Services and systemd

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

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

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

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

Virtual machines on Linux — managed with virsh, networks, storage, console.

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

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

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

nginx serves sites from disk and forwards requests to applications; it picks a site by Host, nginx -t checks the config.

PostgreSQL is a relational DBMS: roles and databases, pg_hba.conf access rules, pg_dump dumps and pg_restore restores.

Cache, queues, and sessions in RAM; KEYS * blocks the server on large databases — use --scan instead.

An NTP client and server: keeps the system clock in step with time servers and can serve time to your machines.

Collecting metrics with a pull model — targets, PromQL, exporters, alerts.

Visualizing metrics and logs — data sources, dashboards, variables, alerts.

An SMB server for Windows-compatible shares; the SMB password is separate from the system one, easy to forget to set.

Sending and receiving mail — config, the queue, logs, typical delivery debugging.

Running your own DNS — zones, records, config checks, resolving debug.

Handing out addresses on a network — pool, reservations, leases, diagnostics.

Load balancing and proxying TCP/HTTP — backends, health checks, statistics.

High availability through a virtual IP — failover between nodes using VRRP.