system(3) Library Functions Manual system(3)
NAME
system - execute a shell command
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
int system(const char *command);
DESCRIPTION
The system() library function behaves as if it used fork(2) to create a
child process that executed the shell command specified in command us-
ing 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
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 re-
turn 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 termina-
tion status of the child shell used to execute command. (The termi-
nation status of a shell is the termination status of the last com-
mand 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
system() can fail with any of the same errors as fork(2).
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ system() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
C11, POSIX.1-2008.
HISTORY
POSIX.1-2001, C89.
NOTES
system() provides simplicity and convenience: it handles all of the de-
tails of calling fork(2), execl(3), and waitpid(2), as well as the nec-
essary 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 cre-
ate 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) (WEXITSTA-
TUS(), etc.) are made available when including <stdlib.h>.
As mentioned, system() ignores SIGINT and SIGQUIT. This may make pro-
grams 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 us-
ing 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 val-
ues for some environment variables might be used to subvert system in-
tegrity. For example, PATH could be manipulated so that an arbitrary
program is executed with privilege. Use the exec(3) family of func-
tions 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 ver-
sion 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
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
sh(1), execve(2), fork(2), sigaction(2), sigprocmask(2), wait(2),
exec(3), signal(7)
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