triggered by http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=283179

[thirdparty/man-pages.git] / man3 / system.3

.\" (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
.\"
.TH SYSTEM 3  2001-09-23 "" "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 \-1 on error (e.g. fork 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
.B "/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 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 NOTES
.PP
As mentioned,
.B system()
ignores SIGINT and SIGQUIT.  This may make programs that call it
from a loop uninterruptable, 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
.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 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.
.SH "SEE ALSO"
.BR sh (1),
.BR signal (2),
.BR wait (2),
.BR exec (3)