system(3) — Linux manual page


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 *) 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         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 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         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │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

           while (something) {
               int ret = system("foo");

               if (WIFSIGNALED(ret) &&
                   (WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))

       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.

       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.

BUGS         top

       If the command name starts with a hyphen, sh(1) interprets the
       command name as an option, and the behavior is undefined.  (See
       the -c option to sh(1).)  To work around this problem, prepend
       the command with a space as in the following call:

               system(" -unfortunate-command-name");

SEE ALSO         top

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

COLOPHON         top

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                               2021-03-22                      SYSTEM(3)

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