Logs, packages, disk space and services on Linux servers and desktops.
Where APT keeps downloaded .deb files, how apt clean, autoclean and autoremove differ, and what is safe to remove on Debian and Ubuntu.
How cron runs scheduled jobs, the five crontab time fields, where system cron files live and how systemd timers compare.
How to read df output, why Use% can reach 100% before Avail is zero for normal users, and how to check inodes with df -i.
How to find what fills a Linux disk with du and the interactive ncdu, with options that stay on one file system.
How to use find to list big or old files on Linux, preview them safely and delete only what you checked.
What an inode is, how to see inode usage with df -i, and how to find directories with millions of small files.
How to read logs with journalctl, check how much space the journal uses and shrink it safely with --vacuum options.
What logrotate does, how its configuration in /etc/logrotate.conf and /etc/logrotate.d works, and how to test a rule safely.
What swap is, how to see it with free and swapon, how to add a swap file, and what vm.swappiness changes.
What a systemd unit is, the common unit types, and how to start, stop, enable and inspect services with systemctl.