> For the complete documentation index, see [llms.txt](https://alicenab.gitbook.io/linux/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alicenab.gitbook.io/linux/redhat/redhat-administration-1-rh124/chapter-9.md).

# Chapter 9

Linux konvensiyasına görə sonu `d` hərfi ilə bitən fayllar daemon fayllarıdır.

systemd linuxda 1 saylı PİD ilə işə düşən ilk daemondur.

Service units

Aşağıdakı komanda vasitəsilə mövcud olan system unitlərə baxmaq mümkündür:

<pre><code>systemctl list-units -all                  # hər tip unitlərə bax
<strong>systemctl list-unit-files --all            # hər tip unitlərə bax
</strong>systemctl list-unit-files --type=service.  # servis tipli unitlərə bax
</code></pre>

1. **.service**:
   * This unit type represents a process or a group of processes. It's used to manage system services.
2. **.path**:
   * Path units are used to trigger other units when file system objects change or when directories appear or disappear.
3. **.scope**:
   * Scope units manage a set of system processes. Unlike service units, scope units manage processes that have already been started.
4. **.mount**:
   * Mount units represent mount points in the filesystem hierarchy. They are used to manage filesystem mount points.
5. **.automount**:
   * Automount units provide an automounting mechanism, which allows on-demand mounting of file systems when they are accessed, and unmounting them when they are no longer needed.
6. **.device**:
   * Device units represent a device in the system, and they encapsulate device nodes present in the `/dev/` directory.
7. **.socket**:
   * Socket units represent network sockets or IPC sockets (inter-process communication). They are used to enable system services to communicate with each other or with other systems.
8. **.swap**:
   * Swap units represent swap spaces in the system. They are used to manage swap partitions or files.
9. **.target**:
   * Target units are used to group other units, creating a logical point of synchronization during the boot process. They do not represent a resource, but are used to model system states.
10. **.timer**:
    * Timer units are used to trigger activation of other units based on timers. They can work on real-time or monotonic time, and can be used to schedule the activation of other units.

|          |                                                                              |
| -------- | ---------------------------------------------------------------------------- |
| Loaded   | Whether the service unit is loaded into memory.                              |
| Active   | Whether the service unit is running and if so, how long it has been running. |
| Main PID | The main process ID of the service, including the command name.              |
| Status   | Additional information about the service.                                    |
|          |                                                                              |

|                  |                                                                         |
| ---------------- | ----------------------------------------------------------------------- |
| loaded           | Unit configuration file has been processed.                             |
| active (running) | Running with one or more continuing processes.                          |
| active (exited)  | Successfully completed a one-time configuration.                        |
| active (waiting) | Running but waiting for an event.                                       |
| inactive         | Not running.                                                            |
| enabled          | Is started at boot time.                                                |
| disabled         | Is not set to be started at boot time.                                  |
| static           | Cannot be enabled, but may be started by an enabled unit automatically. |

```
# Provides a detailed status of the sshd.service, including whether it is active and the most recent log entries.
systemctl status sshd.service

# Checks if the sshd.service is currently active (running).
systemctl is-active sshd.service

# Checks if the sshd.service is enabled to start at boot.
systemctl is-enabled sshd.service

# Checks if the sshd.service has entered a failed state.
systemctl is-failed sshd.service

# Lists all service units that have entered a failed state.
systemctl --failed --type=service

# Starts the sshd service immediately.
systemctl start sshd

# Starts the sshd.service immediately, same as above but with explicit unit type.
systemctl start sshd.service

# Stops the sshd.service immediately.
systemctl stop sshd.service

# Stops and then starts the sshd.service again.
systemctl restart sshd.service

# Reloads the configuration of sshd.service without disrupting current operations.
systemctl reload sshd.service

# Reloads the configuration if possible, otherwise stops and starts the sshd.service again.
systemctl reload-or-restart sshd.service

# Lists all units that sshd.service depends on.
systemctl list-dependencies sshd.service


```

## MASKING AND UNMASKING SERVICES

```
# Prevents the sendmail.service from being started, manually or as a dependency.
systemctl mask sendmail.service

systemctl list-unit-files --type=service
systemctl start sendmail.service

# Reverses the masking of `sendmail.service`, allowing it to be started again, manually or as a dependency.
systemctl unmask sendmail.service 
```