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SYSTEM
Section: Linux Programmer's Manual (3)
Updated: 2010-09-10
Index
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NAME
system - execute a shell command
SYNOPSIS
#include <stdlib.h>
int system(const char *command);
DESCRIPTION
system()
executes a command specified in
command
by calling
/bin/sh -c
command,
and returns after the command has been completed.
During execution of the command,
SIGCHLD
will be blocked, and
SIGINT
and
SIGQUIT
will be ignored.
RETURN VALUE
The value returned is -1 on error (e.g.
fork(2)
failed),
and the return status of the command otherwise.
This latter return status is in the format
specified in
wait(2).
Thus, the exit code of the command will be
WEXITSTATUS(status).
In case
/bin/sh
could not be executed, the exit status will be that of
a command that does
exit(127).
If the value of
command
is NULL,
system()
returns nonzero if the shell is available, and zero if not.
system()
does not affect the wait status of any other children.
CONFORMING TO
C89, C99, POSIX.1-2001.
NOTES
If the
_XOPEN_SOURCE
feature test macro is defined
(before including
any
header files),
then the macros described in
wait(2)
(WEXITSTATUS(),
etc.) are made available when including
<stdlib.h>.
As mentioned,
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.
while (something) {
int ret = system("foo");
if (WIFSIGNALED(ret) &&
(WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
break;
}
Do not use
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
exec(3)
family of functions instead, but not
execlp(3)
or
execvp(3).
system()
will not, in fact, work properly from programs with set-user-ID or
set-group-ID privileges on systems on which
/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
sh.)
In versions of glibc before 2.1.3, the check for the availability of
/bin/sh
was not actually performed if
command
was NULL; instead it was always assumed to be available, and
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
chroot(2)
(which is not specified by POSIX.1-2001).
It is possible for the shell command to return 127, so that code is not
a sure indication that the
execve(2)
call failed.
SEE ALSO
sh(1),
signal(2),
wait(2),
exec(3)
COLOPHON
This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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