From 47704f9952c61519067ac49e081337e5000d1824 Mon Sep 17 00:00:00 2001
From: Michele Cereda <cereda.michele@gmail.com>
Date: Thu, 12 May 2022 23:19:21 +0200
Subject: [PATCH] Added systemd's notes to the knowledge base

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 knowledge base/systemd.md | 197 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 197 insertions(+)
 create mode 100644 knowledge base/systemd.md

diff --git a/knowledge base/systemd.md b/knowledge base/systemd.md
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+# Systemd
+
+## TL;DR
+
+```shell
+# List all available units.
+systemctl list-unit-files
+
+# List failed units only.
+systemctl list-units --state=failed
+
+# Start services.
+sudo systemctl start adb.service
+systemctl --user start keybase.service
+
+# Restart services.
+sudo systemctl restart bluetooth.service
+
+# Stop services.
+sudo systemctl stop cups.service
+
+# Enable services on boot.
+sudo systemctl enable sshd.service
+sudo systemctl enable --now docker.service
+systemctl --user enable --now davmail.service
+
+# Disable services from boot.
+sudo systemctl disable clamav-freshclam.service
+sudo systemctl disable --now gdm.service
+systemctl --user disable --now davmail.service
+
+# Show log entries.
+journalctl
+journalctl -f
+journalctl -n 20
+journalctl -o json-pretty
+journalctl --no-pager
+journalctl --utc
+
+# Show what boots the system has logs about.
+journalctl --list-boots
+
+# Display logs from specific boots only.
+# Persistent logging needs to be enabled.
+journalctl -b
+journalctl -b -3
+
+# Display logs in a specific time window
+journalctl --since yesterday
+journalctl --since "2015-01-10 17:15:00"
+journalctl --since 09:00 --until "1 hour ago"
+journalctl --since "2015-01-10" --until "2015-01-11 03:00"
+
+# Filter logs by unit.
+journalctl -u nginx.service
+journalctl -u nginx.service -u php-fpm.service --since today
+
+# Filter logs by process, user id or group id.
+journalctl _PID=8088
+journalctl _UID=33 --since today
+journalctl -F _GID
+
+# Filter logs by path.
+journalctl /usr/bin/bash
+
+# Display kernel logs only.
+# Works like `dmesg`.
+journalctl -k
+journalctl -k -b -5
+
+# Filter logs by priority.
+journalctl -p err -b
+
+# Truncate the output.
+journalctl --no-full
+
+# Print everything.
+journalctl -a
+
+# Show current logs disk usage.
+journalctl --disk-usage
+
+# Delete old logs.
+sudo journalctl --vacuum-size=1G
+sudo journalctl --vacuum-time=1years
+
+# List available timezones.
+timedatectl list-timezones
+
+# Set timezones.
+sudo timedatectl set-timezone UTC
+sudo timedatectl set-timezone Europe/Dublin
+
+# Set the time.
+sudo timedatectl set-time 15:58:30
+sudo timedatectl set-time '2015-11-20 16:14:50'
+
+# Set the hardware clock to UTC.
+timedatectl set-local-rtc 0
+
+# Set the hardware clock to local timezone.
+timedatectl set-local-rtc 1
+
+# Set automatic time sync.
+sudo timedatectl set-ntp true
+sudo timedatectl set-ntp false
+
+# Check the time and timezones state.
+timedatectl status
+
+# Show the current hostname state.
+hostnamectl
+hostnamectl --pretty status
+hostnamectl --static status
+
+# Set hostnames.
+hostnamectl set-hostname staticky --static
+hostnamectl set-hostname prettiky --pretty
+```
+
+## User services
+
+User's service files should be placed into `~/.config/systemd/user`:
+
+```shell
+cat > "${HOME}/.config/systemd/user/davmail.service" <<EOF
+[Unit]
+Description=Davmail
+
+[Service]
+ExecStart=/usr/bin/davmail -notray
+
+[Install]
+WantedBy=default.target
+EOF
+```
+
+and can be acted upon as normal using `systemctl`'s `--user` switch
+
+```sh
+systemctl --user enable --now davmail.service
+systemctl --user status davmail.service
+```
+
+## Keep past boots record (persistent logging)
+
+Edit the journal configuration file and set the following option:
+
+```ini
+# file /etc/systemd/journald.conf
+[Journal]
+Storage=persistent
+```
+
+## Resolved
+
+### Disable systemd-resolved
+
+1. disable and stop the systemd-resolved service:
+
+   ```shell
+   sudo systemctl disable --now systemd-resolved.service
+   ```
+
+1. set NetworkManager to use the default DNS resolution.
+
+   ```ini
+   # file /etc/NetworkManager/NetworkManager.conf
+   [main]
+   dns=default
+   ```
+
+1. delete `/etc/resolv.conf`:
+
+   ```shell
+   sudo unlink /etc/resolv.conf
+   ```
+
+1. restart NetworkManager
+
+   ```shell
+   sudo service network-manager restart
+   ```
+
+## Sources
+
+- [How to disable systemd-resolved in Ubuntu]
+- [What are the systemctl options to list all failed units?]
+- [How To Use Journalctl to View and Manipulate Systemd Logs]
+- [How to Set Time, Timezone and Synchronize System Clock Using timedatectl Command]
+- [How to Set Hostname Using Hostnamectl Command?]
+
+[how to disable systemd-resolved in ubuntu]: https://askubuntu.com/questions/907246/how-to-disable-systemd-resolved-in-ubuntu
+[how to set hostname using hostnamectl command?]: https://linuxhint.com/set-hostname-using-hostnamectl-command/
+[how to set time, timezone and synchronize system clock using timedatectl command]: https://www.tecmint.com/set-time-timezone-and-synchronize-time-using-timedatectl-command/
+[how to use journalctl to view and manipulate systemd logs]: https://www.digitalocean.com/community/tutorials/how-to-use-journalctl-to-view-and-manipulate-systemd-logs
+[what are the systemctl options to list all failed units?]: https://unix.stackexchange.com/questions/341060/what-are-the-systemctl-options-to-list-all-failed-units/341061#341061