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       system - execute a shell command


       #include <stdlib.h>

       int system(const char *command);


       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.


       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.


       C89, C99, POSIX.1-2001.


       If  the  _XOPEN_SOURCE  feature test macro is defined (before including any header files),
       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 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))

       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.


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


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                                            2010-09-10                                  SYSTEM(3)