Question

Name your source code file sh.c

Write your own simple shell. Your shell should prompt the user for a command, run it, then prompt the user for their next command. If the user types "exit", then the shell should terminate. The shell should ignore Ctrl-C.

Inside the attached parse.h header file, there is a parse function that you can use to split up the command line into separate strings. Recall that execvp accepts two arguments: the name of the command, and then an array of command-line arguments as strings. The parse() function in parse.h accepts the string and returns the array of argument strings.

You can use parse.h or roll your own parse function.

Hints :

Remember the logic for the shell :

while input != exit
parse the command
fork
if parent process :
wait
else
execvp command

You'll need the chdir system call to add support for cd. This needs to be part of the shell as their is no cd system command.

man chdir, execvp, wait, fork, strtok

This may be the most challenging assignment of the quarter for some people. Don't wait to get started on it.

This assignment is a capstone of sorts for the course. Help will be limited to clarification of requirements and concepts.

// splits input into separate strings
// inputs:  input: a string to split up, args: an array of char pointers to
//    store the separate strings
// outputs: an array of strings, with null in the last element, passed back
//    through args
// preconditon: input is a valid c-string, read from the keyboard using fgets
// postcondition:  args contains the separate strings, one 
//    string per element.  Last element contains null
#ifndef PARSE_H
#define PARSE_H

#include "string.h"

static void parse( char* input, char* args[] )
{
  int i = 0;
  
  // fgets reads the \n, so overwrite it
  input[strlen(input)-1] = '\0';  
  
  // get the first token
  args[i] = strtok( input, " " );
  
  // get the rest of them
  while( args[++i] = strtok(NULL, " ") );
}

#endif

Answer 1

//execute.h
#ifndef EXECUTE_H
#define EXECUTE_H

#include <stdio.h>
#include <sys/types.h>
#include"parse.h"
void execute(char **argv)
{
pid_t pid;
int status;

if ((pid = fork()) < 0) { /* fork a child process */
printf("*** ERROR: forking child process failed\n");
return;
}
else if (pid == 0) { /* for the child process: */
if (execvp(*argv, argv) < 0) { /* execute the command */
printf("*** ERROR: exec failed\n");
return;
}
}
else { /* for the parent: */
while (wait(&status) != pid) /* wait for completion */
;
}
}

#endif

//parse.h
// splits input into separate strings
// inputs: input: a string to split up, args: an array of char pointers to
// store the separate strings
// outputs: an array of strings, with null in the last element, passed back
// through args
// preconditon: input is a valid c-string, read from the keyboard using fgets
// postcondition: args contains the separate strings, one
// string per element. Last element contains null
#ifndef PARSE_H
#define PARSE_H

#include "string.h"

static void parse( char* input, char* args[] )
{
int i = 0;
  
// fgets reads the \n, so overwrite it
input[strlen(input)-1] = '\0';
  
// get the first token
args[i] = strtok( input, " " );
  
// get the rest of them
while( args[++i] = strtok(NULL, " ") );
}

#endif

//sh.c
#include <stdio.h>
#include"parse.h"
#include"execute.h"
/* ----------------------------------------------------------------- */
/* The main program starts here */
/* ----------------------------------------------------------------- */

int main(void)
{
char line[1024]; /* the input line */
char *argv[64]; /* the command line argument */

while (1) { /* repeat until done .... */
printf("Shell -> "); /* display a prompt */
fgets(line,sizeof(line),stdin); /* read in the command line */
printf("\n");
parse(line, argv); /* parse the line */
if (strcmp(argv[0], "exit") == 0) /* is it an "exit"? */
return 0; /* exit if it is */
execute(argv); /* otherwise, execute the command */
}
}