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



 

NAME


       system - execute a shell command

 

SYNOPSIS


       #include 

       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(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 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 .

       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(2) call failed.

       If the _XOPEN_SOURCE feature test macro is  defined,  then  the  macros
       described  in  wait(2)  (WEXITSTATUS(),  etc.)  are made available when
       including .

 

SEE ALSO


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

 

COLOPHON


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       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



                                  2004-12-20                         SYSTEM(3)