behave

Record how any CLI behaves. Prove your rewrite behaves the same.

behave treats a command-line tool as a black box. It reads --help (and the man page), generates hundreds of test cases (every flag, flag pairs, edge-case stdin, file arguments, bad values, missing files), records what the tool actually does (stdout, stderr, exit code), and replays those cases against another implementation.

Use it when you are:

• porting a tool to Rust or Go and want proof that the port is compatible,
• refactoring or upgrading a CLI and want a regression net in a few seconds,
• comparing two implementations of the same tool.

Status: early (v0.1). CLI and MCP server work; feedback welcome.

Quick start

Download a binary for Linux or macOS from Releases, or:

go install github.com/emirmrkaljevic/behave/cmd/behave@latest

# 1. record how GNU sort behaves (writes ./behave-sort/)
behave probe sort

# 2. check that the recording is stable
behave run behave-sort

# 3. compare another implementation against it
behave diff behave-sort --impl ./target/release/sort --skip-tag meta

For tools that need a positional argument (a filter, a pattern, a field list), give samples:

behave probe jq --arg . --arg '.a' --arg '.[]' --arg 'map(.+1)' --arg keys
behave diff behave-jq --impl gojq --ignore-stderr

Example output (abridged):

✗ flag --output-delimiter, value -1, arg -f1  0135-flag-output-delimiter-value-1-arg-f1.yaml
  $ cut --output-delimiter -1 -f1
  exit:
    exit: want 0, got 1
  stdout:
    - hello

flags with differences:
  --output-delimiter             2/16  cases differ
flags fully compatible: 9 of 10

cut vs ./uu/cut: 172/175 passed, 3 failed, 0 skipped  (differs in: exit 2, stdout 3)  [0.1s]

What it found on day one

Run on this machine against real tools, generated cases only, --help/--version excluded:

* with --ignore-stderr; error message wording differs almost everywhere. † varies by one or two: some --random-sort cases are only sometimes detected as flaky with tiny inputs.

Many of these are deliberate design choices, not bugs. behave just makes them visible.

MCP server

behave mcp serves the same features over stdio, so a coding assistant can explore the original tool, grow the spec while it works, and check its port after every change.

{
  "mcpServers": {
    "behave": { "command": "behave", "args": ["mcp"] }
  }
}

Spec format

A spec is a directory you can commit and review:

behave-cut/
  behave.yaml           # command, timeout, normalizers
  cases/
    0001-help.yaml
    0135-flag-output-delimiter-value-1-arg-f1.yaml
    ...

name: flag --output-delimiter, value -1, arg -f1
args: [--output-delimiter, "-1", -f1]
stdin: |
  hello
tags: [flag-value, "flag:--output-delimiter"]
expect:
  exit: 0
  stdout: |
    hello
  stderr: ""

Cases can also set env, files (created in a fresh temp dir), and timeout_ms. Add your own with behave add behave-cut --stdin 'a,b' -- -d, -f2.

How it stays deterministic

• every case runs in a new temp directory with a fixed environment (LC_ALL=C.UTF-8, TZ=UTC, NO_COLOR=1, HOME=temp dir)
• each case is recorded 3 times; anything that changes between runs is marked flaky and skipped (e.g. sort --random-sort)
• normalizers remove noise before comparing: ansi, workdir, cmd (jq: vs gojq:), timestamp, hexaddr, duration, pid, plus your own regex rules in behave.yaml
• timeouts kill the whole process group

Commands

run and diff exit with 1 when any case fails, so they drop straight into CI.

Roadmap

• JUnit output, coverage of flags seen in help vs exercised
• smarter value generation from help text (WHEN can be: auto, always, never)

Safety

behave only runs the binaries you point it at, in a temp directory. Don't probe tools that change things outside their working directory (package managers, rm -rf-style tools) without --skip-flag and a sandbox.

Links

• MCP Registry name: io.github.emirmrkaljevic/behave

License

MIT