Master Linux Service Management: Essential Skills Every Sysadmin Needs

Linux Service Management Essentials

Introduction

In Linux, when you install a package that provides a daemon, the system adds a systemd script but does not enable the service automatically, so you must manually start or stop services.

Linux has three well‑known init systems that are still widely used.

Runlevels

Based on Linux/Unix, the init process (PID 1) is the first process started by the kernel. It reads /etc/inittab to determine the runlevel and then launches other processes accordingly. The traditional runlevels are:

0 – Poweroff

1 – Single‑user mode

2 – Multi‑user mode (no NFS)

3 – Full multi‑user mode

4 – Unused

5 – Graphical mode

6 – Reboot

Init Systems

The three most common init systems in Linux are:

System V (SysV)

Upstart

systemd

System V (SysV)

System V is the original Unix init system. The init process is the first program started by the kernel and becomes the parent of all other processes. Most Linux distributions originally used SysV, but newer init systems like systemd have largely replaced it.

Upstart

Upstart is an event‑driven replacement for /sbin/init. It was designed for Ubuntu and can also be used on other Linux distributions to replace the older SysV init. Ubuntu 9.10 through 14.10 and RHEL 6 used Upstart before being superseded by systemd.

systemd

systemd

is a modern init system and service manager used by most major Linux distributions, replacing the traditional SysV init. It is compatible with SysV and LSB scripts, runs as PID 1, and manages services, sockets, devices, mounts, and more. systemd uses .service unit files instead of traditional bash scripts. All daemon processes are placed into cgroups, which can be inspected under /cgroup/systemd.

service command

The service command manages services on many Linux distributions (e.g., Red Hat, Fedora, Mandriva, CentOS).

service --status-all
service vsftpd status
service vsftpd stop
service network restart

chkconfig

chkconfig

is a command‑line tool that enables or disables services for specific runlevels and lists all services with their current settings. It requires root or sudo privileges.

chkconfig --level 35 mysqld on
chkconfig mysqld on
chkconfig --list

systemctl

Concept

systemd

units represent different objects such as services, sockets, and snapshots. Unit files are stored in several directories: /usr/lib/systemd/system/ – Main service unit files (similar to old /etc/init.d) /run/systemd/system/ – Runtime‑generated units with higher priority /etc/systemd/system/ – Administrator‑created symlinks that override lower‑priority units

Unit Types

service

– Files ending with .service define system services target – Files ending with .target emulate runlevels device – Represent kernel‑detected devices mount – Define filesystem mount points socket – Identify inter‑process communication sockets, enabling on‑demand service activation snapshot – Manage system snapshots swap – Identify swap devices automount – Automatic mount points path – Monitor files or directories for changes

You can list units of a specific type with systemctl -t <type>, e.g., systemctl -t service.

Common Commands

systemctl enable httpd.service
systemctl disable httpd.service
systemctl status httpd.service
systemctl start httpd.service
systemctl stop httpd.service
systemctl restart httpd.service
systemctl list-units --type=service
systemctl list-unit-files --type=service
systemctl list-dependencies httpd.service
systemctl mask httpd.service
systemctl unmask httpd.service
systemctl isolate multi-user.target
systemctl isolate graphical.target
systemctl poweroff
systemctl reboot
systemctl suspend
systemctl hibernate
systemctl rescue
systemctl emergency

Service Status Codes

active (running)

– Service is currently running active (exited) – Service finished its one‑time task active (waiting) – Service is waiting for an event inactive – Service is not running dead – Service process has terminated

Enablement States

enabled

– Starts automatically at boot disabled – Does not start at boot static – Cannot be enabled directly but may be pulled in by other units masked – Completely blocked from starting