Disks and Services

Where did my space go?

Two commands answer 90% of disk questions:

df -h        # how full is each filesystem? (-h = human-readable)
du -sh *      # how big is each thing HERE? (-s = summary)

Check: df -h / shows a line with the size, used space, and a Use% column for your root filesystem.

When a disk fills up, drill down: du -sh * in the suspect directory, cd into the biggest one, repeat. The culprit is usually a log file that grew forever or a forgotten download folder.

du -sh ~/.cache/* | sort -h | tail -5   # five biggest things in your cache

Mounting

Linux doesn't have drive letters. Extra disks get mounted — attached at some directory:

lsblk                  # list block devices (disks and partitions)
mount | grep /myData   # where is /myData mounted?

You'll mostly encounter mounting when adding a USB drive or a second disk. The details (fstab entries, mount options) can wait — know that "a disk appears at a directory" is the model, and lsblk shows you the disks.

Services: what starts at boot

A service (or daemon) is a program that runs in the background providing something — networking, printing, a web server. On modern Linux, systemd manages them, and systemctl is how you talk to systemd:

systemctl status ssh          # is the SSH service running?
sudo systemctl start ssh      # start it now
sudo systemctl stop ssh       # stop it now
sudo systemctl restart ssh    # stop, then start (the classic fix)
sudo systemctl enable ssh     # start it automatically at boot
sudo systemctl disable ssh    # don't start it at boot

start vs. enable trips everyone up: start affects right now; enable affects next boot. You usually want both when setting something up permanently.

Check: systemctl is-active ssh prints active or inactive.

Reading logs

When a service misbehaves, its logs are the first place to look:

journalctl -u ssh --since "1 hour ago"   # this service's recent logs
journalctl -f                            # follow the whole log live (Ctrl+C to stop)
dmesg | tail -20                         # kernel messages (hardware, drivers)

journalctl is systemd's log viewer. Most "why won't this start" mysteries end here.

Next: Troubleshooting