system

system - execute a shell command

#include <stdlib.h>
 I int system(const char * command );

R system () executes a command specified in command by calling R /bin/sh -c R command , and returns after the command has been completed. During execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT will be ignored.

The value returned is -1 on error (e.g. fork(2) failed), and the return status of the command otherwise. This latter return status is in the format specified in wait(2). Thus, the exit code of the command will be R WEXITSTATUS(status) . In case /bin/sh could not be executed, the exit status will be that of a command that does R exit(127) .

If the value of command is NULL, R system () returns non-zero if the shell is available, and zero if not.

R system () does not affect the wait status of any other children.

C89, C99, POSIX.1-2001.

If the _XOPEN_SOURCE feature test macro is defined, then the macros described in wait(2) (WEXITSTATUS(), etc.) are made available when including R <stdlib.h> .

As mentioned, R system () ignores SIGINT and R 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. E.g.

    while (something) {
        int ret = system("foo");

        if (WIFSIGNALED(ret) &&
            (WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
                break;
    }

Do not use R system () from a program with set-user-ID or set-group-ID 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). R 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, since bash 2 drops privileges on startup. (Debian uses a modified bash which does not do this when invoked as R sh .)

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 R 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 return 127, so that code is not a sure indication that the execve(2) call failed. If the _XOPEN_SOURCE feature test macro is defined, then the macros described in wait(2) (WEXITSTATUS(), etc.) are made available when including <stdlib.h>.

sh(1), signal(2), wait(2), exec(3)