordexp value, to word‐
free() or wordexp() with the WRDE_APPEND or WRDE_REUSE flag.
If the implementation supports the utilities defined in the Shell and
Utilities volume of IEEE Std 1003.1-2001, 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 substitu‐
tion while expanding words. If WRDE_SHOWERR is set, wordexp() may
write messages to stderr if syntax errors are detected while expanding
words.
The application shall ensure that if WRDE_DOOFFS is set, then pword‐
exp->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 inappro‐
priate 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 untermi‐
nated 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 pword‐
exp->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.
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 filename (or
list of filenames) 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.
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 appli‐
cations.
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 infor‐
mation 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
backslashes, and break into words at unquoted <blank>s. 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 sub‐
stitution. In any case, it would have to scan words for unquoted spe‐
cial characters.
Another proposal is to add the following options to sh:
-w wordlist
This option provides a wordlist expansion service to applica‐
tions. The words in wordlist shall be expanded and the follow‐
ing written to standard output:
1. The count of the number of words after expansion, in deci‐
mal, 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 imple‐
mentation can call wordfree() when WRDE_REUSE is set.
FUTURE DIRECTIONS
None.
SEE ALSOfnmatch() , glob() , the Base Definitions volume of
IEEE Std 1003.1-2001, <wordexp.h>, the Shell and Utilities volume of
IEEE Std 1003.1-2001, Chapter 2, Shell Command Language
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 WORDEXP(P)
