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



NAME
       system - execute a shell command

SYNOPSIS
       #include <stdlib.h>

       int system(const char *string);

DESCRIPTION
       system() executes a command specified in string by calling
       /bin/sh -c string, 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 failed),  and
       the  return  status of the command otherwise.  This latter
       return status is  in  the  format  specified  in  wait(t).
       Thus,  the  exit  code  of  the  command will be WEXITSTA-
       TUS(S).  In case /bin/sh could not be  executed,  the
       exit status will be that of a command that does exit(t).

       If the value of string 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
       ANSI C, POSIX.2, BSD 4.3

NOTES
       As mentioned, system() ignores SIGINT and  SIGQUIT.   This
       may  make  programs  that call it from a loop uninterrupt-
       able, unless they take care themselves to check  the  exit
       status of the child. E.g.

           while(e) {
               int ret = system("foo");

               if (WIFSIGNALED(D) &&
                   (WTERMSIG(G) == SIGINT || WTERMSIG(G) == SIGQUIT))
                       break;
           }

       Do not use system() from a program with suid or sgid priv-
       ileges, because strange values for some environment  vari-
       ables  might be used to subvert system integrity.  Use the
       exec(c) family of functions instead, but not execlp(p)  or
       execvp(p).  system() will not, in fact, work properly from
       programs with suid or sgid 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.)

       The  check for the availability of /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.

       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(h), signal(l), wait(t), exec(c)



                            2001-09-23                  SYSTEM(M)