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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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SYSTEM(3) Linux Programmer's Manual SYSTEM(3)
system - execute a shell command
#include <stdlib.h>
int system(const char *command);
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 *) 0);
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.
* 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.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────┼───────────────┼─────────┤
│system() │ Thread safety │ MT-Safe │
└──────────┴───────────────┴─────────┘
POSIX.1-2001, POSIX.1-2008, C89, C99.
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 arbi‐
trary 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 care‐
fully 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.
sh(1), execve(2), fork(2), sigaction(2), sigprocmask(2), wait(2),
exec(3), signal(7)
This page is part of release 4.15 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/.
2017-09-15 SYSTEM(3)
Pages that refer to this page: execve(2), confstr(3), curs_scr_dump(3x), exec(3), __pmprocessexec(3), popen(3)
Copyright and license for this manual page
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