shell.c

#include <stdio.h>
#include <unistd.h>
#include <string.h>

#define MAX_LINE 80

int main(void) {
  char *args[MAX_LINE/2 + 1]; // an array of 41 pointers to character arrays
  int should_run = 1;

  while(should_run) {
    /*
      Print out the prompt and flush the buffer
    */
    printf("osh > ");
    fflush(stdout);

    /*
      Create a command line to hold arguments
    */
    char command[MAX_LINE];
    gets(command);

    /*
      Parse commands and store into args
    */
    char *command_ptr;
    command_ptr = command;

    int arg_index = 0;
    args[arg_index] = command_ptr;
    arg_index++;

    while(*command_ptr != '\0') {
      if(*command_ptr == ' ' || *command_ptr == '\n') {
        // printf("'%c' is a delimiter, changing it to a \\0\n", *command_ptr);
        *command_ptr = '\0'; // replace spaces with nul-terminators so args words will terminate
        command_ptr++; // move command_ptr to the next char address
        args[arg_index] = command_ptr; // set the next args word address to the command pointer
        arg_index++; // move the arg index to the next word
      } else {
        // printf("'%c' is a not a space, moving to next char address\n", *command_ptr);
        command_ptr++; // move the command pointer to the next char address
      }
    }

    int arg_index = 0;
    args[arg_index] = &command[0]; // need to point to the address of the first char in command
    arg_index++;
    for(int i=0; i<strlen(command); i++) {
      if(command[i] == ' ' || command[i] == '\n') {
        command[i] = '\0';
        args[arg_index] = &command[i + 1]; // need to point to the address at command[i + 1]
        arg_index++;
      }
    }

    /*
      Interpret the command to determine what to do with it
    */
    if(strcmp(args[0], "exit") == 0) {
      should_run = 0;
    } else {
      // Run command
      pid_t pid;
      /* Fork a child process */
      pid = fork();
      if (pid < 0) { /* Error occurred */
        fprintf(stderr, "Fork Failed");
        return 1;
      }
      else if (pid == 0) { /* Child process */
        execvp(args[0], args);
      }
      else {
        /* Parent will wait for the child to complete */
        wait(NULL);
        printf("Child Complete\n");
      }
    }
  }

  return 0;
}