debian man page for system

SYSTEM(3)						     Linux Programmer's Manual							 SYSTEM(3)

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)	(WEXITSTA-
       TUS(), 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 3.44 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/.

								    2010-09-10								 SYSTEM(3)