.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\"
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.\" 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.
.BR fork (2)
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 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"
C89, C99, POSIX.1-2001.
.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
.B SIGINT
and
.BR 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 (2)
call failed.
.SH "SEE ALSO"
.BR sh (1),
.BR signal (2),
.BR wait (2),
.BR exec (3)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
