Overview

A simple shell program. It implements basic features: some built-in commands, all external commands, standard I/O redirection, and pipes. However, I/O redirection and pipes only work in simple cases.

Platform: Ubuntu 14.04 </br> Run MyShell to enter the shell; you’ll see a prompt like [MyShell] zdf@zdf-virtual-machine:~/MyShell$

Supported built-in commands

  • cd — same as cd on Linux: </br>cd .. goes up one directory </br>cd goes to your home directory </br>cd ~/directory goes to a directory under your home </br>cd directory enters the directory
  • exit/quit exits the shell

System calls used here: getpwuid, getuid, chdir.

All external commands

Supports all external commands, with arguments.

System call used here: execvp.

Standard I/O redirection

Redirection only applies to the first command.

System calls used here: open, dup2.

Pipes

Only two commands can be connected by a pipe, e.g. ls -l | sort. So a command line can contain only one |.

System calls used here: fork(), pipe(), dup2.

Misc

When output redirection and a pipe are used together, the piped command must come last, e.g. ls -l > tmp | sort. But the pipe is meaningless in that case, so combining output redirection with pipes is not recommended.

Source code

	#include <stdio.h>
	#include <string.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <sys/stat.h>
	#include <sys/types.h>
	#include <errno.h>
	#include <pwd.h>
	#include <fcntl.h>
	
	#define Max_Name_Len 256
	#define Max_Path_Len 1024
	#define Maxn 100
	
	char *argv[Maxn];
	int len;
	int fd[2];
	
	void type_prompt() {
		char hostname[Max_Name_Len];
		char pathname[Max_Path_Len];
		struct passwd *pwd;
	
		pwd = getpwuid(getuid());
		gethostname(hostname, Max_Name_Len);
		printf("[MyShell] %s@%s:", pwd -> pw_name, hostname);
		getcwd(pathname, Max_Path_Len);
		if (strncmp(pathname, pwd -> pw_dir, strlen(pwd -> pw_dir)) == 0) {
			printf("~%s", pathname + strlen(pwd -> pw_dir));
		}
		else {
			printf("%s", pathname);
		}
		if (geteuid() == 0) {
			printf("#\n");
		}
		else {
			printf("$\n");
		}
		
	}
	
	void read_command()
	{
		char ch = ' ';
		int i = 0;
		memset(argv, 0, sizeof(argv));
		
		while (ch != '\n') {
			char s[Maxn]; // the command length should be less than Maxn
			scanf("%s%c", s, &ch);
			argv[i] = (char*)malloc(sizeof(s));
			strcpy(argv[i++], s);
		}
		len = i;
	}
	
	void RunOutterFunction(int offset) 
	{
		int ret;
		ret = execvp(argv[offset], argv + offset);
		if (ret < 0) {
			printf("command not found\n");
		}
	}
	
	int RunInnerFunction()
	{
		struct passwd *pwd;
		char *cd_path = NULL;
		if (strcmp(argv[0], "exit") == 0 || strcmp(argv[0], "quit") == 0) // exit
			exit(0);
		if (strcmp(argv[0], "cd") == 0) {
			pwd = getpwuid(getuid());
			if (argv[1] == NULL) { // cd
				cd_path = malloc(strlen(pwd -> pw_dir));
				strcpy(cd_path, pwd -> pw_dir);
			}
			else if (argv[1][0] == '~') { // cd ~/...
				cd_path = malloc(strlen(pwd -> pw_dir) + strlen(argv[1]));
				strcpy(cd_path, pwd -> pw_dir);
				strcpy(cd_path + strlen(pwd -> pw_dir), argv[1] + 1);
			}
			else { // normal
				cd_path = malloc(strlen(argv[1] + 1));
				strcpy(cd_path, argv[1]);
			}
			if (chdir(cd_path) != 0) {
				printf("-MyShell: cd: %s:%s\n", cd_path, strerror(errno));
			}
			free(cd_path);
			return 1;
		}
		return 0;
	}
	
	void Redirect()
	{
		int i;
		for (i = 1; i < len; ++i) 
			if (argv[i] != NULL){
				if (strcmp(argv[i], ">") == 0) {
					int output = open(argv[i + 1], O_WRONLY | O_CREAT | O_TRUNC, 0666);
					dup2(output, 1);
					close(output);
					argv[i] = NULL;
					++i;
					continue;
				}
				if (strcmp(argv[i], ">>") == 0) {
					int output = open(argv[i + 1], O_WRONLY | O_APPEND | O_CREAT, 0666);
					dup2(output, 1);
					close(output);
					argv[i] = NULL;
					++i;
					continue;
				}
				if (strcmp(argv[i], "2>") == 0) {
					int output = open(argv[i + 1], O_WRONLY | O_CREAT | O_TRUNC, 0666);
					dup2(output, 2);
					close(output);
					argv[i] = NULL;
					++i;
					continue;
				}
				if (strcmp(argv[i], "2>>") == 0) {
					int output = open(argv[i + 1], O_WRONLY | O_APPEND | O_CREAT, 0666);
					dup2(output, 2);
					close(output);
					argv[i] = NULL;
					++i;
					continue;
				}
				if (strcmp(argv[i], "<") == 0) {
					int input = open(argv[i + 1], O_CREAT | O_RDONLY, 0666);
					dup2(input, 0);
					close(input);
					argv[i] = NULL;
					++i;
					continue;
				}
		}
	}
	
	int hasPipe()
	{
		int i;
		for (i = 1; i < len; ++i)
			if (argv[i] != NULL && strcmp(argv[i], "|") == 0) {
				argv[i] = NULL;
				return i + 1;
			}
		return 0;
	}
	
	void Pipe(int x)
	{
		int pid2;
		if (x == 0) return;
		if ((pid2 = fork()) == 0) {
			//sub process
			close(fd[1]);
			dup2(fd[0], 0);
			RunOutterFunction(x);
		}
		else {
			//parent process
			close(fd[1]);
			waitpid(pid2, NULL, 0);
		}
	}
	
	int main()
	{
		int pid, flag;
		while (1) {
			type_prompt();
			read_command();
			if (RunInnerFunction()) 
				continue;
			if (flag = hasPipe())
				pipe(fd);
			if ((pid = fork()) == 0) {
				//sub process
				if (flag) { 
					close(fd[0]);
					dup2(fd[1], 1);
				}
				Redirect();
				RunOutterFunction(0);
				break;
			}
			else if (pid > 0) {
				//parent process
				Pipe(flag);
				waitpid(pid, NULL, 0);
			}
			else {
				printf("fork failed.");
				break;
			}
		}
		return 0;
	}