man3/system.3

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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  2017-09-15 "" "Linux Programmer's Manual"
.SH NAME
system \- execute a shell command
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.BI "int system(const char *" "command" );
.fi
.SH DESCRIPTION
The
.BR system ()
library function uses
.BR fork (2)
to create a child process that executes the shell command specified in
.I command
using
.BR execl (3)
as follows:
.PP
    execl("/bin/sh", "sh", "-c", command, (char *) 0);
.PP
.BR system ()
returns after the command has been completed.
.PP
During execution of the command,
.B SIGCHLD
will be blocked, and
.B SIGINT
and
.B SIGQUIT
will be ignored, in the process that calls
.BR system ()
(these signals will be handled according to their defaults inside
the child process that executes
.IR command ).
.PP
If
.I command
is NULL, then
.BR system ()
returns a status indicating whether a shell is available on the system.
.SH RETURN VALUE
The return value of
.BR system ()
is one of the following:
.IP * 3
If
.I command
is NULL, then a nonzero value if a shell is available,
or 0 if no shell is available.
.IP *
If a child process could not be created,
or its status could not be retrieved,
the return value is \-1.
.IP *
If a shell could not be executed in the child process,
then the return value is as though the child shell terminated by calling
.BR _exit (2)
with the status 127.
.IP *
If all system calls succeed,
then the return value is the termination status of the child shell
used to execute
.IR command .
(The termination status of a shell is the termination status of
the last command it executes.)
.PP
In the last two cases,
the return value is a "wait status" that can be examined using
the macros described in
.BR waitpid (2).
(i.e.,
.BR WIFEXITED (),
.BR WEXITSTATUS (),
and so on).
.PP
.BR system ()
does not affect the wait status of any other children.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR system ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99.
.SH NOTES
.BR system ()
provides simplicity and convenience:
it handles all of the details of calling
.BR fork (2),
.BR execl (3),
and
.BR waitpid (2),
as well as the necessary manipulations of signals;
in addition,
the shell performs the usual substitutions and I/O redirections for
.IR command .
The main cost of
.BR system ()
is inefficiency:
additional system calls are required to create the process that
runs the shell and to execute the shell.
.PP
If the
.B _XOPEN_SOURCE
feature test macro is defined
(before including
.I any
header files),
then the macros described in
.BR waitpid (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.
For example:
.PP
.in +4n
.EX
while (something) {
    int ret = system("foo");

    if (WIFSIGNALED(ret) &&
        (WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
            break;
}
.EE
.in
.PP
According to POSIX.1, it is unspecified whether handlers registered using
.BR pthread_atfork (3)
are called during the execution of
.BR system ().
In the glibc implementation, such handlers are not called.
.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 terminate with a status of 127,
which yields a
.BR system ()
return value that is indistinguishable from the case
where a shell could not be executed in the child process.
.\"
.SS Caveats
.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 different shell,
.BR dash (1),
which does not do this when invoked as
.BR sh .)
.SH SEE ALSO
.BR sh (1),
.BR execve (2),
.BR fork (2),
.BR sigaction (2),
.BR sigprocmask (2),
.BR wait (2),
.BR exec (3),
.BR signal (7)