First of all, a Linux Service is an application that runs in the background and performs an essential task. One of the advantages to use a service is that you do not need to start it manually, you can configure it to start with the system using this command:
sudo systemctl enable <service_name>
Some of the most commonly used services are:
You can list all the services inside your Server using
systemctl list-units --type=service
Also you can see the status of an specific service using:
systemctl status <service_name>
Let see an example:
[ernesto.lopez@test-vm ~]$ systemctl status chronyd.service ● chronyd.service - NTP client/server Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled) Active: active (running) since Tue 2021-11-02 12:31:04 MST; 3 weeks 3 days ago Docs: man:chronyd(8) man:chrony.conf(5) Main PID: 1212 (chronyd) Tasks: 1 (limit: 98556) Memory: 1.1M CGroup: /system.slice/chronyd.service └─1256 /usr/sbin/chronyd
To explain this output:
- chronyd.service - NTP client/server: is the name of the service and a small description of it.
- Loaded: loaded (/usr/lib/systemd/system/chronyd.service; : means that the services located in the path that appears was loaded in memory
enabled; vendor preset: enabled) : this part indicate that the service is enable, so it will start at boot, this is achieved using the command
sudo systemctl enable
- Active: active (running) : this means that the service is up and running in the system, this line usually appears on green if you are using a normal user. This line also notifies the user when this service was started.
Main PID: 1212 (chronyd) : this line shows the process ID for this service. you can then use
ps 1212to get the command associated with the service.
These lines represent an important aspect to evaluate a service, cGroup deserve a single post entry.
NOTE Before converting anything to a service, make sure you have installed java on your server. A quick using dnf and java 8 example:
sudo dnf -y update sudo dnf -y install java-1.8.0-openjdk-devel java –version #Find your java home path sudo alternatives --config java
Add this line into your profile file (/etc/profile) to easier the following steps:
export JAVA_HOME=$(dirname $(dirname $(readlink $(readlink >$(which javac)))))
Save the file and set the system to use this profile
sudo source /etc/profile
You can verify that the configurations were correct, by running:
echo $JAVA_HOME echo $PATH echo $CLASSPATH
Back to the starting our java app, suppose our app is in a .jar file called my-own-app.jar
JAR stands for Java ARCHIVE, it is a file format used to aggregate many java files into one.
More on the official documentation from Oracle
And for example we have our application stored inside
First thing that we are going to do is to create a script that start the app, imagine we are going to convert our java app in a bash script (Just an imagination exercise!)
cd /opt/my-own-app touch my-own-app.sh vi !$
Then, we need to tell the script to start the app in the .jar file
#!/bin/bash # Script to start MyOwnApp echo "--------------------WAIT-------------------" # Start the app # minimum heap 512MB, maximum heap 1GB cd /opt/my-own-app java -Xms512M -Xmx1G -jar my-own-app.jar &
Here we are creating a script that move tho the directory where the .jar file is located and then execute the java command which is used to launch a java application. The elements to consider:
- -Xms512M specifies the initial sizeo of the memory allocation pool, in this case we append M to indicates it is megabyte. This value must be determined based on app consumption.
- -Xmx1G is the maximum size of the memory allocation pool, this case indicates 1G but can vary depending on your app.
-jar define the location of the jar file, because in the previous step in the script we move to the
/opt/my-own-appdir, we use just the name of the file in this case.
Save the file and change the permissions to make it an executable:
sudo chmod u+rwx my-own-app.sh ls -l | grep my-own-app.sh
Next step is to create a service unit:
Note A service unit describes how to manage a service or application on the server.
Insert the following information on the file:
[Unit] Description=my own app starting After=network.target [Service] Type=forking ExecStart=/opt/my-own-app/my-own-app.sh TimeoutStartSec=0 [Install] WantedBy=default.target
The important aspects of this:
WantedBy=default.target defines how a unit should be enabled, this represents the default and will create a file
- Type=forking tells the systemd that the process is still running even though the parent process exited.
- ExecStart=/opt/my-own-app/my-own-app.sh is the file to be executed.
- After=network.target the service must be started after network services.
The following step is to reload the systemd daemon
sudo systemctl daemon-reload
And finally enable and start the service
sudo systemctl start my-own-app.service sudo systemctl enable my-own-app.service
You can verify if the service started ok or if there was any trouble using
sudo systemctl status my-own-app.service