Bash Unit File

Setup

Clone

git clone https://github.com/LaunchCodeTechnicalTraining/bash-garbage-collector

Review Source Code

ls bash-garbage-collector

cat bash-garbage-collector/empty.sh

#!/bin/bash

dir=/home/student/bash-garbage-collector/trash
log=/home/student/bash-garbage-collector/garbage.log

while true
do
    if [[ $(ls $dir) == "" ]]
    then
        echo "$(date): no contents detected in $dir" >> $log
    else
        echo "$(date): contents detected in $dir...emptying" >> $log
        rm $dir/*
    fi
    sleep 30
done

A simple script that checks the contents of /home/student/bash-garbage-collector/trash every 30 seconds. If contents exist at the location it logs that contents were detected and deletes the contents. If contents were not detected it simply logs that no contents were found.

Run Script

From the project directory run:

bash empty.sh

Feel free to add files to /home/student/bash-garbage-collector/trash after 30 seconds check the contents of /home/student/bash-garbage-collector/garbage.log and the trash/ directory to see the effects of the script.

Stop Script

The script can be stopped by entering ctrl + c to send the interrupt signal to the running process.

Create Unit File

This bash script can be converted into a systemd service by creating a valid unit file. After creating the unit file the service will be controllable with the systemctl tool.

Create a file in /etc/systemd/system/ called bash-garbage-collector.service as the root user:

sudo touch /etc/systemd/system/bash-garbage-collector.service

Add the following contents to the file using your editor of choice (make sure you are still acting as the root user):

[Unit]
Description=An awesome bash garbage collector (files in a certain directory)

[Service]
ExecStart=/usr/bin/bash /home/student/bash-garbage-collector/empty.sh
Restart=never

[Install]
WantedBy=multi-user.target

Breakdown:

• Description: the human readable description of the service
• ExecStart: The command invoked when the service is started (in this case /usr/bin/bash is being used to execute /home/student/bash-garbage-collector/empty.sh)
• Restart: Instructions on how and when the service should restart (in this case never).
• WantedBy: The system level in which the service should be initialized if enabled (in this case just before the user login screen appears (a multi-user environment has been reached))

Service Control via systemctl

Start

sudo systemctl start bash-garbage-collector

After starting the service add contents to /home/student/bash-garbage-collector/trash wait 30 seconds and see how the directory and the garbage.log are affected.

Stop

sudo systemctl stop bash-garbage-collector

After stopping the service the garbage collector is no longer running.

Enable

sudo systemctl enable bash-garbage-collector

After enabling the service the garbage collector should start when the machine reboots.

Disable

sudo systemctl disable bash-garbage-collector

After disabling the service it will no longer start when the computer starts.

Service Failure

Right now the service file instructs systemd to never restart the running service. Even if the service fails and stops running systemd will leave it in a stopped state.

This can be tested by starting, and finding the PID of the service:

systemctl status bash-garbage-collector

Output:

Main PID: 556 (bash)

Simulate Failure with kill

We can simulate a failure by sending a SIGKILL signal with the kill command:

sudo kill -9 [YOUR-SERVICE-PID]

Check Status

After simulating a catrastophic failure check the status of the service:

systemctl status bash-garbage-collector

Configure Service to Restart on Failure

systemd can be told to restart a service when it fails. Change the Restart section of the /etc/systemd/system/bash-garbage-collector.service file to:

Restart=on-failure

Now systemd knows to restart the service when it has experienced a failure.

Stop and Start Service

After making a change to a unit file you will need to stop and start the service again.

sudo systemctl daemon-reload

Kill again

Find the PID of the service again and kill it with a SIGKILL signal (-9).

Check the status of the service. Take note of the new PID the service was assigned when it was restarted.