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Linux - Software: Service

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Linux - Software: Service

Service

service
- a background process that performs specific system tasks or provides a particular functionality.
- These services are typically started automatically during the system boot process and continue running in the background, waiting for requests or events to respond to.
Features
- Background Process:
  - Runs without user interaction.
- Managed by System Daemons:
  - Handled by init systems like systemd, SysVinit, or Upstart.
- Essential for System Functionality:
  - Examples include network management (NetworkManager), file sharing (NFS), and web servers (httpd).
Common Types of Linux Services
- Networking:
  - network, ssh, firewalld
- Web Servers:
  - apache2, nginx
- Database Servers:
  - mysqld, postgresql
- File Sharing:
  - nfs, smb
- System Tools:
  - cron, syslog

`systemd`

systemd
- a system and service manager for Linux operating systems.
- designed to be backwards compatible with SysV init scripts
- provides a number of features such as
  - parallel startup of system services at boot time,
  - on-demand activation of daemons,
  - or dependency-based service control logic.
- acts as the init system that brings up and maintains user space services when run as the first process on boot
- PID: 1
  - known as init and is the first Linux user-mode process created
  - if it is killed, the entire OS will be kill, because all the other of services depends on systemd.
systemd units
- represented by unit configuration files located in one of the directories,
- encapsulate information about system services, listening sockets, and other objects that are relevant to the init system.

Unit File

Systemd Unit Files Locations

Directory	Created	Precedence
`/usr/lib/systemd/system/`	By installed RPM packages.	-
`/run/systemd/system/`	At run time.	over installed service unit files.
`/etc/systemd/system/`	by `systemctl enable` and added for extending a service	over runtime unit files.

Available systemd Unit Types

Unit Type	File Extension	Description
Service unit	`.service`	A system service.
Target unit	`.target`	A group of systemd units.
Automount unit	`.automount`	A file system automount point.
Device unit	`.device`	A device file recognized by the kernel.
Mount unit	`.mount`	A file system mount point.
Path unit	`.path`	A file or directory in a file system.
Scope unit	`.scope`	An externally created process.
Slice unit	`.slice`	A group of hierarchically organized units that manage system processes.
Snapshot unit	`.snapshot`	A saved state of the systemd manager.
Socket unit	`.socket`	An inter-process communication socket.
Swap unit	`.swap`	A swap device or a swap file.
Timer unit	`.timer`	A systemd timer.

unit file status:
- enabled: The unit is enabled to start automatically at boot.
- disabled: The unit is disabled and will not start automatically at boot.
- static: The unit is not directly enabled or disabled, but it is pulled in by other units when needed.
- masked: The unit is masked and cannot be started, either manually or automatically.
units columns:
- UNIT:
  - The name of the unit.
- LOAD:
  - The load status of the unit.
  - This indicates whether the unit is successfully loaded into memory.
- ACTIVE:
  - The current state of the unit.
  - This shows whether the unit is active (running), inactive (stopped), or in another state.
- SUB:
  - The sub-state of the unit, which provides more specific information about its status.
- DESCRIPTION:
  - A short description of the unit, usually indicating its purpose.

# get unit status for a specific system unit
systemctl list-units | grep firewalld

# get all unit file enabled at boot.
systemctl list-unit-files | grep enabled
# get all unit file disabled at boot.
systemctl list-unit-files | grep disabled
# get all unit file sattic at boot.
systemctl list-unit-files | grep sattic
# get all unit file masked at boot.
systemctl list-unit-files | grep masked

`systemctl` and `service` Command

systemctl
- the command-line interface used to interact with systemd.
- allows administrators to manage and query system services and states.
service command
- run a System V init script
- /etc/init.d: The directory containing System V init scripts.

`systemd` Management

CMD	DESC
`systemctl --version`	Used to check if the systemd installed
`ps -ef \\| grep systemd`	Used to check if the systemd running
`systemctl --all` / `systemctl list-units`	Display all available systemd units
`systemctl list-unit-files`	List all unit files

Services vs packages

# shows installed software packages and their versions.
rpm -qa
rpm -qa | wc -l

# shows the status of services and other systemd units
systemctl --all

Unit Management

CMD	DESC
`systemctl` / `systemctl list-units`	Lists active units(in memory) and current states
`systemctl list-units -a`	Lists all units, active + inactive
`systemctl list-units -t socket`	Lists all socke type unit
`systemctl list-units --failed`	Lists all failed unit at last system boot
`systemctl list-unit-files`	Lists `unit files` and states
`systemctl list-sockets`	Lists units of type socket
`systemctl list-dependencies`	Lists dependency tree for all unit
`systemctl list-dependencies crond`	Lists dependency tree for a unit
`systemctl show`	Show all properties
`systemctl show crond`	Show the service’s properties

Service Units Management

CMD	DESC
`systemctl list-units -t service`	Lists all serivces type unit
`systemctl daemon-reload`	Re-reads and reloads all unit configuration files
`systemctl status crond`	Check the status of a service
`systemctl is-enabled crond`	Check if a service is enabled
`systemctl is-active crond`	Checks whether a unit is running
`systemctl is-failed crond`	Checks whether a unit is in the failed state
`systemctl start crond`	Start a service
`systemctl stop crond`	Stop a service
`systemctl reload crond`	Reload the configuration of a service
`systemctl restart crond`	Stop and Start a service after a configuration change
`systemctl enable crond`	Enable a service at boot time
`systemctl disable crond`	Disable a service at boot time
`systemctl mask crond`	Disable a service completely including dependencies
`systemctl unmask crond`	Enable a service completely including dependencies
`systemctl kill unit_name`	Terminates all processes for a unit

Target Unit Management

rescue and emergency targets are for troubleshooting and system recovery purposes
poweroff and halt are similar to shutdown, and hibernate is suitable for mobile devices.

CMD	DESC
`systemctl list-units -t target`	Lists all target type unit
`systemctl list-units -t target --state active`	Get the current ative target units
`systemctl get-default`	Shows the default boot target
`systemctl set-default graphical.target`	Set the system to boot into a graphical environment.
`systemctl isolate rescue.target`	Switch to Rescue Mode (Start one unit and stop all others)
`systemctl reboot`	Reboots a Linux system
`systemctl poweroff`	Shut down the system

Lab: Change target

Default target

systemctl get-default
# graphical.target

systemctl set-default multi-user
# Removed "/etc/systemd/system/default.target".
# Created symlink /etc/systemd/system/default.target → /usr/lib/systemd/system/multi-user.target.

systemctl set-default graphical.target
Removed "/etc/systemd/system/default.target".
Created symlink /etc/systemd/system/default.target → /usr/lib/systemd/system/graphical.target.

Switch currrent target

# To switch into multi-user
systemctl isolate multi-user
# To return to the graphical target:
systemctl isolate graphical

Start/Stop a service

Start a Service

systemctl start service_name command
- used to start a service immediately on a system managed by systemd.
1. Unit Activation
- The systemd manager reads the service’s unit file (usually located in /usr/lib/systemd/system/ or /etc/systemd/system/) to determine how to start the service.
1. Dependency Resolution
- systemd checks for and activates any dependencies required by the service (defined in the Requires= or Wants= directives of the unit file).
- If the service depends on other services or system targets, they are started first.
1. Service Initialization
- systemd executes the commands specified in the [Service] section of the unit file.
- This often includes:
  - Running the ExecStart= command to initialize the service.
  - Setting up environment variables, if specified in the unit file.
1. Immediate Effect
  - The service starts immediately and runs in the current session.
  - This does not affect the service’s startup behavior after a reboot unless you also use the enable command.

Stop a Service

systemctl stop service_name command
- used to stop a running service immediately on a system managed by systemd.
Service Termination
- systemd sends a signal to the service’s main process to terminate it.
- By default, it uses the SIGTERM signal, but this can be overridden in the unit file.
Stops Related Services
- If the service has dependencies defined in the unit file (e.g., Requires= or PartOf=), they may also be stopped, depending on the configuration.
Updates Service State
- The service’s state changes to inactive. This means it is no longer running.

Enable/Disable a Service

Enable a Service

systemctl enable service_name command
- the system sets up the specified service (service_name) to start automatically at boot time.
Creation of Symlinks
- The command creates symbolic links in the appropriate systemd target directory (e.g., /etc/systemd/system/<target>.wants/).
- These links point to the service’s unit file, usually located in /usr/lib/systemd/system/ or /etc/systemd/system/.
  - /etc/systemd/system/multi-user.target.wants/httpd.service -> /usr/lib/systemd/system/httpd.service
Service Dependency Configuration
- The system associates the service with a specific target, such as multi-user.target (typically equivalent to runlevel 3 or 5 in traditional init systems).
- This ensures the service starts when the system reaches the specified target during boot.
Persistence
- The change is persistent, meaning the service will start automatically at every subsequent boot, even after a system reboot.

Disable a Service

systemctl disable service_name command
- the service is disabled
- it will no longer start automatically at boot time.
- However, the service can still be started manually or by another service that depends on it.
Removes Symlinks
- The command removes the symbolic links that were created when the service was enabled.
- These symlinks are located in the target directory (e.g., /etc/systemd/system/<target>.wants/).
  - /etc/systemd/system/multi-user.target.wants/httpd.service
Stops Automatic Startup
- The service will no longer start automatically when the system boots into the specified target (like multi-user.target or graphical.target).
Service Remains Available
- The service is not completely disabled. You can still:
  - Start it manually using systemctl start service_name.
  - Start it automatically if triggered by another service

Mask/Unmask a Service

Mask a Service

systemctl mask service_name
- To mask a service.
- the service is completely disabled and cannot be started, either manually or automatically, even by another service that depends on it.
Symlink Creation to /dev/null
- The systemctl mask command creates a symbolic link for the service unit file that points to /dev/null.
- /etc/systemd/system/service_name.service -> /dev/null
- This effectively prevents the service from being started, as the system cannot read the configuration for the service unit.
Overrides Enablement
- Even if the service is enabled or has dependencies set up, masking overrides them, ensuring the service cannot start.
Prevents Manual Starts
- Attempting to start a masked service manually with systemctl start service_name will fail with an error:
- Failed to start service_name.service: Unit service_name.service is masked.

Unmask a Service

systemctl unmask service_name command
- it removes the masking from a service, allowing it to be started again manually, automatically, or by other dependent services.
Removes the Symlink to /dev/null
- systemctl mask service_name: its unit file is replaced with a symbolic link pointing to /dev/null.
  - This prevents the service from starting.
- systemctl unmask service_name removes this symlink, restoring the ability for the system to access the original unit file.
Restores Service Availability
- After unmasking, the service can be:
  - Started manually (systemctl start service_name).
  - Enabled to start at boot (systemctl enable service_name).
  - Started by dependent services if required.
Does Not Automatically Enable or Start the Service
- Unmasking does not re-enable or start the service. You must explicitly enable or start it if needed.

Lab: Manage `nginx`

Install `nginx` Package

su -

dnf update -y
dnf upgrade -y
dnf install nginx -y

# confirm
rpm -qa | grep nginx
# nginx-mod-http-image-filter-1.14.1-9.module+el8.0.0+4108+af250afe.x86_64
# nginx-1.14.1-9.module+el8.0.0+4108+af250afe.x86_64
# nginx-mod-http-xslt-filter-1.14.1-9.module+el8.0.0+4108+af250afe.x86_64
# nginx-mod-stream-1.14.1-9.module+el8.0.0+4108+af250afe.x86_64
# nginx-filesystem-1.14.1-9.module+el8.0.0+4108+af250afe.noarch
# nginx-mod-mail-1.14.1-9.module+el8.0.0+4108+af250afe.x86_64
# nginx-all-modules-1.14.1-9.module+el8.0.0+4108+af250afe.noarch
# nginx-mod-http-perl-1.14.1-9.module+el8.0.0+4108+af250afe.x86_64
systemctl list-unit-files | grep nginx
# nginx.service                              disabled

Start/Stop `nginx`

systemctl show nginx
# Type=forking
# Restart=no
# PIDFile=/run/nginx.pid
# NotifyAccess=none
# RestartUSec=100ms
# TimeoutStartUSec=1min 30s
# TimeoutStopUSec=5s
# RuntimeMaxUSec=infinity
# WatchdogUSec=0
# ...

systemctl --all | grep nginx
# none
systemctl start nginx
# display status
systemctl status nginx
# ● nginx.service - The nginx HTTP and reverse proxy server
#    Loaded: loaded (/usr/lib/systemd/system/nginx.service; disabled; vendor preset: disabled)
#    Active: active (running) since Sun 2024-12-01 00:46:26 EST; 1min 26s ago
#   Process: 73366 ExecStart=/usr/sbin/nginx (code=exited, status=0/SUCCESS)
#   Process: 73364 ExecStartPre=/usr/sbin/nginx -t (code=exited, status=0/SUCCESS)
#   Process: 73362 ExecStartPre=/usr/bin/rm -f /run/nginx.pid (code=exited, status=0/SUCCESS)
#  Main PID: 73367 (nginx)
#     Tasks: 9 (limit: 22878)
#    Memory: 16.6M
#    CGroup: /system.slice/nginx.service
#            ├─73367 nginx: master process /usr/sbin/nginx
#            ├─73368 nginx: worker process
#            ├─73369 nginx: worker process
#            ├─73370 nginx: worker process
#            ├─73371 nginx: worker process
#            ├─73372 nginx: worker process
#            ├─73373 nginx: worker process
#            ├─73374 nginx: worker process
#            └─73375 nginx: worker process

# Dec 01 00:46:26 rhelhost.localdomain systemd[1]: Starting The nginx HTTP and reverse proxy server.>
# Dec 01 00:46:26 rhelhost.localdomain nginx[73364]: nginx: the configuration file /etc/nginx/nginx.>
# Dec 01 00:46:26 rhelhost.localdomain nginx[73364]: nginx: configuration file /etc/nginx/nginx.conf>
# Dec 01 00:46:26 rhelhost.localdomain systemd[1]: Started The nginx HTTP and reverse proxy server.
systemctl --all | grep nginx
# nginx.service    loaded    active   running   The nginx HTTP and reverse proxy server
# verify
curl 127.0.0.1
# return the default nginx website

# stop service
systemctl stop nginx
# display status
systemctl status nginx
# ● nginx.service - The nginx HTTP and reverse proxy server
#    Loaded: loaded (/usr/lib/systemd/system/nginx.service; disabled; vendor preset: disabled)
#    Active: inactive (dead)

# Dec 01 00:36:22 rhelhost.localdomain systemd[1]: nginx.service: Unit cannot be reloaded because it>
# Dec 01 00:46:26 rhelhost.localdomain systemd[1]: Starting The nginx HTTP and reverse proxy server.>
# Dec 01 00:46:26 rhelhost.localdomain nginx[73364]: nginx: the configuration file /etc/nginx/nginx.>
# Dec 01 00:46:26 rhelhost.localdomain nginx[73364]: nginx: configuration file /etc/nginx/nginx.conf>
# Dec 01 00:46:26 rhelhost.localdomain systemd[1]: Started The nginx HTTP and reverse proxy server.
# Dec 01 00:56:03 rhelhost.localdomain systemd[1]: Stopping The nginx HTTP and reverse proxy server.>
# Dec 01 00:56:03 rhelhost.localdomain systemd[1]: nginx.service: Succeeded.
# Dec 01 00:56:03 rhelhost.localdomain systemd[1]: Stopped The nginx HTTP and reverse proxy server.
systemctl --all | grep nginx
# none
curl 127.0.0.1
# curl: (7) Failed to connect to 127.0.0.1 port 80: Connection refused

Enable/Disable `nginx`

# Check link before operations
ll /etc/systemd/system/multi-user.target.wants | grep nginx*
# none
ll /etc/systemd/system/ | grep nginx*
# drwxr-xr-x. 2 root root    6 Aug 30  2019 nginx.service.d

systemctl enable nginx.service
# Created symlink /etc/systemd/system/multi-user.target.wants/nginx.service → /usr/lib/systemd/system/nginx.service.
systemctl is-enabled nginx.service
# enabled
systemctl list-unit-files | grep nginx.service
# nginx.service                              enabled
ll /etc/systemd/system/multi-user.target.wants | grep nginx*
# lrwxrwxrwx. 1 root root 37 Dec  1 00:38 nginx.service -> /usr/lib/systemd/system/nginx.service
ll /etc/systemd/system/ | grep nginx*
# drwxr-xr-x. 2 root root    6 Aug 30  2019 nginx.service.d

systemctl disable nginx.service
# Removed /etc/systemd/system/multi-user.target.wants/nginx.service.
systemctl is-enabled nginx.service
# disabled
systemctl list-unit-files | grep nginx.service
# nginx.service                              disabled
ll /etc/systemd/system/multi-user.target.wants | grep nginx*
# none
ll /etc/systemd/system/ | grep nginx*
# drwxr-xr-x. 2 root root    6 Aug 30  2019 nginx.service.d

Mask/Unmask `nginx`

systemctl mask nginx.service
# Created symlink /etc/systemd/system/nginx.service → /dev/null.
systemctl list-unit-files | grep nginx.service
# nginx.service                              masked
ll /etc/systemd/system/multi-user.target.wants | grep nginx*
# none
ll /etc/systemd/system/ | grep nginx*
# lrwxrwxrwx. 1 root root    9 Dec  1 00:41 nginx.service -> /dev/null
# drwxr-xr-x. 2 root root    6 Aug 30  2019 nginx.service.d

systemctl unmask nginx.service
# Removed /etc/systemd/system/nginx.service.
systemctl list-unit-files | grep nginx.service
# nginx.service                              disabled
ll /etc/systemd/system/multi-user.target.wants | grep nginx*
# none
ll /etc/systemd/system/ | grep nginx*
# drwxr-xr-x. 2 root root    6 Aug 30  2019 nginx.service.d

Clear up

dnf remove -y nginx
ll /etc/systemd/system/ | grep nginx*
# none

Service Log

journalctl -u service_name

TOP

Note_Tech

Linux - Software: Service

Service

systemd

Unit File

systemctl and service Command

systemd Management

Unit Management

Service Units Management

Target Unit Management

Lab: Change target

Default target

Switch currrent target

Start/Stop a service

Start a Service

Stop a Service

Enable/Disable a Service

Enable a Service

Disable a Service

Mask/Unmask a Service

Mask a Service

Unmask a Service

Lab: Manage nginx

Install nginx Package

Start/Stop nginx

Enable/Disable nginx

Mask/Unmask nginx

Clear up

Service Log

`systemd`

`systemctl` and `service` Command

`systemd` Management

Lab: Manage `nginx`

Install `nginx` Package

Start/Stop `nginx`

Enable/Disable `nginx`

Mask/Unmask `nginx`