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system(3) — Linux manual page

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) to
 create a child process that executed 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
 attributes(7).
 ┌──────────────────────────────────────┬───────────────┬─────────┐
 │ 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), 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.
 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) (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.

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

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

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