A basic command interpreter. (For teaching purposes.)

shell.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

void my_system(char **);
int my_builtins(char *);

int main(void)
{
  unsigned int i, j, arr_count;
  char c, *cmd, **cmd_args;

  while(printf("> "))
  {
    arr_count = (int) (cmd = (char *) (cmd_args = (char **) 0));
    for(i = 0; (c = getchar()) != '\n'; i++)
    {
      if(!cmd)
      {
        if(!(cmd = (char *) malloc(1))) goto err;
        *cmd = c;
        continue;
      }

      if(!(cmd = (char *) realloc(cmd, i + 1))) goto err;
      cmd[i] = c;
    }
  
    if(cmd)
    {
      cmd = (char *) realloc(cmd, i + 1);
      cmd[i] = 0;
    }
    else continue;

    for(i = 0; cmd[i]; i++)
    {
      if(cmd[i] != ' ')
      {
        for(j = 0; cmd[i] && cmd[i] != ' '; j++)
          i++;

        if(!cmd_args)
        {
          if(!(cmd_args = (char **) malloc(sizeof(char *)))) goto err;
        }
        else if(!(cmd_args = (char **) realloc(cmd_args, sizeof(char *) * (arr_count + 1)))) goto err;

        cmd_args[arr_count] = (char *) malloc(j + 1);
        snprintf(cmd_args[arr_count++], j + 1, "%s", cmd + i - j);
      }

      if(!cmd[i])
      {
        if(!(cmd_args = (char **) realloc(cmd_args, sizeof(char *) * (arr_count + 1)))) goto err;
        cmd_args[arr_count] = (char *) 0;

        break;
      }
    }

    switch(my_builtins(*cmd_args))
    {
      case 1:
        exit(0);
      case 2:
        if(!cmd_args[1])
        {
          if(chdir(getenv("HOME")) < 0)
            fprintf(stderr, "chdir: Cannot change to HOME.\n");
        }
        else if(chdir(cmd + strlen(*cmd_args) + 1) < 0)
          fprintf(stderr, "chdir: Cannot change to %s.\n", cmd + strlen(*cmd_args) + 1);

        break;
      default:
        my_system(cmd_args);
    }

    for(i = 0; cmd_args[i]; i++)
      free(cmd_args[i]);
    free(cmd_args);
    free(cmd);
  }

  return 0;

  err:
    fprintf(stderr, "Failed to allocate memory.\n");

    return -1;
}

void my_system(char **param)
{
  if(fork()) wait(NULL);
  else if(execvp(*param, param) < 0)
  {
    fprintf(stderr, "%s: Command not found.\n", *param);
    exit(0);
  }
}

int my_builtins(char *param)
{
  int i, ret = 0;

  for(i = 0; param[i] && "exit"[i]; i++)
  {
    if(param[i] == "exit"[i]) ret++;
    else
    {
      ret = 0;
      break;
    }
  }

  if(strlen(param) == ret && strlen("exit") == ret) return ret = 1;
  else for(i = 0; param[i] && "chdir"[i]; i++)
  {
    if(param[i] == "chdir"[i]) ret++;
    else
    {
      ret = 0;
      break;
    }
  }

  if(strlen(param) == ret && strlen("chdir") == ret) return ret = 2;
  else return ret = 0;
}