C Terminal Shell

Overview

EasyShell is a basic bash-like shell implemented in C that that provides a subset of features from bash. It features include running commands, I/O redirection, background processing, and built-in signal handling.

Features

• Prompt for Commands: Provides a colon (:) as the prompt for user input.
• Command Execution: Runs any valid command using fork(), exec(), and/or waitpid().
• Built-in Commands:
  • exit: Exits the shell, terminating all child processes.
  • cd: Changes the current working directory.
  • status: Displays the exit status or the signal of the last foreground process.
• Input and Output Redirection: Supports both input and output redirection using < and >.
• Foreground and Background Execution: Runs commands in the foreground or background depending on the presence of &.
• Signal Handling:
  • SIGINT (Ctrl+C): Terminates foreground processes but does not affect the shell.
  • SIGTSTP (Ctrl+Z): Toggles between allowing and disallowing background execution.

How to Use

1. Command Prompt

The shell uses : as the prompt. The general command format is:

command [arg1 arg2 ...] [< input_file] [> output_file] [&]

• Commands are words separated by spaces.
• Redirection symbols <, >, and & must be surrounded by spaces.
• $$(PID expansion) replaces instances of $$ with the process ID of the shell.

Example:

ls -l > output.txt &

2. Built-in Commands

exit

Exit the shell and terminate all background processes.

exit

cd [directory]

Change the current directory. Without arguments, changes to the home directory.

cd /path/to/directory

status

Displays the exit status or terminating signal of the last foreground command.

status

3. Input & Output Redirection

Redirect input and output of commands using < and >. For example, to read from input.txt and write to output.txt:

sort < input.txt > output.txt

4. Foreground & Background Execution

• Foreground: Default for commands without &. The shell waits for these commands to finish.

  sleep 5

• Background: Append & to run commands in the background.

  sleep 5 &

5. Signal Handling

SIGINT (Ctrl+C)

• Foreground processes are terminated.
• Background processes and the shell ignore the signal.

SIGTSTP (Ctrl+Z)

• Toggles background execution. When background execution is disabled, commands with & run in the foreground.

Message for background suspension:

Entering foreground-only mode (& is now ignored)

Message to resume background execution:

Exiting foreground-only mode

Compilation

To compile EasyShell, use the following command:

gcc -std=gnu99 main.c -o EasyShell

Running EasyShell

Run the shell by executing:

./EasyShell

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Example Usage

: ls -l > directory_listing.txt
: cat < directory_listing.txt
: sleep 10 &
: status
: exit