CI "ruff: command not found" - Linter Not Installed

The job called ruff but no such executable is on PATH. Either the linter was never installed in this environment, or it was installed somewhere PATH does not include.

What this error means

A lint step fails instantly with ruff: command not found (or black/mypy). The tool may be in a venv or ~/.local/bin that the step never put on PATH, or it was simply not installed.

CI log

Run ruff check .
/home/runner/work/_temp/script.sh: line 1: ruff: command not found
Error: Process completed with exit code 127.

Common causes

The linter is not installed in this environment

A dependency group that includes ruff was not installed, so the executable does not exist for this job.

Installed but not on PATH

ruff was installed into a venv or ~/.local/bin that the current step does not have on PATH, so the bare command is unresolved.

How to fix it

Install the linter (or invoke as a module)

Install ruff and run it; calling it via python -m sidesteps PATH issues.

Terminal

pip install ruff
ruff check .
# PATH-independent:
python -m ruff check .

Put the script directory on PATH

If installed into a venv/user location, add that bin to the job PATH.

.github/workflows/ci.yml

- run: |
    pip install --user ruff
    echo "$HOME/.local/bin" >> "$GITHUB_PATH"
- run: ruff check .

How to prevent it

Pin and install linters in the CI dependency set (e.g. a [lint] extra).
Prefer python -m ruff to avoid PATH problems.
Use pre-commit or the official ruff action so the tool is provisioned consistently.