Shell-Emulator

Overview

Simple command-line shell emulator written in C.
It replicates the basic functionality of a UNIX shell — including executing external programs, handling built-in commands (cd, pwd, exit), managing I/O redirection, and supporting multiple pipes in a single command line.

The goal of this project is to gain an understanding of process control and system call interfaces in Linux, while maintaining modular and readable C code.

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Features Implemented

Built-in Commands

Command	Description
cd [dir]	Changes the current working directory. If no argument is provided, it defaults to $HOME
pwd	Prints the current working directory
exit [status]	Exits the shell (optionally accepts an exit code)

External Command Execution

• Executes any valid executable in your $PATH.
• Handles invalid commands gracefully with error messages.

I/O Redirection

Operator	Description	Example
>	Redirects standard output to a file (creates/truncates file)	ls > out.txt
<	Redirects standard input from a file	sort < data.txt

Pipes

Supports multiple pipes, allowing chained commands:

cat input.txt | grep error | sort | uniq

Status Substitution

Supports $? to access the exit status of the last executed command:

echo $?

Error Handling

• Detects missing filenames or syntax errors in redirections
• Detects empty pipe commands (e.g., | ls or ls | | grep) and ignores them (ls || grep becomes ls | grep)
• Prints descriptive errors using strerror(errno)
• Can exit out of running commands using Ctrl+C

Interactive & Script Modes

• Interactive mode: Standard command-line interface ($ prompt)
• Batch mode: Execute commands from a file:

./shell commands.txt

How to run

1. To compile the shell

make

2. Start the shell in interactive mode

./shell

3. To clean the directory

make clean

Current Limitations / Future Improvements

Command	Description
Background jobs (&)	Allow commands to run asynchronously in the background
Command history	Support !!, !n, and arrow-key navigation
Environment variables	Expand $PATH, $HOME, etc., and support export
Append redirection (>>)	Add output appending functionality
Logical operators	Allow sequential and conditional command execution