> ## Documentation Index > Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt > Use this file to discover all available pages before exploring further. # Manage the startup process and services In Services Configuration > This guide teaches how to inspect, configure, and control services on a Linux system using systemd. In this guide, you’ll learn how to inspect, configure, and control services on a Linux system using **systemd**—the modern init system. We cover unit files, service management commands, and best practices for maintaining your server’s services. *** ## Table of Contents 1. [Introduction to systemd](#introduction-to-systemd) 2. [Unit File Types](#unit-file-types) 3. [Service Unit Structure](#service-unit-structure) 4. [Inspecting the SSH Daemon Service](#inspecting-the-ssh-daemon-service) 5. [Editing and Reverting a Unit](#editing-and-reverting-a-unit) 6. [Monitoring Service Status](#monitoring-service-status) 7. [Managing Service Lifecycle](#managing-service-lifecycle) 8. [Enabling and Disabling at Boot](#enabling-and-disabling-at-boot) 9. [Masking and Unmasking Services](#masking-and-unmasking-services) 10. [Listing All Service Units](#listing-all-service-units) 11. [Further Reading](#further-reading) *** ## Introduction to systemd When Linux boots, **systemd** orchestrates service startup in parallel while respecting dependencies. If a critical service fails, systemd can restart it automatically, ensuring high availability. All behavior is defined by **unit files**—plain-text configurations that tell systemd how to manage resources. Learn more about systemd on the [Official Freedesktop Wiki](https://www.freedesktop.org/wiki/Software/systemd/). *** ## Unit File Types Unit files end in `.service`, `.socket`, `.device`, `.timer`, and more. Here’s a quick overview: | Unit Type | Description | | --------- | ------------------------------------------------ | | service | Defines how to start, stop, and manage a service | | socket | Configures socket activation | | target | Groups units and handles synchronization | | timer | Schedules tasks similar to cron | *** ## Service Unit Structure Service units reside in `/etc/systemd/system/` or `/usr/lib/systemd/system/`. They have three main sections: ```ini theme={null} [Unit] Description=Human-readable description After=network.target [Service] Type=simple ExecStart=/usr/bin/myapp Restart=on-failure [Install] WantedBy=multi-user.target ``` Key directives: * **ExecStart**: Command to launch the service * **ExecReload**: How to reload without a full restart * **Restart**: Policy (`no`, `on-failure`, `always`) Consult the manual for all options: ```bash theme={null} man systemd.service ``` *** ## Inspecting the SSH Daemon Service Most servers run the OpenSSH daemon (`sshd`) as a systemd service. View its complete unit file: ```bash theme={null} $ systemctl cat sshd.service # /usr/lib/systemd/system/sshd.service [Unit] Description=OpenSSH server daemon Documentation=man:sshd(8) man:sshd_config(5) After=network.target sshd-keygen.target Wants=sshd-keygen.target [Service] Type=notify EnvironmentFile=/etc/crypto-policies/back-ends/opensshserver.config EnvironmentFile=-/etc/sysconfig/sshd ExecStart=/usr/sbin/sshd -D $OPTIONS $CRYPTO_POLICY ExecReload=/bin/kill -HUP $MAINPID KillMode=process Restart=on-failure RestartSec=42s [Install] WantedBy=multi-user.target ``` *** ## Editing and Reverting a Unit To fully edit the `sshd.service` file: ```bash theme={null} $ sudo systemctl edit --full sshd.service ``` To discard changes and restore the vendor-provided unit: ```bash theme={null} $ sudo systemctl revert sshd.service ``` *** ## Monitoring Service Status Check detailed status and recent logs: ```bash theme={null} $ sudo systemctl status sshd.service ``` Sample output: ```text theme={null} ● sshd.service - OpenSSH server daemon Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled) Active: active (running) since Wed 2021-12-08 16:48:53 CST; 15min ago Docs: man:sshd(8) man:sshd_config(5) Main PID: 1031 (sshd) Tasks: 1 (limit: 23555) Memory: 2.1M CGroup: /system.slice/sshd.service └─1031 /usr/sbin/sshd -D -oCiphers=aes256-gcm@openssh.com,chacha20-poly1305 Dec 08 16:48:53 LFCS-CentOS systemd[1]: Starting OpenSSH server daemon... Dec 08 16:48:53 LFCS-CentOS sshd[1031]: Server listening on 0.0.0.0 port 22. Dec 08 16:48:53 LFCS-CentOS systemd[1]: Started OpenSSH server daemon. ``` Press `Q` to exit. *** ## Managing Service Lifecycle Use `systemctl` for day-to-day operations: | Operation | Command | Description | | ----------------- | ----------------------------------------------- | ----------------------------------------- | | Start | `sudo systemctl start sshd.service` | Launch the service immediately | | Stop | `sudo systemctl stop sshd.service` | Stop the service | | Restart | `sudo systemctl restart sshd.service` | Stop and then start the service | | Reload | `sudo systemctl reload sshd.service` | Reload configuration without full restart | | Reload or Restart | `sudo systemctl reload-or-restart sshd.service` | Try reload, else restart | *** ## Enabling and Disabling at Boot Control automatic startup: ```bash theme={null} # Enable at boot $ sudo systemctl enable sshd.service # Disable at boot $ sudo systemctl disable sshd.service # Check status $ sudo systemctl is-enabled sshd.service ``` Combine with `--now` to apply immediately: ```bash theme={null} # Enable and start now $ sudo systemctl enable --now sshd.service # Disable and stop now $ sudo systemctl disable --now sshd.service ``` Disabling and stopping `sshd.service` will prevent all SSH logins. *** ## Masking and Unmasking Services Prevent any activation—manual or dependency-driven—by masking: ```bash theme={null} $ sudo systemctl mask atd.service ``` Reverse masking: ```bash theme={null} $ sudo systemctl unmask atd.service ``` Attempting to start a masked service results in: ```text theme={null} Failed to enable unit: Unit file /etc/systemd/system/atd.service is masked. Failed to start atd.service: Unit atd.service is masked. ``` *** ## Listing All Service Units View every service, regardless of state: ```bash theme={null} $ systemctl list-units --type=service --all ``` *** ## Further Reading * [systemd.service Manual](https://www.freedesktop.org/software/systemd/man/systemd.service.html) * [Kubernetes Basics](https://kubernetes.io/docs/concepts/overview/what-is-kubernetes/) * [Docker Hub](https://hub.docker.com/) * [Terraform Registry](https://registry.terraform.io/)