Simulates a Linux shell that can execute built-in commands like changing directories or deleting files as well as customized commands like listing command history as a foreground or background process.

Linux Shell Simulator

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h> 
#include <sys/wait.h>
#include "parse_args.h"
#include "history_queue.h"

/*
 * Simulates a Linux shell that can execute simple commands as foreground or background processes
 */

void signal_handler(int sig); // defines a signal handler

int main( ){ 

	char cmdline[MAXLINE];
	char *argv[MAXARGS];
	int bg;
	int cmd_num = 1;
	
	// signal handler for SIGCHLD
	void signal_handler(int sig){
                        while (waitpid((pid_t)(-1), 0, WNOHANG) > 0) {}
	}
	// registers SIGCHLD to be handled by signal_handler 
	signal(SIGCHLD, signal_handler);

	while(1) {
		//  print the shell prompt
		fprintf(stdout, "ttsh> ");  
		fflush(stdout);

		// read in the next command entered by the user
		if ((fgets(cmdline, MAXLINE, stdin) == NULL) 
				&& ferror(stdin)) {
			perror("fgets error");
		}

		if (feof(stdin)) { /* End of file (ctrl-d) */
			fflush(stdout);
			exit(0);
		}

		HistoryEntry he; // history queue
		he.cmd_num = cmd_num; // history process id
		strcpy(he.cmdline, cmdline);
		bg = parseArguments(cmdline, argv);

		// if no input will not exit shell.
		if(argv[0] == NULL){
			printf("No command entered\n");
			continue;
		}
		// if exit, shell will exit
		else if(!strcmp(argv[0], "exit")){
			exit(0);
		}
		// if history, will add to history queue and print last 10 commands
		else if(!strcmp(argv[0], "history")){
			add_queue(he);
			cmd_num++;
			print_queue();
		}
		// if !<num>, will execute command with a matching command ID
		// from command history.
		else if(argv[0][0] == '!'){
			// check to make sure proper usage (i.e. !5)
			if (strlen(cmdline) > 2){
				char num[strlen(cmdline)];
				int n;
				for(n=1; n <= strlen(cmdline); n++){
					num[n-1] = (argv[0][n]);
				}
				unsigned int i = atoi(num);
				char* cmdHist = commandLine_history(i);
				strcpy(he.cmdline, commandLine_history(i));
				bg = parseArguments(commandLine_history(i), argv);
				if(!strcmp(cmdHist, "History ID not found!\n")){
					printf("ID not found: %s", cmdline);
				}
				else if(!strcmp(argv[0], "history")){
					add_queue(he);
					cmd_num++;
					print_queue();
				}
				else {
					int pid = fork();
					int ret;
					if(pid == 0){
						ret = execvp(argv[0], argv);
						if(ret == -1){
							exit(0);
						}
					}
					// if '&' in command line
					// run process in background
					if(bg == 1){
						kill(pid, SIGCHLD);
					}
					// else run process in foreground
					else waitpid(pid, NULL, 0);	
					cmd_num++;
               		         	add_queue(he);	
				}
			}
			// improper call to !<num>
			else {
				printf("Invalid history entry\n");
				printf("Usage: !<num>\n");
				printf("Example: !5\n");			
			}
		}
		// run built in shell commands
		else {
			int ret;
			add_queue(he);
			int pid = fork();
			if(pid == 0){
				ret = execvp(argv[0], argv);
				if(ret == -1){
					printf("%s: Command not found\n", argv[0]);
					exit(0); 
				} 
			}
			// if '&' included in command line
			// run process in background
                        if(bg == 1){
                        	kill(pid, SIGCHLD);
			}
			// else run process in foreground		
			else waitpid(pid, NULL, 0);
			cmd_num++;
		}	
	}
	return 0;
}