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When a Linux server breaks, the answer is usually already written down. The problem is that /var/log holds two completely different kinds of thing, and each one needs its own command.
Most of what is in there is plain text. A program appends a line to a file, and tail -f follows it live. But sitting in the same directory is the journal, and that is not a file you open. It is a structured, indexed store that you query.
That difference is why journalctl can do something tail cannot. Every entry the journal stores carries fields the program never typed, attached by journald itself, so you can filter by which program wrote the line, which boot it came from, and how bad it was.
Instead of searching text for the word “error”, you ask for the level:
journalctl -p err -b
That returns this boot’s errors AND everything more important, whatever wording the program happened to use.
And one thing catches people out. tail -f follows a file HANDLE, not a name. When the log is rotated the file is renamed out from under it, so tail keeps happily reading a file nothing is writing to any more, and your terminal simply goes quiet.
where Linux logs actually live, why /var/log holds two different shapes of log, how to read the journal with journalctl, how the priority levels work, and the tail -f gotcha nobody warns you about.
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Keywords :
how to read linux logs, journalctl, journalctl tutorial, var log, /var/log explained, linux logging, systemd journal, journalctl -p err, tail -f, tail command linux, linux troubleshooting, sysadmin basics, linux for beginners, devops tutorial, linux terminal, log files linux, journalctl vs tail, linux command line
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This one gets everybody at least once. tail -f follows the file HANDLE, not the name.
Rotate the log and your terminal goes silent while the service is still writing. Use
tail –follow=name if you want it to survive a rotation, or just use journalctl -f.
What is the first thing you check when a box misbehaves, journalctl or /var/log?