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.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" License.
.\" Modified Sat Jul 24 17:51:15 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified 11 May 1998 by Joseph S. Myers (jsm28@cam.ac.uk)
.\" Modified 14 May 2001, 23 Sep 2001 by aeb
.\" 2004-12-20, mtk
.\"
.TH SYSTEM 3 2004-12-20 "" "Linux Programmer's Manual"
.SH NAME
system \- execute a shell command
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "int system(const char *" "command" );
.fi
.SH DESCRIPTION
.BR system ()
executes a command specified in
.I command
by calling
.BR "/bin/sh \-c"
.IR command ,
and returns after the command has been completed.
During execution of the command,
.B SIGCHLD
will be blocked, and
.B SIGINT
and
.B SIGQUIT
will be ignored.
.SH "RETURN VALUE"
The value returned is \-1 on error (e.g. fork failed),
and the return status of the command otherwise.
This latter return status is in the format
specified in
.BR wait (2).
Thus, the exit code of the command will be
.IR WEXITSTATUS(status) .
In case
.I "/bin/sh"
could not be executed, the exit status will be that of
a command that does
.IR exit(127) .
.PP
If the value of
.I command
is
.BR NULL ,
.BR system ()
returns non-zero if the shell is available, and zero if not.
.PP
.BR system ()
does not affect the wait status of any other children.
.SH "CONFORMING TO"
ANSI C, POSIX.2, BSD 4.3
.SH NOTES
.PP
If the
.B _XOPEN_SOURCE
feature test macro is defined, then the macros described in
.BR wait (2)
.RB ( WEXITSTATUS() ,
etc.) are made available when including
.IR <stdlib.h> .
.PP
As mentioned,
.BR system ()
ignores SIGINT and SIGQUIT. This may make programs that call it
from a loop uninterruptible, unless they take care themselves
to check the exit status of the child. E.g.
.br
.nf
while(something) {
int ret = system("foo");
if (WIFSIGNALED(ret) &&
(WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
break;
}
.fi
.PP
Do not use
.BR system ()
from a program with set-user-ID or set-group-ID privileges,
because strange values for some environment variables
might be used to subvert system integrity.
Use the
.BR exec (3)
family of functions instead, but not
.BR execlp (3)
or
.BR execvp (3).
.BR system ()
will not, in fact, work properly from programs with set-user-ID or
set-group-ID privileges on systems on which
.I /bin/sh
is bash version 2, since bash 2 drops privileges on startup.
(Debian uses a modified bash which does not do this when invoked as
.BR sh .)
.PP
In versions of glibc before 2.1.3, the check for the availability of
.I /bin/sh
was not actually performed if
.I command
was NULL; instead it was always assumed to be available, and
.BR system ()
always returned 1 in this case.
Since glibc 2.1.3, this check is performed because, even though
POSIX.1-2001 requires a conforming implementation to provide
a shell, that shell may not be available or executable if
the calling program has previously called
.BR chroot (2)
(which is not specified by POSIX.1-2001).
.PP
It is possible for the shell command to return 127, so that code is not
a sure indication that the
.BR execve ()
call failed.
.SH "SEE ALSO"
.BR sh (1),
.BR signal (2),
.BR wait (2),
.BR exec (3)
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