MeeShell

Pronounced as "michelle", a reference to The Beatles' song Michelle.

Build & Execution

make # Build
./meeshell # Run

Features

Tokenization

• Following are special tokens: >, >>, >>>, &. The input is split into tokens by spaces, special tokens, or quoted strings.
• When a special token is encountered it is added to the token list as a separate token, if it is not quoted.
• & token must be the last token in the input, otherwise an error is thrown.

Prompt

• Prompt replaces ~ with the home directory of the user.
• Input is read to a buffer of size 1024.

Aliases

• Aliases are stored in .meeshrc file.
• At the beginning of the program, .meeshrc file is read and aliases are stored in a hash map, see dictionary.h for implementation.
• Aliases can be added with alias key=value syntax, and removed with alias key="" syntax.
• After an expression is matched with above syntax the rest of the line is ignored.
• Aliases can be overwritten.
• Aliases are expanded once, no recursive expansion.
• All the session aliases are written to .meeshrc file at the end of the program.

bello

• Current Shell holds the name of parent process, if it is not a shell (for example logind, then its value is logind). Alternatives: $SHELL environment variable stores the login shell, which may not be the parent shell.
• Number of processes shows the number of running processes spawned by the shell, except the dummy processes created for >>>.
• bello output can be redirected and run in background.

cd

• cd is a built-in command, changes the current working directory.

exit

• exit is a built-in command, exits the shell. <Ctrl-D> also exits the shell.
• <Ctrl-C> does not exit the shell, prints a new line.

Command Execution

• All external commands are executed with fork() and execv(). Commands are first searched in the aliases, if not found, then in the PATH environment variable, it is assumed that it is an absolute/relative path.
• Background processes are kept in user->bg_pids list. Number of processes in bello is the length of this list. When a background process terminates, it is removed from the list, and reaped with waitpid() in a signal handler.

Redirection

• > and >> redirections are handled by dup2() calls.
• >>> redirection in background is handled by creating a dummy process, which waits for the command to finish, and then exits. Dummy processes are not included in bello process count. Dummy process reads from a pipe, reverses the input, and writes to the file.

Implementation

The project is implemented in OOP style: Main:

• main.c contains the main loop, and the signal handlers.

Classes:

• tokenizer.c contains the tokenizer implementation.
• dictionary.c contains the hash map implementation.
• repl.c contains the REPL implementation.
• user.c contains the user object implementation.

Utilities:

• utils.c contains the utility functions, for executing commands, redirections, and user info. Please refer to header files for documentation.