Linux Services

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  1. Linux Services from Linux command line for you and me

What is a service?

A service is a process or application which is running in the background, either doing some predefined task or waiting for some event.

Daemon is the actual term for those long-running background processes. A service actually consists of one or more daemons.

What is systemd

Systemd is a system and service manager for Linux operating systems. The systemd process has PID 1. It is the first process to run in our system; it then starts all the required processes, including services.

If you look at Unix/Linux history, you will find the first process which starts up, is also known as init process. This process used to start other processes by using the rc files from /etc/rc.d directory. In the modern Linux systems, systemd has replaced the init system.

Units in systemd

systemd manages various parts of a system with the abstraction units .

There are different kinds of units:

Unit Type File Extension Description
Service .service Defines and manages a system service.
Socket .socket Describes a socket for inter-process communication.
Device .device Describes a device that has been recognized by udev.
Mount .mount Defines a mount point for a filesystem.
Target .target Groups units for easier management or dependencies.
Path .path Monitors a file or directory for changes.
Timer .timer Schedules and runs units based on time events.
Swap .swap Manages swap space on the system.
Automount .automount Configures an automount point for a filesystem.

See here for detailed explanation.

.service units in systemd

.service units are service units, which explains how to manage a particular service in the system. In our daily life, we generally only have to work with these unit files.

Here are commands using systemctl to manage services

  • List all the service units:

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    systemctl --type=service

  • Start the service:

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    sudo systemctl start <service name>

  • Enable a service from starting at boot:

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    sudo systemctl enable <service name>

  • Disable a service from starting at boot:

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    sudo systemctl disable <service name>

journalctl

systemd runs the systemd-journald.service, which stores logs in the journal from the different services maintained by systemd in binary format.

To read these logs,

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sudo journalctl

You can add users into systemd-journal group to avoid using sudo.

Advanced usage:

  • Find the logs of a service:

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    sudo journalctl -u <service name>

  • List previous boots:

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    sudo journalctl --list-boots

  • Continuous stream of logs:

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    sudo journalctl -f -u <service name>

service command

Other than systemd, you may also see people manage services using the service command. The latter is an older utility and is more of a compatibility wrapper that can be used to manage services in a generic way, regardless of the underlying init system (which could be SysVinit, Upstart, or systemd).

How to create a unit file

A unit file is a plain text ini-format file that encodes information about a unit.

Below is a simple example of a systemd service unit file that might be used to run a web application.

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[Unit]
Description=My Web Application
After=network.target

[Service]
ExecStart=/usr/bin/python3 /path/to/mywebapp/app.py
WorkingDirectory=/path/to/mywebapp
Environment="FLASK_ENV=production"
User=www-data
Group=www-data
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

Breakdown of the sections:

  1. [Unit]
  • Description: A brief description of the service.
  • After: Specifies the order in which units are started. This service will start after network.target, meaning it will only start after the network is up.
  1. [Service]
    • ExecStart: The command that will be executed to start the service. In this case, it runs a Python web application.
    • WorkingDirectory: Sets the working directory for the service.
    • Environment: Sets environment variables for the service. Here, it's setting the Flask environment to production.
    • User and Group: The user and group under which the service will run. This is typically set to a non-privileged user like www-data for security reasons.
    • Restart: Configures when the service should be restarted. always means it will be restarted if it crashes or is stopped.
    • RestartSec: Time to wait before restarting the service.
  2. [Install]
    • WantedBy: Specifies the target that this service should be included in. multi-user.target is a common target for services that need to run on a typical system.

How to use an unit file

Suppose the unit file is for a service,

  1. Create the unit file: Save the above content as /etc/systemd/system/<service name>.service.

  2. Reload systemd:

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    sudo systemctl daemon-reload

  3. Start the service:

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    sudo systemctl start <service name>.service