Add built-in commands: pwd, echo, and export

[adityaanand05]

Proposed change

This PR extends the shell’s built-in command support by adding the following commands to handle_builtins():

pwd – Prints the current working directory using getcwd()

echo – Prints arguments passed to the command, separated by spaces

export – Sets environment variables using KEY=VALUE syntax

These additions improve basic shell usability while keeping the implementation simple and consistent with the existing built-in structure.

Details

pwd uses a fixed-size buffer and reports errors via perror

echo iterates over argv starting from index 1

export supports:

export KEY=VALUE

export KEY (sets an empty value)

No changes were made to existing built-ins (cd, exit)

Type of change

Existing code improvement (feature enhancement)

Non-breaking change

Notes

This implementation focuses on minimal POSIX-like behavior and does not yet handle advanced cases (e.g., echo -n, variable expansion, multiple exports in one command).