UNIX_shell_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 (args in Figure 3.36). 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. The C library function strtok() can be used to parse the command entered by the user into arguments.

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."