Linux

Logs, packages, disk space and services on Linux servers and desktops.

20 Articles

Built-in tools

UFW (Uncomplicated Firewall)How UFW manages the Linux firewall on Ubuntu, how to allow SSH before enabling it, and how to list and delete rules.

apt autoremoveWhat apt autoremove removes, why packages become 'no longer required', and how to check the list before removing anything.
df (disk free)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.
Disk usage with du and ncduHow to find what fills a Linux disk with du and the interactive ncdu, with options that stay on one file system.
File permissions: chmod and chownHow Linux read, write and execute permissions work, what 755 and 644 mean, and how to change owner and mode safely.
Finding large and old files with findHow to use find to list big or old files on Linux, preview them safely and delete only what you checked.
journalctl and the systemd journalHow to read logs with journalctl, check how much space the journal uses and shrink it safely with --vacuum options.
lsblk and block devicesHow to list disks and partitions with lsblk, read sizes, file systems and mount points, and identify a new disk safely.
rsyncHow rsync copies and synchronises files, what the trailing slash changes, and how to test with --dry-run before --delete.

Inodes and 'No space left on device'What an inode is, how to see inode usage with df -i, and how to find directories with millions of small files.
Load averageWhat the three load average numbers mean, how they relate to CPU cores, and when a high load is a problem.
OOM killerWhat the Linux out-of-memory killer does, how to find out which process it killed, and how to make it less likely.
SSH keysHow SSH key authentication works, how to create a key with ssh-keygen, install it on a server and protect it.
Swap space on LinuxWhat swap is, how to see it with free and swapon, how to add a swap file, and what vm.swappiness changes.

/etc/fstabWhat each field of /etc/fstab means, why UUIDs are used, and how to test a new entry before rebooting.

cron and crontabHow cron runs scheduled jobs, the five crontab time fields, where system cron files live and how systemd timers compare.
logrotateWhat logrotate does, how its configuration in /etc/logrotate.conf and /etc/logrotate.d works, and how to test a rule safely.
systemd units and systemctlWhat a systemd unit is, the common unit types, and how to start, stop, enable and inspect services with systemctl.

/tmp and temporary filesWhat /tmp and /var/tmp are for, when Linux cleans them, and how systemd-tmpfiles controls their age limits.
APT package cache (apt clean and autoclean)Where APT keeps downloaded .deb files, how apt clean, autoclean and autoremove differ, and what is safe to remove on Debian and Ubuntu.