systemd

Service

What a service is, how it differs from an ordinary program, and how to manage it.

journalctl — reading the systemd journal

Reading the systemd journal (journald) — filters by service, time, priority.

systemd

The init system and service manager (PID 1): units, targets, dependencies, journals.

System boot and GRUB

How Linux boots: firmware, the GRUB bootloader, the kernel, systemd targets and emergency mode.

OOM killer: when memory runs out

A kernel mechanism: when memory runs out on the machine or in a cgroup with a limit, it kills a process with SIGKILL.

Boot targets: default, multi-user, rescue and emergency

A target is a group of units systemd brings the boot up to; rescue and emergency are the targets for repairs.

systemd-analyze: what slows the boot and what is wrong in a unit

Breaks the boot down by time and by the chain of unit dependencies, and checks a unit file before the service runs.

systemctl — managing systemd services

The main interface to systemd. enable turns on autostart, start runs it now — these are different operations.

Cron — the task scheduler

Running tasks on a schedule: crontab syntax, classic cron and modern systemd timers.

ulimit and process limits: open files, processes, core

Process resource limits — open files, processes, core: ulimit, /proc/<pid>/limits and a service LimitNOFILE.

Service hardening: User=, ProtectSystem, NoNewPrivileges

Unit settings that strip a service of root, system writes and new privileges; systemd-analyze security rates them.

systemd timers — scheduled jobs

A systemd timer is a .timer unit that starts the service of the same name on a schedule; its output goes to the journal.

timedatectl — time, time zone, and synchronization

Manages system time and NTP; set-time requires turning off set-ntp first, or the time reverts.

hostnamectl — host name and machine info

View and change the host name and machine info; /etc/hostname and /etc/hosts.