Building a Shell
Systems Programming
COP-3402

• String input
• Program name and arguments, e.g., ls ./
• Built-in commands, e.g., cd or exit

• Prompt for another command after execution

  do
    command = read(userinput)
    interpret(command)
  while command != exit
  

• Break up command into its parts
• Create new process
• Execute new command
• Wait for command to finish

• Tokenize whitespace-delimited inputs

• (Diagram)

  ls ./ ../
  

#include <stdio.h>
#include <inttypes.h>  // intmax_t
#include <string.h>    // strcspn(), strtok
#include <stdlib.h>    // exit()

int main() {
  const int BUFSIZE = 1024;
  char *prog;
  char *arg;
  char *buffer;
  while (1) {
    buffer = malloc(sizeof(char) * BUFSIZE);

    printf("$ ");
    // if you don't flush the buffer, it gets printed out in child as well apparently!

    // read in command with maximum size
    if (!fgets(buffer, BUFSIZE, stdin)) {
      // use ctrl-d to end the input
      printf("\nno more input\n");
      exit(EXIT_SUCCESS);
    }
                printf("length: %" PRIdMAX "\n", strlen(buffer));
                printf("buffer: %s\n", buffer);

    // remove newline
    buffer[strcspn(buffer, "\n")] = '\0';

    if (strlen(buffer) == 0) {
      continue;
    }

    prog = strtok(buffer, " ");
    if (NULL == prog) {
      fprintf(stderr, "empty program name\n");
      break;
      /* exit(EXIT_FAILURE); */
    }

    printf("program name: %s\n", prog);

    while (NULL != (arg = strtok(NULL, " "))) {
      printf("arg: %s\n", arg);
    }
  }
}

What syscalls can we use for this?

#include <stdio.h>
#include <unistd.h>    // fork(), execvp()
#include <stdlib.h>    // exit()
#include <inttypes.h>  // intmax_t

// man 2 execvp

int main(int argc, char **argv) {
  char *prog = "ls";
  const int MAXARGV = 32 + 1 + 1;  // including the NULL terminator and the progname
  char **newargv = malloc(sizeof(char *) * (MAXARGV));
  newargv[0] = prog;
  newargv[1] = "./";
  newargv[2] = "../";
  newargv[3] = NULL;  // NULL-terminate the argv list
  execvp(prog, newargv);
}

• Break up command by pipes
• Create pipe for each pipe symbol
• For each pipe component
  • Break up command into its parts
  • Create new process
  • Redirect I/O
  • Execute new command
• Wait for all commands to finish