Shrimp

Shellzao Rust Implementation

Test

cargo test -- --test-threads=1

Design Notes

General

• Basic Unit of execution is the Pipeline.
• The Pipeline is composed by Steps (other shells sometimes use "Simple Command" as terminology).
• Steps are either a built-in functions, or external programs
• Redirections (and possibly pipes?) require whitespace as delimitators

Steps

• Implemented as an enum instead of trait
  • Known and limited number of variants (either a Built-in, or external command)
  • No need for extensibility (one of the main uses for Traits and Trait Objects)
  • More performant than trait objects due to lack of dynamic dispatching (at least in theory, not sure how big the impact would be in such a small case)
• Pipes connect the output of one Step with the input of the next by passing byte streams (Vec<u8>)
  • Flexible, we can use Readers and Writeers

Sources

• https://gitlab.com/monaco/posixeg/-/blob/master/exercises/shell/foosh.txt
• https://doc.rust-lang.org/std/process/index.html
• https://www.joshmcguigan.com/blog/build-your-own-shell-rust/
• https://www.gnu.org/software/bash/manual/html_node/Basic-Shell-Features.html#Basic-Shell-Features
• https://github.com/psinghal20/rush
• https://hyperpolyglot.org/unix-shells
• http://zsh.sourceforge.net/Doc/Release/Shell-Grammar.html
• https://github.com/Swoorup/mysh
• https://adriann.github.io/rust_parser.html

Built-in Commands

• cd <path> makes the directory 'path' the current directory
• exit terminates foosh
• quit same as exit
• fg [job-id] sends job identified by jobid to foreground. If jobid is not specified, defaults to job which sate has been most recently modified.
• bg [job-id] sends job identified by jobid to background. If jobid is not specified, defaults to job which sate has been most recently modified.
• jobs output a list of currently active jobs If a built-in command conflicts with the name of an external program, the built in command prevails --- unless the program path is explicitly given.
• echo

Basic features

• Command execution: built-ins and external
• Pipelining
• IO Redirection
• Background Execution / Job management
• Basic Scripting

More features

• Prompt customization
• Profiles / configs w/ variables
• History
• Expansion
• Proper parsing

Advanced features

No guarantee of implementing

• Scripting compatibility w/ bash
• Autocompletion
• Parsing (Lexer -> Parser)

Lexer

• The Lexical analyzer separates input into tokens.
• The lexer will read the input character by character and it will try to match the input with each token
• Regular expressions describe each token