system

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 *) NULL);

       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 and errno is set to indicate the
          error.

       *  If a shell could not be executed in the child process, then the  re-
          turn  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.

ERRORS
       system() can fail with any of the same errors as fork(2).

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(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 de-
       tails of calling fork(2), execl(3), and waitpid(2), as well as the nec-
       essary 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;
           }

       According to POSIX.1, it is unspecified whether handlers registered us-
       ing 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.

   Caveats
       Do not use system() from a privileged program (a  set-user-ID  or  set-
       group-ID  program, or a program with capabilities) because strange val-
       ues for some environment variables might be used to subvert system  in-
       tegrity.   For  example, PATH could be manipulated so that an arbitrary
       program is executed with privilege.  Use the exec(3)  family  of  func-
       tions  instead, but not execlp(3) or execvp(3) (which also use the PATH
       environment variable to search for an executable).

       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 ver-
       sion 2: as a security measure, bash  2  drops  privileges  on  startup.
       (Debian  uses  a  different shell, dash(1), which does not do this when
       invoked as sh.)

       Any user input that is employed as part of command should be  carefully
       sanitized,  to ensure that unexpected shell commands or command options
       are not executed.  Such risks are especially grave when using  system()
       from a privileged program.

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

COLOPHON
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       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

                                  2019-03-06                         SYSTEM(3)