Custom C Shell

Overview

This project implements a simple Unix-style shell in the C programming language, called customshell.

The project is delivered in 3 stages (Stage 1, Stage 2, Stage 3) with incremental functionality.

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Features

Stage 1

• Internal Commands
  • cd – change current directory (or display if no argument). Updates $PWD.
  • clr – clear the terminal screen.
  • dir – list directory contents (like ls -al).
  • environ – display all environment variables.
  • echo – print arguments (collapsing multiple spaces/tabs).
  • help – display user manual with more filter.
  • pause – pause execution until Enter is pressed.
  • quit – exit the shell.
• Batch Mode
  • If invoked as ./customshell batchfile, commands are read and executed from the file.
• Shell Environment
  • Sets environment variable shell=/path/to/customshell.

Stage 2

• External Commands
  • Launch programs using fork + exec, with parent=/path/to/customshell added to environment.
• I/O Redirection
  • Input < inputfile
  • Output overwrite > outputfile
  • Output append >> outputfile
  • Works for both internal (dir, environ, echo, help) and external commands.
• Background Execution
  • Commands ending with & run in the background while shell prompt remains available.

Stage 3

• Demonstration Video
  • 5-minute video showing:
    • Operation of internal commands.
    • Running external commands in background (e.g. ./sleepy 5 &).
    • Demonstration of I/O redirection.
    • Code walkthrough (focus on external commands, fork/exec, redirection with freopen / dup).

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Project Structure

• makefile – builds the customshell binary.
• customshell.c – main shell implementation.
• utility.c – supporting functions.
• customshell.h – header definitions.
• Stage1/, Stage2/, Stage3/ – submission directories for each project stage.

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Building

Compile the shell using the provided makefile:

make

This generates the executable:

./customshell

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Usage

Interactive Mode Run the shell directly:

./customshell

Prompt will show the current working directory.

Batch Mode

Provide a batch file:

./customshell batchfile

Executes all commands in the file sequentially.

Example Commands

cd /home/user
dir
echo Hello World
environ
pause
quit

I/O Redirection Examples

dir > out.txt
echo Hello >> log.txt
./myprogram < input.txt > output.txt

Background Execution

./sleepy 5 &
dir

User Manual (Beginner - Friendly)

• cd [dir] – change directory, or show current if none given.
• clr – clear screen.
• dir [dir] – list contents of a directory.
• environ – show all environment variables.
• echo [text] – print text.
• help – open manual.
• pause – wait until Enter is pressed.
• quit – exit the shell.
• [program] [args] – run external program.
• [command] < infile > outfile – redirect input/output.
• [command] & – run process in background.