Shell Interpreter

Description

This project is about creating a simple shell interpreter. The shell is a command that reads lines from either a file or the terminal, interprets them, and generally executes other commands. The shell implements a limited set of features. The shell is not a replacement for the sh shell, but rather a simple version of it.

Usage

The shell works like this in interactive mode:

$ ./hsh
($) /bin/ls
hsh main.c shell.c
($)
($) exit
$

And in non-interactive mode:

$ echo "/bin/ls" | ./hsh
hsh main.c shell.c test_ls_2
$
$ cat test_ls_2
/bin/ls
/bin/ls
$
$ cat test_ls_2 | ./hsh
hsh main.c shell.c test_ls_2
hsh main.c shell.c test_ls_2
$

Features

The shell interpreter provides the following features:

• Display a prompt and wait for user input.
• Execute commands entered by the user.
• Handle errors gracefully.
• Detect the "end of file" condition (Ctrl+D).
• Support simple command lines with no advanced features like semicolons, pipes, redirections, or arguments.
• Find executables in the system and print an error message if not found.
• Implement the exit built-in command to exit the shell.
• Implement the env built-in command to print the current environment.
• Implement the cd built-in command to change the current directory.
• Support the cd - command to switch between the current and previous directories.
• Update the PWD environment variable when changing directories.
• Handle commands separated by ;.
• Support logical operators && and ||.
• Implement the alias built-in command to define and print aliases.
• Handle variable replacement.
• Support the $? variable to access the exit status of the last command.
• Support the $$ variable to access the current process ID.
• Ignore comments prefixed with #.
• Accept a command file as a command line argument. Execute the commands in the file and exit without printing a prompt.

Built-in Commands

The shell interpreter provides the following built-in commands:

• exit: Exit the shell.

$ exit

You can optionally provide an integer status to exit with a specific status code.

$ exit 0

• env: Print the current environment.

$ env

• cd: Change the current directory.

$ cd [DIRECTORY]

If no directory is provided, the command is interpreted as cd $HOME. The - argument can be used to switch between the current and previous directories.

• setenv: Initializes a new environment variable or modifies an existing one.

$ setenv [VARIABLE] [VALUE]

• unsetenv: Remove an environment variable.

$ unsetenv [VARIABLE]

• alias: Allows you to define, print, or modify aliases. To print a list of all aliases:

$ alias

To print specific aliases:

$ alias [NAME]...

To define an alias:

$ alias [NAME]=[VALUE] [...]

To remove an alias:

$ unalias [NAME]...

Command Line Arguments

The shell interpreter supports running commands from a file by providing the filename as a command line argument. The file should contain one command per line.

$ simple_shell [FILE]

In this mode, the shell does not display a prompt and does not read from standard input. Instead, it executes the commands in the file and exits.

Contributors

• Girma Eshete
• Toby Salau