UNIX Shell Project

Overview

This project involves creating a custom UNIX shell for an Operating Systems lab assignment. The shell simulates essential shell features, allowing users to execute commands, manage background processes, use input/output redirection, manage command history, and handle shell variables. Each version of the shell builds upon the previous, gradually adding more functionality until a fully functional shell environment is achieved.

Features by Version

Version 01: Basic Shell

• Objective: Implement the foundational shell capabilities.

• Features:

  • Displays a custom prompt.
  • Parses user input to execute basic commands.
  • Uses fork() and exec() to execute commands in a child process.
  • Allows the shell to exit using the exit command or <CTRL+D>.

• Usage:

  • Run simple commands such as ls, pwd, whoami.
  • Exit by typing exit or pressing <CTRL+D>.

Version 02: I/O Redirection

• Objective: Add input and output redirection.

• Features:

  • Supports output redirection using >.
  • Supports input redirection using <.
  • Allows combined input and output redirection, e.g., command < input.txt > output.txt.

• Usage:

  • Redirect output: ls > output.txt (writes the output of ls to output.txt).
  • Redirect input: sort < unsorted.txt (reads from unsorted.txt).
  • Combined redirection: sort < unsorted.txt > sorted.txt (sorts contents of unsorted.txt and writes to sorted.txt).

Version 03: Background Processes

• Objective: Implement background processing.

• Features:

  • Enables background process execution by appending & to commands.
  • Displays background job details (PID and job ID) when a job is started.
  • Prevents "zombie" processes by handling background processes in a non-blocking way using waitpid() with WNOHANG.

• Usage:

  • Start a background job: sleep 30 & (runs sleep in the background).
  • List background jobs: jobs.
  • Kill a background job: kill <pid> (replace <pid> with the process ID).

Version 04: Command History

• Objective: Provide a command history feature.

• Features:

  • Tracks up to the last 10 commands.
  • Displays command history using history.
  • Supports re-execution of a specific command from history by typing !number, where number is the command’s position in the history list.

• Usage:

  • View history: history (lists the last 10 commands).
  • Execute a previous command: !1 (executes the first command from history).

Version 05: Built-In Commands

• Objective: Implement core built-in commands without forking.

• Features:

  • Implements essential built-in commands: cd, jobs, kill, and help.
    • cd <directory>: Changes the current directory.
    • jobs: Lists all active background jobs.
    • kill <pid>: Terminates a background process by its PID.
    • help: Displays available built-in commands and their usage.

• Usage:

  • Change directory: cd /tmp (moves to /tmp directory).
  • View background jobs: jobs.
  • Terminate a process: kill <pid>.
  • Get help: help.

Version 06: Shell Variables

• Objective: Add shell variable support for setting, retrieving, and unsetting variables.

• Features:

  • Enables users to define variables with set <name>=<value>.
  • Retrieves variable values by typing the variable name directly.
  • Lists all defined shell variables using printenv.
  • Unsets (removes) a variable with unset <name>.

• Usage:

  • Set a variable: set myvar=hello (creates a variable myvar with the value hello).
  • Access a variable: myvar (prints hello).
  • List all variables: printenv.
  • Unset a variable: unset myvar (removes myvar).

How to Run the Project

1. Compile the Code:

   • Open a terminal in the directory containing your code file (e.g., shell.c).
   • Compile the shell with:

     gcc -o myshell shell.c

2. Run the Shell:

   • Start the shell by running:

     ./myshell

Testing Each Version

Version 01: Basic Shell

1. Run commands such as ls, pwd, and whoami to confirm basic execution.
2. Type exit or press <CTRL+D> to exit the shell.

Version 02: I/O Redirection

1. Output redirection: ls > output.txt. Check output.txt for output.
2. Input redirection: Create unsorted.txt, then run sort < unsorted.txt.
3. Combined redirection: sort < unsorted.txt > sorted.txt. Check sorted.txt.

Version 03: Background Processes

1. Start a background job: sleep 30 &.
2. View active jobs with jobs.
3. Kill a job using kill <pid>, where <pid> is from jobs.

Version 04: Command History

1. Run several commands (ls, pwd, whoami).
2. Type history to view the last 10 commands.
3. Use !number (e.g., !1) to repeat a command from history.

Version 05: Built-In Commands

1. cd /tmp to test changing directories.
2. Start a background job (sleep 30 &), then check with jobs.
3. Terminate a job with kill <pid> from jobs.
4. Type help to view all built-in commands.

Version 06: Shell Variables

1. Set a variable: set myvar=hello.
2. Access it by typing myvar (should output hello).
3. List all variables with printenv.
4. Unset myvar with unset myvar and verify it no longer displays.

Example Usage

Here’s an example session showing the use of various features across all versions:

PUCITshell@/home/user:- ls
file1.txt  file2.txt  shell.c
PUCITshell@/home/user:- set myvar=hello
PUCITshell@/home/user:- myvar
hello
PUCITshell@/home/user:- pwd > output.txt
PUCITshell@/home/user:- sleep 30 &
[Background] Started process with PID 1234
PUCITshell@/home/user:- jobs
[1] PID: 1234
PUCITshell@/home/user:- kill 1234
Process 1234 terminated.
PUCITshell@/home/user:- history
1: ls
2: set myvar=hello
3: myvar
4: pwd > output.txt
5: sleep 30 &
6: jobs
PUCITshell@/home/user:- !1
file1.txt  file2.txt  shell.c
PUCITshell@/home/user:- help
Built-in Commands:
cd <directory> - Change directory
exit - Exit the shell
jobs - List background jobs
kill <pid> - Terminate background process with PID
history - Display command history
set <name>=<value> - Set a shell variable
unset <name> - Remove a shell variable
printenv - List shell variables
help - List built-in commands
PUCITshell@/home/user:- printenv
myvar=hello
PUCITshell@/home/user:- unset myvar
PUCITshell@/home/user:- myvar
Error executing command: No such file or directory
PUCITshell@/home/user:- exit

Known Limitations

• Limited History: Only stores the last 10 commands.
• Limited Variables: Supports a maximum of 50 shell variables.
• Background Process Limit: Only allows 10 concurrent background processes.

Summary

This project emulates core functionalities of a UNIX shell, building progressively with each version:

• Basic command execution.
• I/O redirection.
• Background job control.
• Command history management.
• Built-in commands.
• Shell variable management.

Through this project, users gain a deep understanding of process control, input/output management, and command handling in a UNIX-like environment, which are crucial skills for systems programming and operating systems development.

Acknowledgments

Thanks to the Operating Systems Lab course for providing foundational concepts and guidance, and to online resources on fork(), execvp(), dup2(), and signal handling for aiding the development of this shell.

---