lsh — a minimal Bash-like shell

lsh is a compact POSIX shell written in pure C with no external dependencies beyond the standard C library and POSIX APIs available on typical Linux/macOS/gcc environments (including Coderpad).

Build

make

Run

./lsh              # interactive mode
./lsh script.sh    # run a script file
./lsh -c 'echo hi' # run a single command

Test

make test

Features

Implemented

• Command execution — external programs via fork/execvp
• Builtins — cd, pwd, echo, exit, export, unset, true, false, :, type, kill, history, source/., jobs, fg, bg
• Pipelines — cmd1 | cmd2 | ...
• Redirections — <, >, >>, 2>, 2>>, &>
• Variables — NAME=value prefixes, $VAR, ${VAR}, export, unset
• Special parameters — $? (last exit status), $$/$PPID, $! (last background PID)
• Logical operators — &&, ||, ;
• Background jobs — cmd &, job table, jobs/fg/bg
• Quoting — single quotes, double quotes, backslash escapes
• Comments — # ...
• Globbing — * and ? in unquoted words
• Tilde expansion — ~ and ~/path
• History — up/down arrow recall in interactive mode
• Full-screen programs — less, vim, and similar TUI apps receive the controlling terminal
• Emacs line editing (interactive):
  • Ctrl-A / Ctrl-E — beginning/end of line
  • Ctrl-B / Ctrl-F — backward/forward character
  • Option-Left / Option-Right (or Alt-b / Alt-f) — backward/forward word
  • Ctrl-P / Ctrl-N — previous/next history (also arrow keys)
  • Ctrl-K — kill to end of line
  • Ctrl-U — kill whole line
  • Ctrl-W — kill word backward
  • Backspace / Ctrl-D — delete char / EOF
  • Tab — complete commands (first word or after |) or file paths
• Signal handling — Ctrl-C forwards to foreground process group
• Scripts — lsh file.sh and source file

Unimplemented features

These are intentionally omitted or only partially supported:

• Full Bash compatibility — syntax and semantics differ in many edge cases
• Subshells — ( commands )
• Command substitution — $(...) and backticks
• Arithmetic expansion — $(( ... ))
• Brace expansion — {a,b,c}
• Process substitution — <(...) and >(...)
• Here documents — <<EOF
• Aliases — alias/unalias
• Programmable completion — complete builtin, context-specific completion
• Advanced globbing — [...] character classes, **, extended globs
• Job control signals — full Ctrl-Z / tty stop-start parity with Bash
• Coprocesses, select, case statements, for/while/until loops
• Functions — name() { ... }
• Local variables, readonly, declare, set -o
• Trap builtin and user-defined signal handlers
• Path hashing, command -v caching
• Locale/multibyte — ASCII-oriented input editing
• Restricted mode, shopt, getopts
• Word splitting on $IFS for all contexts (only basic splitting)
• eval, exec, wait, times, ulimit
• Regex matching — =~
• Array variables
• Prompt escape sequences — \w, \h, etc. in PS1 (literal prompt string only)

Contributions welcome for any of the above.

Project layout

include/lsh/          Public headers (installed-style API)
  shell.h             Core types and shell state
  ast.h lexer.h parser.h
  execute.h expand.h jobs.h
  builtin.h vars.h signals.h
  input.h complete.h

src/
  core/               Process lifecycle and signals
    main.c            Entry point, REPL, script driver
    shell.c           Shell init/teardown
    signals.c         Terminal and signal handling
  parse/              Lexing and parsing
    lexer.c parser.c ast.c
  runtime/            Execution engine
    execute.c         Pipelines, redirects, fork/exec
    expand.c          Variables, globs, tilde
    jobs.c            Background job management
  builtins/           Shell builtin commands
    builtin.c
  env/                Variable and environment table
    vars.c
  frontend/           Interactive UI
    input.c           Line editor and history
    complete.c        Tab completion

build/obj/            Compiled object files (mirrors src/)
tests/
  run_tests.sh        Integration test suite

License

MIT