/* Placed in the public domain 2009 by Sam Trenholme */

/* Yash: A usable command shell in under 4k (yet another shell)
 *
 * This is a simple thought experiment: How small can we make a *NIX command
 * line shell?  This is as simple as it gets; it will run a command typed
 * in if it's an external command like "ls" or "cc", with arguments separated
 * by space; if one types in "-cd {directory}", it will go to that directory.
 *
 * CNTL-C exits the shell
 *
 * BUGS: CNTL-D reruns the previous command instead of exiting the shell.
 */

#define LINEMAX 80
#define ARGMAX 16

#include <stdio.h>
#include <unistd.h>

/* Given a pointer to arguments, destroy the string */
void yash_zap_args(char **zap) {
	int a = 0;

	if(zap == 0) {
		return;
	}

	for(a = 0; a < ARGMAX; a++) {
		if(zap[a] != 0) {
			free(zap[a]);
			zap[a] = 0;
		}
	}

	free(zap);
}

/* Given a line separated by whitespace, return an array of strings
 * of the individual arguments */

char **yash_args(char *in) {
	char **out = 0;
	int a = 0, b = 0;

	/* Sanity checks */
	if(in == 0) {
		return 0;
	}

	out = malloc(ARGMAX * sizeof(char *));
	for(a = 0; a < ARGMAX; a++) {
		out[a] = 0;
	}
	a = 0;

	if(out == 0) {
		return 0;
	}

	while(*in != 0) {
		if(*in != ' ' && *in != '\n') {
			if((out[a] = malloc(LINEMAX)) == 0) {
				goto catch_yash_args;
			}
			b = 0;
			while(b < LINEMAX - 2 && *in != ' ' && *in != 0 &&
                              *in != '\n') {
				out[a][b] = *in;
				b++;
				in++;
			}
			out[a][b] = 0;
			a++;
			if(a >= ARGMAX) {
				goto catch_yash_args;
			}
		}
		in++;
	}	

	return out;
		
catch_yash_args:
	yash_zap_args(out);
	return 0;
}

/* Meta commands for yash; right now this is just the cd command */ 
void yash_meta(char **args) {
	if(args == 0 || args[0] == 0) {
		return;
	}

	if(args[0][1] == 'c' && args[0][2] == 'd') {
		if(args[1] == 0) {
			chdir("/"); /* Should be $HOME, isn't */
		} else {
			chdir(args[1]);
		}
		return;
	}
}
		
main() {
	char in[LINEMAX], **args = 0;

	for(;;) {
		printf("$ ");
		fgets(in,LINEMAX,stdin);
		args = yash_args(in);
		if(args != 0 && args[0] != 0) {
			if(*args[0] == '-') {
				yash_meta(args);
			}
			else if(fork()) {
				wait(0);
			} else {
				execvp(args[0],args);
				printf("Command not found\n");
				exit(-1);
			}
		}		
		yash_zap_args(args);
	}
}