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wordexp.3p (15246B)

'\" et
.TH WORDEXP "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
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.
.\"
.SH NAME
wordexp,
wordfree
\(em perform word expansions
.SH SYNOPSIS
.LP
.nf
#include <wordexp.h>
.P
int wordexp(const char *restrict \fIwords\fP, wordexp_t *restrict \fIpwordexp\fP,
    int \fIflags\fP);
void wordfree(wordexp_t *\fIpwordexp\fP);
.fi
.SH DESCRIPTION
The
\fIwordexp\fR()
function shall perform word expansions as described in the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Section 2.6" ", " "Word Expansions",
subject to quoting as described in the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Section 2.2" ", " "Quoting",
and place the list of expanded words into the structure pointed to by
.IR pwordexp .
.P
The
.IR 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
.IR words
were the part of a command line representing the arguments to a
utility. Therefore, the application shall ensure that
.IR words
does not contain an unquoted
<newline>
character or any of the unquoted shell special characters
.BR '|' ,
.BR '&' ,
.BR ';' ,
.BR '<' ,
.BR '>' 
except in the context of command substitution as specified in the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "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
.IR words
which it expects to have expanded by
\fIwordexp\fR()
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
.IR words .
If the argument
.IR words
contains an unquoted comment character (\c
<number-sign>)
that is the beginning of a token,
\fIwordexp\fR()
shall either treat the comment character as a regular character, or
interpret it as a comment indicator and ignore the remainder of
.IR words .
.P
The structure type
.BR wordexp_t
is defined in the
.IR <wordexp.h> 
header and includes at least the following members:
.TS
center box tab(!);
cB | cB | cB
lw(1.25i)B | lw(1.25i)I | lw(2.5i).
Member Type!Member Name!Description
_
size_t!we_wordc!Count of words matched by \fIwords\fP.
char **!we_wordv!Pointer to list of expanded words.
size_t!we_offs!T{
Slots to reserve at the beginning of \fIpwordexp\fP\->\fIwe_wordv\fR.
T}
.TE
.P
The
\fIwordexp\fR()
function shall store the number of generated words into
\fIpwordexp\fP\->\fIwe_wordc\fP and a pointer to a list of pointers to
words in \fIpwordexp\fP\->\fIwe_wordv\fP. Each individual field created
during field splitting (see the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Section 2.6.5" ", " "Field Splitting")
or pathname expansion (see the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Section 2.6.6" ", " "Pathname Expansion")
shall be a separate word in the \fIpwordexp\fP\->\fIwe_wordv\fP
list. The words shall be in order as described in the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "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\(hy2017,
.IR "Section 2.5.2" ", " "Special Parameters"
is unspecified.
.P
It is the caller's responsibility to allocate the storage pointed to by
.IR pwordexp .
The
\fIwordexp\fR()
function shall allocate other space as needed, including memory
pointed to by \fIpwordexp\fP\->\fIwe_wordv\fP. The
\fIwordfree\fR()
function frees any memory associated with
.IR pwordexp
from a previous call to
\fIwordexp\fR().
.P
The
.IR flags
argument is used to control the behavior of
\fIwordexp\fR().
The value of
.IR flags
is the bitwise-inclusive OR of zero or more of the following constants,
which are defined in
.IR <wordexp.h> :
.IP WRDE_APPEND 14
Append words generated to the ones from a previous call to
\fIwordexp\fR().
.IP WRDE_DOOFFS 14
Make use of \fIpwordexp\fP\->\fIwe_offs\fP. If this flag is set,
\fIpwordexp\fP\->\fIwe_offs\fP is used to specify how many null
pointers to add to the beginning of \fIpwordexp\fP\->\fIwe_wordv\fP.
In other words, \fIpwordexp\fP\->\fIwe_wordv\fP shall point to
\fIpwordexp\fP\->\fIwe_offs\fP null pointers, followed by
\fIpwordexp\fP\->\fIwe_wordc\fP word pointers, followed by a null
pointer.
.IP WRDE_NOCMD 14
If the implementation supports the utilities defined in the Shell and Utilities volume of POSIX.1\(hy2017,
fail if command substitution, as specified in the Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Section 2.6.3" ", " "Command Substitution",
is requested.
.IP WRDE_REUSE 14
The
.IR pwordexp
argument was passed to a previous successful call to
\fIwordexp\fR(),
and has not been passed to
\fIwordfree\fR().
The result shall be the same as if the application had called
\fIwordfree\fR()
and then called
\fIwordexp\fR()
without WRDE_REUSE.
.IP WRDE_SHOWERR 14
Do not redirect
.IR stderr
to
.BR /dev/null .
.IP WRDE_UNDEF 14
Report error on an attempt to expand an undefined shell variable.
.P
The WRDE_APPEND flag can be used to append a new set of words to those
generated by a previous call to
\fIwordexp\fR().
The following rules apply to applications when two or more calls to
\fIwordexp\fR()
are made with the same value of
.IR pwordexp
and without intervening calls to
\fIwordfree\fR():
.IP " 1." 4
The first such call shall not set WRDE_APPEND. All subsequent calls
shall set it.
.IP " 2." 4
All of the calls shall set WRDE_DOOFFS, or all shall not set it.
.IP " 3." 4
After the second and each subsequent call,
\fIpwordexp\fP\->\fIwe_wordv\fP shall point to a list containing the
following:
.RS 4 
.IP " a." 4
Zero or more null pointers, as specified by WRDE_DOOFFS and
\fIpwordexp\fP\->\fIwe_offs\fP
.IP " b." 4
Pointers to the words that were in the \fIpwordexp\fP\->\fIwe_wordv\fP
list before the call, in the same order as before
.IP " c." 4
Pointers to the new words generated by the latest call, in the
specified order
.RE
.IP " 4." 4
The count returned in \fIpwordexp\fP\->\fIwe_wordc\fP shall be the
total number of words from all of the calls.
.IP " 5." 4
The application can change any of the fields after a call to
\fIwordexp\fR(),
but if it does it shall reset them to the original value before a
subsequent call, using the same
.IR pwordexp
value, to
\fIwordfree\fR()
or
\fIwordexp\fR()
with the WRDE_APPEND or WRDE_REUSE flag.
.P
If the implementation supports the utilities defined in the Shell and Utilities volume of POSIX.1\(hy2017,
and
.IR words
contains an unquoted character\(em\c
<newline>,
.BR '|' ,
.BR '&' ,
.BR ';' ,
.BR '<' ,
.BR '>' ,
.BR '(' ,
.BR ')' ,
.BR '{' ,
.BR '}' \(em\c
in an inappropriate context,
\fIwordexp\fR()
shall fail, and the number of expanded words shall be 0.
.P
Unless WRDE_SHOWERR is set in
.IR flags ,
\fIwordexp\fR()
shall redirect
.IR stderr
to
.BR /dev/null
for any utilities executed as a result of command substitution while
expanding
.IR words .
If WRDE_SHOWERR is set,
\fIwordexp\fR()
may write messages to
.IR stderr
if syntax errors are detected while expanding
.IR words ,
unless the
.IR 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
.IR stderr
error indicator or the value of
.IR errno .
.P
The application shall ensure that if WRDE_DOOFFS is set, then
\fIpwordexp\fP\->\fIwe_offs\fP has the same value for each
\fIwordexp\fR()
call and
\fIwordfree\fR()
call using a given
.IR pwordexp .
.P
The results are unspecified if WRDE_APPEND and WRDE_REUSE are
both specified.
.br
.P
The following constants are defined as error return values:
.IP WRDE_BADCHAR 14
One of the unquoted characters\(em\c
<newline>,
.BR '|' ,
.BR '&' ,
.BR ';' ,
.BR '<' ,
.BR '>' ,
.BR '(' ,
.BR ')' ,
.BR '{' ,
.BR '}' \(em\c
appears in
.IR words
in an inappropriate context.
.IP WRDE_BADVAL 14
Reference to undefined shell variable when WRDE_UNDEF is set in
.IR flags .
.IP WRDE_CMDSUB 14
Command substitution requested when WRDE_NOCMD was set in
.IR flags .
.IP WRDE_NOSPACE 14
Attempt to allocate memory failed.
.IP WRDE_SYNTAX 14
Shell syntax error, such as unbalanced parentheses or unterminated
string.
.SH "RETURN VALUE"
Upon successful completion,
\fIwordexp\fR()
shall return 0. Otherwise, a non-zero value, as described in
.IR <wordexp.h> ,
shall be returned to indicate an error. If
\fIwordexp\fR()
returns the value WRDE_NOSPACE, then \fIpwordexp\fP\->\fIwe_wordc\fP
and \fIpwordexp\fP\->\fIwe_wordv\fP shall be updated to reflect any
words that were successfully expanded. In other error cases, if the
WRDE_APPEND flag was specified, \fIpwordexp\fP->\fIwe_wordc\fP and
\fIpwordexp\fP->\fIwe_wordv\fP shall not be modified.
.P
The
\fIwordfree\fR()
function shall not return a value.
.SH ERRORS
No errors are defined.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
The
\fIwordexp\fR()
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
\fIwordexp\fR()
to process the input, the user could respond with anything that would
be valid as input to the shell.
.P
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.
.P
POSIX.1\(hy2008 does not require the
\fIwordexp\fR()
function to be thread-safe if passed an expression referencing an
environment variable while any other thread is concurrently modifying
any environment variable; see
.IR "\fIexec\fR\^".
.P
Even though the WRDE_SHOWERR flag allows the implementation to write
messages to
.IR stderr
during command substitution or syntax errors, this standard does not
provide any way to detect write failures during the output of such
messages.
.P
Applications which use wide-character output functions with
.IR stderr
should ensure that any calls to
\fIwordexp\fR()
do not write to
.IR stderr ,
by avoiding use of the WRDE_SHOWERR flag.
.SH RATIONALE
This function was included as an alternative to
\fIglob\fR().
There had been continuing controversy over exactly what features should
be included in
\fIglob\fR().
It is hoped that by providing
\fIwordexp\fR()
(which provides all of the shell word expansions, but which may be slow
to execute) and
\fIglob\fR()
(which is faster, but which only performs pathname expansion, without
tilde or parameter expansion) this will satisfy the majority of
applications.
.P
While
\fIwordexp\fR()
could be implemented entirely as a library routine, it is expected that
most implementations run a shell in a subprocess to do the expansion.
.P
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.
.P
One proposal is to extend the
.IR echo
utility by adding a
.BR \-q
option. This option would cause
.IR echo
to add a
<backslash>
before each
<backslash>
and
<blank>
that occurs within an argument. The
\fIwordexp\fR()
function could then invoke the shell as follows:
.sp
.RS 4
.nf
(void) strcpy(buffer, "echo -q");
(void) strcat(buffer, \fIwords\fP);
if ((flags & WRDE_SHOWERR) == 0)
    (void) strcat(buffer, "2>/dev/null");
f = popen(buffer, "r");
.fi
.P
.RE
.P
The
\fIwordexp\fR()
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,
\fIwordexp\fR()
would have to scan
.IR words
before starting the subshell to make sure that there would be no
command substitution. In any case, it would have to scan
.IR words
for unquoted special characters.
.P
Another proposal is to add the following options to
.IR sh :
.IP "\fB\-w\fP\ \fIwordlist\fR" 6
.br
This option provides a wordlist expansion service to applications. The
words in
.IR wordlist
shall be expanded and the following written to standard output:
.RS 6 
.IP " 1." 4
The count of the number of words after expansion, in decimal, followed
by a null byte
.IP " 2." 4
The number of bytes needed to represent the expanded words (not
including null separators), in decimal, followed by a null byte
.IP " 3." 4
The expanded words, each terminated by a null byte
.P
If an error is encountered during word expansion,
.IR sh
exits with a non-zero status after writing the former to report any
words successfully expanded
.RE
.IP "\fB\-P\fP" 6
Run in ``protected'' mode. If specified with the
.BR \-w
option, no command substitution shall be performed.
.P
With these options,
\fIwordexp\fR()
could be implemented fairly simply by creating a subprocess using
\fIfork\fR()
and executing
.IR sh
using the line:
.sp
.RS 4
.nf
execl(<\fIshell path\fP>, "sh", "-P", "-w", \fIwords\fP, (char *)0);
.fi
.P
.RE
.P
after directing standard error to
.BR /dev/null .
.P
It seemed objectionable for a library routine to write messages to
standard error, unless explicitly requested, so
\fIwordexp\fR()
is required to redirect standard error to
.BR /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.
.P
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
\fIwordfree\fR()
when WRDE_REUSE is set.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIexec\fR\^",
.IR "\fIfnmatch\fR\^(\|)",
.IR "\fIglob\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<wordexp.h>\fP"
.P
The Shell and Utilities volume of POSIX.1\(hy2017,
.IR "Chapter 2" ", " "Shell Command Language"
.\"
.SH 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 .
.PP
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 .