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, 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.
If the _XOPEN_SOURCE feature test macro is defined, then the macros
described in wait(2) (WEXITSTATUS(), etc.) are made available when
including <stdlib.h>.
SEE ALSO
sh(1), signal(2), wait(2), exec(3)
COLOPHON
This page is part of release 3.24 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/.
2004-12-20