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 cases (every flag, flag pairs, listed values, edge case stdin, missing files, bad values), records stdout, stderr and exit code, and replays the cases against another implementation.

brew install emirmrkaljevic/tap/behave
behave probe sort
behave run behave-sort
behave diff behave-sort --impl ./target/release/sort

Guides

• Test a Rust or Go rewrite of a CLI against the original
• Golden tests for CLI tools without writing them by hand
• Case study: what behave found in uutils coreutils

Source, releases and the MCP server: github.com/emirmrkaljevic/behave