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 termination 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

   For  an  explanation  of  the  terms  used   in   this   section,   see
   attributes(7).

   
   Interface  Attribute      Value   
   
   system()   Thread safety  MT-Safe 
   

CONFORMING TO

   POSIX.1-2001, POSIX.1-2008, C89, C99.

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 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 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),  execve(2),  fork(2),  sigaction(2),  sigprocmask(2),   wait(2),
   exec(3), signal(7)

COLOPHON

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

                              2016-10-08                         SYSTEM(3)

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