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



NAME
       system - execute a shell command

SYNOPSIS
       #include <stdlib.h>

       int system(const char *command);

DESCRIPTION
       The  system()  library  function  uses fork(2) to create a child process that executes the
       shell command specified in command using execl(3) as follows:

           execl("/bin/sh", "sh", "-c", command, (char *) 0);

       system() returns after the command has been completed.

       During execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT  will  be
       ignored,  in  the  process that calls system() (these signals will be handled according to
       their defaults inside the child process that executes command).

       If command is NULL, then system() returns a status indicating whether a shell is available
       on the system

RETURN VALUE
       The return value of system() is one of the following:

       *  If  command  is NULL, then a nonzero value if a shell is available, or 0 if no shell is
          available.

       *  If a child process could not be created, or its status  could  not  be  retrieved,  the
          return value is -1.

       *  If  a  shell  could  not  be executed in the child process, then the return value is as
          though the child shell terminated by calling _exit(2) with the status 127.

       *  If all system calls succeed, then the return value is the  termination  status  of  the
          child  shell used to execute command.  (The termination status of a shell is the termi‐
          nation status of the last command it executes.)

       In the last two cases, the return value is a "wait status" that can be examined using  the
       macros described in waitpid(2).  (i.e., WIFEXITED() WEXITSTATUS() and so on).

       system() does not affect the wait status of any other children.

ATTRIBUTES
   Multithreading (see pthreads(7))
       The system() function is thread-safe.

CONFORMING TO
       C89, C99, POSIX.1-2001.

NOTES
       system()  provides  simplicity  and  convenience: it handles all of the details of calling
       fork(2), execl(3), and waitpid(2), as well as the necessary manipulations of  signals;  in
       addition,  the  shell  performs  the usual substitutions and I/O redirections for command.
       The main cost of system() is inefficiency: additional system calls are required to  create
       the process that runs the shell and to execute the shell.

       If  the  _XOPEN_SOURCE  feature test macro is defined (before including any header files),
       then the macros described in waitpid(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.  For example:

           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 actu‐
       ally  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 terminate with a status of 127, which yields a
       system() return value that is indistinguishable from the case where a shell could  not  be
       executed in the child process.

SEE ALSO
       sh(1), sigaction(2), sigprocmask(2), fork(2), wait(2), exec(3), signal(7)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.



                                            2014-06-13                                  SYSTEM(3)