NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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

NAME         top

       system - execute a shell command

SYNOPSIS         top

       #include <stdlib.h>

       int system(const char *command);

DESCRIPTION         top

       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         top

       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         top

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

       ┌──────────┬───────────────┬─────────┐
       │Interface Attribute     Value   │
       ├──────────┼───────────────┼─────────┤
       │system()  │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘

CONFORMING TO         top

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

NOTES         top

       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.)

       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         top

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

COLOPHON         top

       This page is part of release 4.11 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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