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System

Notes target Debian and Ubuntu. Other distributions differ, mainly in the package manager — see Package management.

Version and hardware​

lsb_release -a          # distribution and release
hostnamectl | grep Kernel
uname -mrs              # kernel name, release, machine
nproc                  # number of available cores
egrep '(vmx|svm)' /proc/cpuinfo   # CPU supports hardware virtualization

Ubuntu 22.04 notable changes​

  • Wayland is the default display server for most users.
  • Linux kernel 5.15, GNOME 42.
  • PHP 8.1, Ruby 3.0, Python 3.10, OpenJDK 18.
  • nftables is the default firewall backend, replacing iptables.
  • Firefox ships only as a Snap package.
  • RDP replaces VNC as the default remote desktop protocol.
  • scp now supports the SFTP transfer mode.
  • BIND 9.18 supports DNS over TLS/HTTPS and zone transfers over TLS.

Reboot and shutdown​

sudo reboot -h 0       # reboot immediately
poweroff
halt

systemd services​

# Status and logs
systemctl status SERVICE
journalctl -u SERVICE -b        # this boot
journalctl -u app.service -b

# Start a templated service with an argument
systemctl start 'autontfsfix@arg1 arg2.service'

# Enable / disable
systemctl enable  SERVICE
systemctl disable SERVICE

# Remove a service completely
systemctl stop SERVICE
systemctl disable SERVICE
rm /etc/systemd/system/SERVICE
rm /usr/lib/systemd/system/SERVICE
systemctl daemon-reload
systemctl reset-failed

systemd-oomd can kill a user session and drop you to the login screen. To disable it:

sudo systemctl disable --now systemd-oomd
sudo systemctl mask systemd-oomd
# Re-enable
sudo systemctl unmask systemd-oomd
systemctl enable systemd-oomd

Environment variables​

VAR1="var1"            # local: child shells cannot see it
export VAR2="var2"     # exported: inherited by child processes
export                 # list exported variables in this session

Timezone​

# Debian / Ubuntu: reconfigure tzdata rather than editing files by hand
sudo dpkg-reconfigure tzdata
date                   # verify

Disable the screen lock​

gsettings set org.gnome.desktop.lockdown disable-lock-screen true

Logs​

/var/log/auth.log                       # authentication / login
tail -f /var/log/syslog | grep CRON     # follow cron activity

screen​

A terminal multiplexer that keeps sessions alive after disconnect.

screen -version
screen -S SESSION_NAME     # start a named session
echo $STY                  # non-empty when inside screen
screen -ls                 # list sessions
screen -r                  # reattach (single session)
screen -r 1164             # reattach by id
screen -x pts-1.localhost  # attach to an already-attached session
# Detach: Ctrl-A then D
sudo screen -X -S 1164 quit    # kill a session

# Start a detached session, e.g. from crontab
/usr/bin/screen -dmS nomp /bin/bash -c 'nvm use v0.10.48; cd ~/nomp; node init.js; exec /bin/bash'

crontab​

Changes take effect without a restart. Each user (including root) has a separate crontab.

crontab -e             # edit the current user's crontab
sudo crontab -e        # edit root's crontab
# ┌───────────── minute       (0 - 59)
# │ ┌─────────── hour         (0 - 23)
# │ │ ┌───────── day of month (1 - 31)
# │ │ │ ┌─────── month        (1 - 12)
# │ │ │ │ ┌───── day of week  (0 - 7, 0 is Sunday, 6 is Saturday, 7 is also Sunday)
# │ │ │ │ │
# * * * * * /path/to/command
*/10 * * * * /bin/do                              # every 10 minutes
0 3 * * 1,3,5 truncate -s 0 /root/.forever/*.log  # 03:00 on Mon/Wed/Fri
0 0 1 */2 * /usr/bin/certbot renew --quiet --no-self-upgrade --nginx

Special shorthands:

ShorthandEquivalent
@rebootOnce after every reboot
@yearly / @annually0 0 1 1 *
@monthly0 0 1 * *
@weekly0 0 * * 0
@daily0 0 * * *
@hourly0 * * * *

Benchmark​

time ./script.sh       # wall-clock time of a command

# Geekbench
mkdir geekbench && cd geekbench
wget https://cdn.geekbench.com/Geekbench-5.4.4-Linux.tar.gz
tar -xzvf Geekbench-5.4.4-Linux.tar.gz
cd Geekbench-5.4.4-Linux
./geekbench5