Simple UNIX shell

seashell.c

/*
 * Graham Patterson
 * Simple shell program
 */
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define MAX_SIZE 256

void parse_command(char *cmd, char **parsed);
void die(char *message);
int exec_cmd(char **cmd_args);
int cd(char *path);

int main(int argc, char **argv)
{
    char command[MAX_SIZE];
    char *parsed[MAX_SIZE/sizeof(char *)];

    while(1)
    {
        printf("SeaShell--> ");
        if (fgets(command, sizeof(command), stdin) == NULL) {
            die("fgets failed");
        }

        // skip execution if just pressing enter
        if (strcmp(command, "\n")) {
            parse_command(command, parsed);

            if (!strcmp(*parsed, "cd")) {
                cd(parsed[1]);
            } else {
                exec_cmd(parsed);
            }
        }
    }
}


/*
 * Execute the command { cmd_args }
 */
int exec_cmd(char **cmd_args)
{
    if (cmd_args == NULL) {
        die("exec_cmd failed");
    }

    pid_t pid = fork();
    if (pid < 0) {
        die("fork failed");
    } else if (pid == 0) {
        execvp(*cmd_args, cmd_args);
        die("Error! Unknown command");
    } else {
        waitpid(pid, 0, 0);
    }

    return 0;
}


/*
 * kill a process on failure
 */
void die(char *message)
{
    fprintf(stderr, "%s\n", message);
    exit(-1);
}


/*
 * Implementing cd because without changing
 * the directory of the shell, ie the parent
 * process, you'll only change the directory
 * of some child process. In this case, we're
 * never even calling exec
 */
int cd(char *path) { return chdir(path); }


/*
 * Parses up to 32 space delimited
 * command and options
 *
 * Fills in the {parsed} paramter with the parsed commands
 * from {input}
 */
void parse_command(char *input, char **parsed)
{
    char *sep = " ";
    char *cmd;
    int i = 0;

    cmd = strtok(input, sep);
    while(cmd != NULL)
    {
        // make sure that cmd is null terminated
        if (cmd[strlen(cmd)-1] == '\n') {
            cmd[strlen(cmd)-1] = '\0';
        }

        // only add a command if it isn't a space
        if (strcmp(cmd,"") && strcmp(cmd," ") && strcmp(cmd, "\n")) {
            parsed[i++] = cmd;
        }

        cmd = strtok(NULL, sep);
    }
    parsed[i] = NULL;
}