system(3) - Linux manual page (original) (raw)

system(3) Library Functions Manual system(3)

NAME top

   system - execute a shell command

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <stdlib.h>**

   **int system(const char ***_command_**);**

DESCRIPTION top

   The **system**() library function behaves as if it used [fork(2)](../man2/fork.2.html) to
   create a child process that executed the shell command specified
   in _command_ using [execl(3)](../man3/execl.3.html) 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](../man3/errno.3.html)_ 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)](../man2/%5Fexit.2.html) 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)](../man2/waitpid.2.html).  (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)](../man2/fork.2.html).

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **system**()                             │ Thread safety │ MT-Safe │
   └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   C11, POSIX.1-2008.

HISTORY top

   POSIX.1-2001, C89.

NOTES top

   **system**() provides simplicity and convenience: it handles all of
   the details of calling [fork(2)](../man2/fork.2.html), [execl(3)](../man3/execl.3.html), and [waitpid(2)](../man2/waitpid.2.html), 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)](../man2/waitpid.2.html) (**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)](../man3/pthread%5Fatfork.3.html) are called during the execution
   of **system**().  In the glibc implementation, such handlers are not
   called.

   Before glibc 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)](../man2/chroot.2.html) (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)](../man1/dash.1.html), 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)](../man2/execve.2.html), [fork(2)](../man2/fork.2.html), [sigaction(2)](../man2/sigaction.2.html), [sigprocmask(2)](../man2/sigprocmask.2.html), [wait(2)](../man2/wait.2.html),
   [exec(3)](../man3/exec.3.html), [signal(7)](../man7/signal.7.html)

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Linux man-pages 6.10 2024-07-23 system(3)

Pages that refer to this page:execve(2), confstr(3), curs_scr_dump(3x), exec(3), __pmprocessexec(3), popen(3)