MASH

Matthew's Shell, aims to be a bash compatible, or at least bash-like shell.

Not necessarily trying to bring any unique features to the table, I just want to write a shell, cause I'm bored.

Features

Commands/Builtins

• Change directory with cd
• Set environment variables (or move shell variables to the environment) with export
• Flow control: if, and while (supports multiple commands in conditional)
• Aliases: alias and unalias
• Removing environment variables with unset
• POSIX exec (only for executing commands, does not have file descriptor functionality)
• shift to shift out positional parameters (arguments) - most useful in scripts
• break and continue to stop, or return to the top of a while loop

Others

• Run scripts (can be used as a shebang)
• Version info --version
• Environment and shell variables with $varname
• Run single command with -c command
• Subshells with $(command) - if inside double quotes, you will get the exact output contents (otherwise it is tokenized)
• Redirection. Input with < and <<< (file and string literal), and output with > and >> (overwrite and append).
• Set prompt with $PS1, supports bash prompt expansion tokens. Also supports $PROMPT_COMMAND which if set, will always execute before displaying your prompt (for fancier things like powerline).
• Pipes via |
• Cursor around and edit current command text, via GNU Readline
• Math statements with $((...)), i.e.: echo $((num * 5)).
• Proper handling of SIGINT, so ^C won't kill the shell, it kills the running command.

TODO

Command Parser

• For loops
• Read another line if the line ends with a \, then concatenate them together
• Chain commands together based on exit status with && and ||
• More advanced (recursion) math statement parsing, matching the abilities of other shells.
• Does not error when an if ... statement is entered with no then... I swear it used to do this.
• When the command parser gets an error, don't just print the location, print a reason (I see a new enum in our future...).

Behind the Scenes

• Create $XDG\_CONFIG\_HOME/mash/config.mash?
• Keep history loaded in memory to allow for ! statements and possibly arrow keys (up/down).
• Improve syntax error output messages
• Jobs (should also fix issue with defunct processes resulting from pipes...)
• Split commandExecute into multiple functions, and use those functions where appropriate to improve performance (subshells don't need to parse aliases because there won't be any!)
• Output of subshells ($(...)), when not in double quotes, should become multiple arguments, not just a single argument - as in, in bash/zsh for x in $(echo 'hello world'); do "echo $x"; done will echo hello and world separately, on new lines
• Consider moving away from stdio FILEs and exclusively using unix file descriptors

Code Improvements

• More comments

Other

• Create testbed script that runs through as many mash features as possible to verify they still work, or work period on unfamiliar systems

OS' and Architectures Tested

x86

• Manjaro, Arch, Ubuntu (Linux) - no reason to believe others wouldn't work, except for Void (haven't tested musl)
• macOS - Big Sur (I think), running in a KVM - compatibility.c contains any GNU libc extensions I used that aren't available in the C library included in macOS
• OpenBSD - 7.0, also in a KVM - Makefile does not work out of the box, presumably due to OpenBSD not shipping with GNU make - compatibility.c may also be required for the same reason as macOS

ARM

• Android - via Termux - not the most authentic environment, but as best I can tell it is being natively compiled and executed - compatibility.c also required since Termux ships with bionic libc

PowerPC

• Debian, Lubuntu - running on a Wii and Wii U respectively, yes these are my only ppc machines - outside of being out-of-date versions of these OS', mash compiled and ran fine

Misc

An idea I had before finding out about environ(7):
Environment variable dictionary using binary search tree (still store in array, but use this to easily find its location if it exists) Builtins were going to use suffix trees, and did up until commit 0cead82c where I finally deleted the code. Although janky in its implementation I was still somewhat proud of them. I was going to manually tweak the terminal and implement my own line editing system, but found readline and decided to use that instead. I may revisit this in the future for flexibility. Zsh and fish both use their own line editor.