system(3)



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 termina-
          tion status of the child shell used to execute command.  (The termi-
          nation  status of a shell is the termination status of the last com-
          mand 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 cre-
       ate 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) (WEXITSTA-
       TUS(), etc.) are made available when including <stdlib.h>.

       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.  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  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.)

       According  to  POSIX.1,  it  is unspecified whether handlers registered
       using pthread_atfork(3) are called during the  execution  of  system().
       In the glibc implementation, such handlers are not called.

       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
       This  page  is  part of release 4.10 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
       https://www.kernel.org/doc/man-pages/.

                                  2017-03-13                         SYSTEM(3)

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