Configuring Unit Files in Linux

Linux Introduction

Chapter 1. Linux Basics

What Is OS?

CUI and GUI

Linux Distributions

Package Manager

Kernel and Shell

Current Working Directory

Linux Directory Structure

Absolute Path and Relative Path (Linux OS)

Linux Command Syntax

Special Characters and Escape Character

Chapter 2. Linux Key Commands

Setting Up Linux Environment on AWS

pwd (Print Working Directory)

cd (Change Directory)

ls (List Contents of Directory)

mkdir (Make Directory)

rmdir (Remove Directory)

touch (Create File)

rm (Remove File)

mv (Move File and Directory)

cp (Copy File and Directory)

cat (Display File Content)

sort (Sort File Contents)

grep (Global Regular Expression Print)

Regular Expression

find (Find File and Directory)

Wildcard

ln (Create Link to File and Directory)

Chapter 3. Vim Editor

What Is Vim and How to Launch It?

Normal, Insert and Visual Mode

Normal Mode (1) – Move Cursor

Normal Mode (2) – Delete

Normal Mode (3) – Copy and Paste

Normal Mode (4) – Undo and Redo

Normal Mode (5) – Search Phrase

Normal Mode (6) – Replace Phrase

Normal Mode (7) – Save and Exit

Insert Mode

Visual Mode

Chapter 5. Redirection, Pipe and Shell Script

Standard Input Output and Redirection

Pipe (Combine Commands)

less (Display Content with Pager)

tr (Replace Characters)

cut (Extract Data Sections)

uniq (Extract Unique Data Lines)

Shell Script

echo (Echo input)

read (Read and Store Input)

Shell Variable and Environmental Variable

source (Execute Shell Script and Refresh Environmental Variables)

Chapter 6. Commands for Command Management

history (Check Command History)

alias (Create Command Shortcuts)

man (Display Manual)

type, which and whereis (Display Command Information)

Package Manager Command

tree (Display Directory Tree)

Chapter 7. SSH Remote Connection

SSH (Secure Shell)

Locate .ssh Directory

SSH Remote Login (1) – Use Key Pair Generated by Server

SSH Remote Login (2) – Use Key Pair Generated by Client

SSH Config File

SSH Remote Login with Visual Studio Code

SCP (Secure Copy Protocol)

SCP with SSH Config File

SFTP (Secure File Transfer Protocol)

Other File Transfer Commands

Chapter 8. Process Management

Process and Job

Foreground and Background Jobs

jobs and ps (Display Jobs and Processes)

Signals

Create, Stop and Terminate Jobs

Daemon Processes

What Is Service on Linux?

Systemd

Unit File

Systemctl Sub-Commands

Create Custom Unit and Start at Boot

Firewall

UFW (Uncomplicated Firewall)

Web Server

Launch Apache Web Server

Unit file structure

There are three sections in a unit file. The second section is a unit-type-specific section. For example, if the unit is a service unit, [Service] is the section’s name. If it is used for a socket unit, [Socket] is the section’s name. The unit-type-specific sections differ by type. Here are explanations for the unit file of a service unit.

[Unit]: This section is used to describe general directives that are not unit-type specific. Directives in this section include dependencies, order of unit file executions, conflicting units, etc.

[Service]: This section includes directives for commands to be triggered when the unit is started, stopped, or reloaded. It also covers unit behaviors when the process is completed.

[Install]: This section is an optional section and is used to set the behavior of a unit when it is enabled.

This is an example of a unit file for sshd.service.

sshd.service

[Unit] Description=OpenBSD Secure Shell server Documentation=man:sshd(8) man:sshd_config(5) After=network.target auditd.service ConditionPathExists=!/etc/ssh/sshd_not_to_be_run [Service] EnvironmentFile=-/etc/default/ssh ExecStartPre=/usr/sbin/sshd -t ExecStart=/usr/sbin/sshd -D $SSHD_OPTS ExecReload=/usr/sbin/sshd -t ExecReload=/bin/kill -HUP $MAINPID KillMode=process Restart=on-failure RestartPreventExitStatus=255 Type=notify RuntimeDirectory=sshd RuntimeDirectoryMode=0755 [Install] WantedBy=multi-user.target Alias=sshd.s