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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 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
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
system() can fail with any of the same errors as fork(2).
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│system() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2001, POSIX.1-2008, C99.
NOTES
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
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)