'python: command not found' (but python3 works) or 'pip: command not found' on Linux and macOS. Which command should I use, and how do I fix it properly?

Cause: many current Linux distributions and macOS ship Python 3 only as python3 and pip3; the bare python command either doesn't exist or (on old systems) points to Python 2. pip may not be installed at all for the system Python, or its scripts directory isn't on PATH (for user installs it's ~/.local/bin on Linux, or the user base bin folder on macOS). Scripts and tutorials written for 'python' and 'pip' then fail. Fix (best first): 1. Use python3, and run pip through the interpreter: python3 -m pip install ... . The Python Packaging User Guide recommends python3 -m pip because it guarantees pip installs into the Python you're running. 2. Better for projects: create a virtual environment (python3 -m venv .venv, then activate it). Inside an activated venv, python and pip both exist and point to the right interpreter. 3. If pip is missing for the system Python, install it from your OS (Debian/Ubuntu: sudo apt install python3-pip python3-venv) or bootstrap it with python3 -m ensurepip --upgrade where available. On Debian/Ubuntu, global pip installs into the system Python are blocked by the externally-managed-environment rule; use a venv or pipx. 4. If you want a bare 'python' command: on Debian/Ubuntu the python-is-python3 package provides it; otherwise use a shell alias, or a Python version manager (pyenv, uv) that provides python and pip shims. 5. If a command installed with pip --user isn't found, add the user scripts directory to PATH (python3 -m site --user-base shows the base; its bin folder holds the scripts). Verify: python3 --version and python3 -m pip --version work; inside your venv, which python and which pip point into .venv. Sources: https://docs.python.org/3/using/unix.html https://packaging.python.org/en/latest/tutorials/installing-packages/