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
          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.

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
       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;
           }

       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.

   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
       values for some environment variables might be used to subvert system
       integrity.  For example, PATH could be manipulated so that an arbitrary
       program is executed with privilege.  Use the exec(3) family of
       functions 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
       version 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
       This page is part of release 5.02 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/.



                                  2019-03-06                         SYSTEM(3)