meiksh (pronounced /maiksh/) is a new Unix shell written in Rust with no dependencies beyond std and libc. It is the only shell that fully implements the POSIX shell specification.

meiksh passes 100% of its in-house POSIX conformance test suite (tests/matrix/), which covers the Shell Command Language (XCU Chapter 2), all required builtins, and the interactive line-editing and job-control features specified by POSIX.

For comparison, every other major shell has verified non-compliances against the same test suite:

Note: meiksh has not been officially certified by The Open Group. The compliance claim is based solely on the project's own test suites, which are written directly from the POSIX.1-2024 (Issue 8) specification text.

When started interactively, meiksh sources the following files, in this order, skipping any that do not exist:

1. /etc/profile
2. $HOME/.profile
3. $ENV — its value is subjected to parameter expansion and sourced only if the result is an absolute path (POSIX requirement for interactive shells).

Non-interactive shells (including sh script.sh and sh -c '...') source none of these files, so scripts always run in a predictable, user-independent environment.

Before any file is sourced, meiksh exports the shell variable MEIKSH_VERSION (set to the crate's SemVer string, e.g. 0.1.0), following the convention established by BASH_VERSION, ZSH_VERSION, and KSH_VERSION. Unlike those shells, meiksh exports the marker so that /etc/profile and ~/.profile — which are commonly shared across shells — can detect meiksh reliably. Portable startup scripts can branch on shell identity with the standard idiom:

if [ -n "${MEIKSH_VERSION:-}" ]; then
    # meiksh-specific setup
    set -o bash_prompts  # opt into bash-style prompt escapes (\u, \h, \w, \D{…}, …)
    PS1="\u@\h:\w \\$ " # classic bash-style prompt: user@host:cwd $
fi

(emacs editing mode is on by default, matching bash, so there is no need to enable it from ~/.profile.)

Setuid/setgid shells skip all three files (the same conservative guard POSIX requires for $ENV, extended to the profile files) but still export the MEIKSH_VERSION marker.

See docs/features/startup-files.md for the full normative specification.

• fully implement the POSIX shell specification (Issue 8)
• keep the implementation limited to std and libc
• use explicit, auditable Unix bindings instead of external abstraction crates
• maintain at least 99.5% production-code line coverage as reported by ./scripts/coverage.sh

The local docs/posix/ mirror defined by docs/posix-manifest.txt is the only requirements source of truth used for conformance work.

• src/main.rs, src/lib.rs: CLI entry point and crate root
• src/bstr.rs: byte-string utilities for handling arbitrary (non-UTF-8) shell data
• src/shell/: shell state, option parsing, top-level execution flow, and job table
• src/syntax/: tokenizer, parser, AST, alias handling, and here-doc collection
• src/expand/: parameter, command, arithmetic, field-splitting, and pathname expansion
• src/exec/: command execution, pipelines, redirections, and compound-command runtime
• src/builtin/: builtin dispatch and builtin implementations
• src/interactive/: prompt loop, startup-file sourcing (/etc/profile, ~/.profile, $ENV), vi-/emacs-mode line editing, inputrc, and history
• src/sys/: handwritten Unix FFI behind a mockable SystemInterface (fd/process/fs/locale/env helpers)
• docs/: policy, traceability, and local POSIX reference instructions
• tests/matrix/: .md-driven POSIX conformance suites, expect_pty driver, and per-shell compliance reports
• tests/integration/, tests/perf/: integration and performance tests
• scripts/: repository automation (coverage, benchmarks, POSIX-obligation extraction)

The implementation is driven by local POSIX reference material under docs/posix/, which is intentionally not committed for copyright reasons. The required local mirror is defined by docs/posix-manifest.txt, and docs/fetch-posix-docs.sh populates it from the upstream archive. See docs/README.md for the workflow and expected local layout.

Run production-only coverage with:

./scripts/coverage.sh

This prints the production-code coverage summary on stdout and writes detailed artifacts under target/coverage/.