fhtagn
fhtagn is a CLI tool for testing command-line programs.
If the test succeeds it exits silently with code 0.
But if the test fails, fhtagn will output a diff between the expected output and the actual output.

Syntax
$ command to be run
| success output
@ expected error
? expected exit code
Notes
The script uses /usr/bin/awk which is not where awk is located on NixOS.
The workaround is a script that calls
awk -f fhtagn.awk
since that will find the PATH version of awk.