This project consists of designing a C program to serve as a shell interface that accepts user commands and then executes each command in a separate process. This project can be completed on any Linux, UNIX, or Mac OS X system.

A shell interface gives the user a prompt, after which the next command
is entered. The example below illustrates the prompt osh> and the user’s
next command: cat prog.c. (This command displays the file prog.c on the
terminal using the UNIX cat command.)
  
  osh> cat prog.c

One technique for implementing a shell interface is to have the parent process first read what the user enters on the command line (in this case, cat prog.c), and then create a separate child process that performs the command.

Unless otherwise specified, the parent process waits for the child to exit
before continuing. This is similar in functionality to the new process creation. However, UNIX shells typically also allow the child process to run in the background, or concurrently. To accomplish this, we add
an ampersand (&) at the end of the command. Thus, if we rewrite the above
command as 
  
  osh> cat prog.c &

the parent and child processes will run concurrently.

The separate child process is created using the fork() system call, and the user’s command is executed using one of the system calls in the exec() family (as described in Section 3.3.1).

A C program that provides the general operations of a command-line shell
is supplied in Figure 3.36. The main() function presents the prompt osh>
and outlines the steps to be taken after input from the user has been read. The main() function continually loops as long as should run equals 1; when the user enters exit at the prompt, your program will set should run to 0 and terminate.

This project is organized into two parts: (1) creating the child process and executing the command in the child, and (2) modifying the shell to allow a history feature.

  Part I— Creating a Child Process

The first task is to modify the main() function in Figure 3.36 so that a child process is forked and executes the command specified by the user. This will require parsing what the user has entered into separate tokens and storing the tokens in an array of character strings. 

For example, if the user enters the command 

  ps -ael 

at the osh> prompt, the values stored in the args array are:

  args[0] = "ps"
  args[1] = "-ael"
  args[2] = NULL

This args array will be passed to the execvp() function, which has the
following prototype:

  execvp(char *command, char *params[]);

Here, command represents the command to be performed and params stores the
parameters to this command. For this project, the execvp() function should
be invoked as execvp(args[0], args). Be sure to check whether the user
included an & to determine whether or not the parent process is to wait for the child to exit.


