.\" (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)
.TH SYSTEM 3  "11 May 1998" "GNU" "Linux Programmer's Manual"
.SH NAME
system \- execute a shell command
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "int system (const char * " "string" ");"
.fi
.SH DESCRIPTION
.B system()
executes a command specified in
.I string
by calling
.BR "/bin/sh -c"
.IR string ,
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 127 if the
.B execve()
call for
.B /bin/sh
fails, \-1 if there was another error and the return code of the
command otherwise.
.PP
If the value of
.I string
is 
.BR NULL ,
.B system()
returns nonzero if the shell is available, and zero if not.
.PP
.B system()
does not affect the wait status of any other children.
.SH "CONFORMING TO"
ANSI C, POSIX.2, BSD 4.3
.SH "BUGS"
.PP
It is extremely unfortunate that the libc version of
.B system()
ignores interrupts.  This makes programs that call it
from a loop uninterruptable.  This means that for such
purposes one should not use
.B system()
but a private version like (warning: untested code!)
.br
.nf

int my_system (const char *command) {
    int pid, status;

    if (command == 0)
        return 1;
    pid = fork();
    if (pid == -1)
        return -1;
    if (pid == 0) {
        char *argv[4];
        argv[0] = "sh";
        argv[1] = "-c";
        argv[2] = command;
        argv[3] = 0;
        execve("/bin/sh", argv, environ);
        exit(127);
    }
    do {
        if (waitpid(pid, &status, 0) == -1) {
            if (errno != EINTR)
                return -1;
        } else
            return status;
    } while(1);
}
.fi
.PP
Do not use
.B system()
from a program with suid or sgid 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).
.B system()
will not, in fact, work properly from programs with suid or sgid
privileges on systems on which
.B /bin/sh
is
.B 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
The check for the availability of
.B /bin/sh
is not actually performed; it is always assumed to be available.  ISO
C specifies the check, but POSIX.2 specifies that the return shall
always be non-zero, since a system without the shell is not
conforming, and it is this that is implemented.
.PP
It is possible for the shell command to return 127, so that code is not a sure
indication that the
.B execve()
call failed; check
.I errno
to make sure.
.SH "SEE ALSO"
.BR sh (1),
.BR signal (2),
.BR exec (3)