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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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system(3) Library Functions Manual system(3)
system - execute a shell command
Standard C library (libc, -lc)
#include <stdlib.h>
int system(const char *command);
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.
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.
system() can fail with any of the same errors as fork(2).
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ system() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
POSIX.1-2001, C89.
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.
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");
sh(1), execve(2), fork(2), sigaction(2), sigprocmask(2), wait(2),
exec(3), signal(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
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2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-05-02 system(3)
Pages that refer to this page: execve(2), confstr(3), curs_scr_dump(3x), exec(3), __pmprocessexec(3), popen(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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