最简单的shell程序——yash

/* 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);}}

源自：http://samiam.org/software/yash.html