Project 2-SP22: Developing a Linux Shell

• Justin Gallagher, Project 2: The C Shell
• Due Mar 23 by 1pm Points 8.6
• Submitting a file upload Available Jan 25 at 6pm - Apr 27 at 11:59pm 3 months
• CIS 3207, Section 004 //Spring 2022 //Instructor: Professor Gene Kwatny

The full Project 2 description is here.

Installation (Using the make file)

• Using prompt> ./>make clean will remove the following files from the unpackDirectory. This should be completed first to ensure there are no leftover files in the directory
  • rm myshell myshell.o myshellfunc.o
• Using prompt> ./>make will compile the .c file using the following flags 'Wall and Werror'
  • gcc -c -o myshell.o myshell.c -I.
  • gcc -c -o myshellfunc.o myshellfunc.c -I.
  • gcc -o myshell myshell.o myshellfunc.o -Wall -Werror

#Usage Interactive Mode

The shell can be invoked with either no arguments (interactive). Here is the no-argument way:

• prompt> ./myshell

At this point, myshell is running, and ready to accept commands. Type away!

Batch Mode

• prompt> ./myshell batch.txt

The shell was also be invokes with a single argument (batch0; anything else is an error. There is a difference between batch and interactive modes: in interactive mode, a prompt is printed (myshell> ). In batch mode, no prompt should be printed during execution of commands.

Program Specifications

Basic Supported Shell Commands:

• a. cd - Change the current default directory to . If the argument is not present, report the current directory. If the directory does not exist an appropriate error should be reported. This command should also change the PWD environment variable.
• b. clr - Clear the screen.
• c. dir - List the contents of directory .
• d. environ - List all the environment strings.
• e. echo - Display on the display followed by a new line (multiple spaces/tabs may be reduced to a single space).
• f. help - Display the user manual using the more filter.
• g. pause - Pause operation of the shell until ‘Enter’ is pressed.
• h. quit - Quit the shell.

Custom Modifiers

Piping

• A pipe is a form of redirection (transfer of standard output to some other destination) that is used in Linux and other Unix-like operating systems to send the output of one command/program/process to another command/program/process for further processing. usage: myshell> command_1 | command_2 | command_3 | .... | command_N

Background

• An ampersand (&) at the end of the command line indicates that the shell should return to the command line prompt immediately after launching that program. usage:

      myshell> command_1 & command_2 & command_3 &
          This will execute three commands.
              command_1 will run in the background
              command_2 will run in the background
              command_3 will run in the background
  
      myshell> command_1 & command_2 & command_3
          This will execute three commands.
              command_1 will run in the background
              command_2 will run in the background
              command_3 will run in the foreground
  

I/O Redirection

• The ‘<‘ symbol is used for input(STDIN) redirection
• The ‘>‘ symbol is used for output (STDOUT) redirection.
• If you do not want a file to be overwritten but want to add more content to an existing file, then you should use ‘>>‘ operator.