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 II—Creating a History Feature

The next task is to modify the shell interface program so that it provides
a history feature that allows the user to access the most recently entered
commands. The user will be able to access up to 10 commands by using the
feature. The commands will be consecutively numbered starting at 1, and
the numbering will continue past 10. For example, if the user has entered 35
commands, the 10 most recent commands will be numbered 26 to 35.

The user will be able to list the command history by entering the command
history at the osh> prompt. 

As an example, assume that the history consists of the commands (from most to least recent):

  ps, ls -l, top, cal, who, date

The command history will output:

  6 ps
  5 ls -l
  4 top
  3 cal
  2 who
  1 date

Your program should support two techniques for retrieving commands
from the command history:

1. When the user enters !!, the most recent command in the history is
executed.

2. When the user enters a single ! followed by an integer N, the Nth
command in the history is executed.

Continuing our example from above, if the user enters !!, the ps command
will be performed; if the user enters !3, the command cal will be executed.
Any command executed in this fashion should be echoed on the user’s screen.
The command should also be placed in the history buffer as the next command.
The program should also manage basic error handling. If there are
no commands in the history, entering !! should result in a message “No
commands in history.” If there is no command corresponding to the number
entered with the single !, the program should output "No such command in
history."



