Simple Shell Application

A custom shell implementation for CMPT 201 Systems Programming

Key System Calls Used

This shell was implemented using several core system calls:

fork()

• Used to create a new child process for executing external commands.
• Enables running commands in the foreground or background by forking separate processes.

exec()

• Used within child processes to replace the process image with the specified command and its arguments.
• Ensures the child process runs the desired program.

wait()

• Used in foreground execution to wait for the child process to complete before returning control to the shell.
• In background execution, waitpid() is used in a non-blocking loop to clean up terminated child processes and prevent zombie processes.

getcwd()

• Used to retrieve and display the current working directory as part of the shell prompt and for the pwd command.

chdir()

• Used to implement the cd command to change the current working directory.

strtok_r()

• Used to tokenize and parse user input into commands and arguments safely.

read() and write()

• Used for reading user input and writing shell output, ensuring signal-safe I/O operations.

Signal Handling (SIGINT)

• A custom handler for SIGINT (triggered by CTRL-C) prevents the shell from terminating.
• Displays help information and re-displays the shell prompt when interrupted.

Features

Command Execution

• Execute external programs with arguments.
• Support for background execution using &.

Internal Commands

• exit: Exits the shell program.
  • Prints an error if any arguments are provided (exit: too many arguments).
• pwd: Prints the current working directory.
  • Prints an error if arguments are provided (pwd: too many arguments).
• cd: Changes the current working directory.
  • ~: Changes to the home directory (e.g., cd ~/tmp).
  • -: Changes to the previous directory.
  • Handles errors such as invalid directories or too many arguments.
• help: Displays help information about internal commands.
  • No arguments: Lists all supported internal commands and their descriptions.
  • First argument: Displays help for a specific internal command.
  • Errors: Displays help: too many arguments if more than one argument is provided.

History Feature

• history: Displays up to the 10 most recent commands executed in the shell.
  • Outputs command numbers and full command details, including arguments.
  • Background commands (&) are included.
• !n: Re-runs the command with the history number n.
  • Prints an error if n is invalid (history: command invalid).
• !!: Re-runs the last command.
  • Prints an error if no command has been entered (history: no command entered).

Signal Handling

• CTRL-C: Does not terminate the shell. Instead:
  • Displays help information (same as the help command).
  • Re-displays the shell prompt.

Shell Prompt

• Displays the current working directory as the shell prompt (e.g., /home/user$).
• Uses getcwd() to retrieve the working directory and handles errors gracefully.