How to issue a C SHELL command within a C program

[Roger Droz]

There have been some excellent responses to this question.  Our system
has expired the article that first mentioned using execlp(), which I
thought was not only the best solution, but a fine example of techinical
writing as well.

I too, like to use execlp() because of the efficiency of directly
invoking the shell of choice without invoking the Bourne shell first,
and because it gets around the quoting problem mentioned by
Guy Harris in <3923 at auspex.auspex.com>.

The two cents I want to add to this thread is that the parent process should
disable the INT and QUIT signals, so that the user can type ^C (or your
favorite INT signal key) to kill the child without killing the parent.

The following code is taken from my patches to MicroEMACS 3.10e, which I
plan to release on the world in the next week or so:

/** Callout to system to perform command **/
/** RLD 07-24-90: modified to use $SHELL */
/** cmd == "" means interactive shell */
int callout(cmd)
char * cmd;			/* Command to execute	*/
{
	int status;
	char *sh;		/* Shell program. */
	void (*oldsigint)(), (*oldsigquit)();
	extern char *getenv();

	/* Get shell path */
	sh = getenv("SHELL");
	if (!sh)
#if BSD || SUN
		sh = "/bin/csh";
#endif /* BSD || SUN */
#if USG || SMOS || HPUX || XENIX
		sh = "/bin/sh";
#endif /* USG || SMOS || HPUX || XENIX */

	if (fork() == 0) {
		/* Child decides what to do. */
		if (*cmd != '\0')
			/* Do a command. */
			execlp(sh, sh, "-c", cmd, NULL);
		else
			/* Fork an interactive shell */
			execlp(sh, sh, NULL);
		exit(1); /* Child shouldn't get here. */	
	}

#if USG || SMOS || HPUX || XENIX
	/* Parent disables signals and waits. */
	oldsigint = signal(SIGINT, SIG_IGN);
	oldsigquit = signal(SIGQUIT, SIG_IGN);
	wait(&status);
	signal(SIGINT, oldsigint);
	signal(SIGQUIT, oldsigquit);
#else /* BSD || SUN */
	/* BSD should disable signals while waiting for child too,  */
	/* but I can't remember how. -- RLD 08-28-90		    */
	wait(&status);
#endif /* USG || SMOS || HPUX || XENIX */

}
____________
               Roger Droz       UUCP: uw-beaver!gtisqr!roger
()       ()    Maverick MICRoSystems / Global Technology International
 (_______)     Mukilteo, WA 
  (     )      
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