NAME¶
system - execute a shell command
SYNOPSIS¶
#include <stdlib.h>
int system (const char * string);
DESCRIPTION¶
system() executes a command specified in
string by calling
/bin/sh -c string, and returns after the command has been
completed. During execution of the command,
SIGCHLD will be blocked,
and
SIGINT and
SIGQUIT will be ignored.
RETURN VALUE¶
The value returned is 127 if the
execve() call for
/bin/sh fails,
-1 if there was another error and the return code of the command otherwise.
If the value of
string is
NULL,
system() returns nonzero if
the shell is available, and zero if not.
system() does not affect the wait status of any other children.
ANSI C, POSIX.2, BSD 4.3
BUGS¶
It is extremely unfortunate that the libc version of
system() ignores
interrupts. This makes programs that call it from a loop uninterruptable. This
means that for such purposes one should not use
system() but a private
version like (warning: untested code!)
int my_system (const char *command) {
int pid, status;
if (command == 0)
return 1;
pid = fork();
if (pid == -1)
return -1;
if (pid == 0) {
char *argv[4];
argv[0] = "sh";
argv[1] = "-c";
argv[2] = command;
argv[3] = 0;
execve("/bin/sh", argv, environ);
exit(127);
}
do {
if (waitpid(pid, &status, 0) == -1) {
if (errno != EINTR)
return -1;
} else
return status;
} while(1);
}
Do not use
system() from a program with suid or sgid privileges, because
strange values for some environment variables might be used to subvert system
integrity. Use the
exec(3) family of functions instead, but not
execlp(3) or
execvp(3).
system() will not, in fact, work
properly from programs with suid or sgid privileges on systems on which
/bin/sh is
bash version 2, since bash 2 drops privileges on
startup. (Debian uses a modified bash which does not do this when invoked as
sh.)
The check for the availability of
/bin/sh is not actually performed; it
is always assumed to be available. ISO C specifies the check, but POSIX.2
specifies that the return shall always be non-zero, since a system without the
shell is not conforming, and it is this that is implemented.
It is possible for the shell command to return 127, so that code is not a sure
indication that the
execve() call failed; check
errno to make
sure.
SEE ALSO¶
sh(1),
signal(2),
exec(3)