Miscellany

/*
 * This code implements a simple shell program
 * It supports the internal shell command "exit",
 * backgrounding processes with "&", input redirection
 * with "<" and output redirection with ">".
 */

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <errno.h>
#include <signal.h>

extern char **getline();

// Handle exit signals from child processes
void sig_handler(int signal) {
	int status;
	int result = wait(&status);
	printf("Wait returned %d\n", result);
}


// The main shell function
main() {
	int i;
	char **args; 
	int result;
	int block;
	int output;
	int input;
	char *output_filename;
	char *input_filename;

	// Set up the signal handler
	// sigset(SIGCHLD, sig_handler);

	// Loop forever
	while(1) {
		// Print out the prompt and get the input
		printf("->");
		args = getline();

		// No input, continue
		if(args[0] == NULL)
			continue;

		// Check for internal shell commands, such as exit
		if(internal_command(args))
			continue;

		// Check for an ampersand
		block = (ampersand(args) == 0);

		// Check for redirected input
		input = redirect_input(args, &input_filename);

		switch(input) {
			case -1:
				printf("Syntax error!\n");
				continue;
				break;
			case 0:
				break;
			case 1:
				printf("Redirecting input from: %s\n", input_filename);
				break;
		}

		// Check for redirected output
		output = redirect_output(args, &output_filename);

		switch(output) {
			case -1:
				printf("Syntax error!\n");
				continue;
				break;
			case 0:
				break;
			case 1:
				printf("Redirecting output to: %s\n", output_filename);
				break;
		}

		// Do the command
		do_command(args, block, 
			input, input_filename, 
			output, output_filename);
	}
}

// Check for ampersand as the last argument
int ampersand(char **args) {
	int i;

	for(i = 1; args[i] != NULL; i++) ;

	if(args[i-1][0] == '&') {
		free(args[i-1]);
		args[i-1] = NULL;
		return 1;
	} else {
		return 0;
	}
	return 0;
}

// Check for internal commands
// Returns true if there is more to do, false otherwise 
int internal_command(char **args) {
	if(strcmp(args[0], "exit") == 0) {
		exit(0);
	}
	return 0;
}

// Do the command
int do_command(char **args, int block,
		int input, char *input_filename,
		int output, char *output_filename) {

	int result;
	pid_t child_id;
	int status;

	// Fork the child process
	child_id = fork();

	// Check for errors in fork()
	switch(child_id) {
		case EAGAIN:
			perror("Error EAGAIN: ");
			return;
		case ENOMEM:
			perror("Error ENOMEM: ");
			return;
	}

	if(child_id == 0) {

		// Set up redirection in the child process
		if(input)
			freopen(input_filename, "r", stdin);

		if(output)
			freopen(output_filename, "w+", stdout);

		// Execute the command
		result = execvp(args[0], args);

		exit(-1);
	}

	// Wait for the child process to complete, if necessary
	if(block) {
		printf("Waiting for child, pid = %d\n", child_id);
		result = waitpid(child_id, &status, 0);
	}
}

// Check for input redirection
int redirect_input(char **args, char **input_filename) {
	int i;
	int j;

	for(i = 0; args[i] != NULL; i++) {

		// Look for the <
		if(args[i][0] == '<') {
			free(args[i]);

			// Read the filename
			if(args[i+1] != NULL) {
				*input_filename = args[i+1];
			} else {
				return -1;
			}

			// Adjust the rest of the arguments in the array
			for(j = i; args[j-1] != NULL; j++) {
				args[j] = args[j+2];
			}

			return 1;
		}
	}
	return 0;
}

// Check for output redirection
int redirect_output(char **args, char **output_filename) {
	int i;
	int j;

	for(i = 0; args[i] != NULL; i++) {

		// Look for the >
		if(args[i][0] == '>') {
			free(args[i]);

			// Get the filename 
			if(args[i+1] != NULL) {
				*output_filename = args[i+1];
			} else {
				return -1;
			}

			// Adjust the rest of the arguments in the array
			for(j = i; args[j-1] != NULL; j++) {
				args[j] = args[j+2];
			}

			return 1;
		}
	}
	return 0;
}