#include <stdlib.h> int system(const char *command);
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.
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.
| Interface | Attribute | Value |
| system() | Thread safety | MT-Safe |
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.
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.