A zsh-compatible shell written in Rust. Frost aims for behavioral parity with zsh 5.9, validated against the vendored zsh test suite.

Alpha — core execution works, expanding feature coverage toward full zsh compatibility.

• 289 unit tests passing
• 53+ built-in commands
• 113 shell options
• 33/70 zsh compatibility tests passing
• 11 crates in the workspace

• Full compound command support — if/elif/else/fi, for/while/until, case, select, C-style for (( )), repeat, try-always
• Parameter expansion — $var, ${var:-default}, ${var##pattern}, ${var/old/new}, ${var:offset:len}, ${#var}, array subscripts ${arr[n]}
• Brace expansion — {a,b,c}, {1..10}, {a..z}, step values, zero-padding
• Command substitution — $(command), arithmetic $((expr))
• Pipelines — cmd1 | cmd2 |& cmd3, $pipestatus array, PIPE_FAIL
• Redirections — <, >, >>, &>, &>>, <>, <<<, <<, fd duplication
• Arrays — indexed (1-based) and associative, subscript assignment arr[n]=val
• Shell options — 113 options with setopt/unsetopt, wired to behavior
• Conditionals — [[ ]] with file/string/integer/regex tests, -o option checks
• 53+ builtins — echo, cd, typeset, printf, eval, source, trap, read, export, kill, getopts, pushd/popd, umask, and more
• Trap infrastructure — signal handlers, pseudo-signals (EXIT, DEBUG, ERR, ZERR)
• Job control — job table, jobs, fg, bg, wait, disown (stubs)

cargo build --release
cargo test --workspace --lib          # unit tests
cargo test --test ztst_bridge         # zsh compatibility tests

nix build                             # build frost
nix run                               # launch frost shell
nix run .#test                        # cargo test
nix run .#clippy                      # cargo clippy
nix run .#fmt                         # cargo fmt --check
nix run .#ci                          # all checks
nix run .#compat                      # zsh compatibility suite
nix flake check                       # pure nix-sandbox checks

# Interactive shell
frost

# Run a command
frost -c 'echo hello world'

# Run a script
frost script.zsh

Frost is organized as a Cargo workspace with 11 crates:

frost-lexer     → Token stream
frost-parser    → AST (Program, Command, Word, ParamExpansion, ...)
frost-expand    → Word expansion (tilde, param, brace, glob, cmd sub)
frost-exec      → Execution engine (fork/exec, pipes, env, traps, jobs)
frost-builtins  → 53+ built-in commands
frost-options   → 113 shell options
frost-glob      → Glob matching (planned)
frost-zle       → Line editor (planned)
frost-complete  → Tab completion (planned)
frost-compat    → Zsh test runner
frost            → Binary entry point

See DECISIONS.md for detailed technical decisions.

Frost validates against the vendored zsh 5.9 test suite (Test/*.ztst). Current progress:

MIT