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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 exe-
       cution  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() 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 non-zero 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 pro-
       grams 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 func-
       tions 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() call failed.

SEE ALSO
       sh(1), signal(2), wait(2), exec(3)



				  2004-12-20			     SYSTEM(3)
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