Linux · zero to on-call
The machine under everything.
Processes, files, permissions, systemd and networking, on a server that misbehaves on purpose. Free, in your browser, nothing to install.
Learning Linux?
Start from zero, step by step.
Levels in order, each with a hands-on incident to fix in your browser.
Something broken right now?
Paste the error, get the fix.
The cause, the commands to confirm it and the fix, for every common Linux error.
Everything you need, from the first command to the 3 a.m. incident.
Fix a broken server. Right here.
60 real incidents, real commands, the errors Linux really prints.
Your path
Progress is saved in this browser only. Tick a level when you have done its lab.
Quiz
Scenario-based questions for every level. You see right or wrong as soon as you answer, with the reasoning, which is where the learning is.
Unit file X-ray
Paste a systemd unit file, a drop-in, a timer or a crontab. Every line is explained, risky lines are flagged, and missing settings are listed.
Stuck on an error?
Paste the error you see. Each answer explains why it happens, how to confirm it and how to fix and prevent it, then gives you a safe scenario to break and fix it yourself, so you recognise it next time. Every answer has its own link.
A plain copy of this reference, readable without JavaScript: problems.html.
Cheat sheet
The commands people actually use. Try runs one in the playground, opening the scenario it needs. Lab marks the ones that need a real Linux machine.
Linux internals
From your shell down to the disk: what each layer does, how it fails, and the commands that show it. Point at a part, or tab to it, to read about it.