How to run script on startup using systemd in Linux

In this article I will share a sample systemd unit file which you can use to run script at startup with systemd without using crontab in RHEL/CentOS 7/8 Linux.

Some more articles on similar topic:

• How to execute a command or script with systemd at shutdown only and not at reboot in Linux
• How to execute a command or script with systemd right before login prompt appears on terminal in Linux
• How to execute a command or script at system startup using systemd without using cronjob in Linux
• How to execute a command or script after N minutes of boot up (system startup) with systemd in Linux
• How to halt system reboot or shutdown and read user input during boot up stage in Linux
• How to execute a command or script using systemd right before shutdown happens in Linux
• How to run a service as a specific user or group using systemd in Linux

There can be various scenarios when you expect a script or command to be called at startup such as

• Execute a script after waiting for N minutes of startup
• Execute a script after all the systemd services are loaded
• Execute a script immediately after login prompt appears
• Execute a script just before the login prompt appears

In this article I will cover below two topics as they are almost similar

• Run script at startup with systemd after network is reachable
• Execute script at starup after all the systemd services are loaded

I will be using CentOS/RHEL 7/8 Linux node to verify the steps from this article to run script with systemd right before login prompt.

Step 1: Overview on systemd

I hope you are already familiar with below topics

• Overview on systemd and how it is different from legacy SysV scripts
• How to create a systemd unit file in Linux

Step 2: Create Sample Script

Now to run script at startup with systemd firstly we need a script or command. For the sake of this article I have create a dummy shell script /tmp/startup_script.sh which we will use for testing this article.

[root@centos-8 ~]# cat /tmp/startup_script.sh
#!/bin/bash
SCRIPT_NAME=$(basename -- "$0")

z=0
for i in {1..5}; do
    sleep 1m
    ((z++))
    wall $SCRIPT_NAME: finished minute ${z}
done
wall $SCRIPT_NAME: COMPLETELY FINISHED

This script will continue to run for 5 minutes and will print an echo statement on the screen every minute as a broadcast message using wall command for all Linux users on the respective node. And at the end of 5th minute it will print a completed broadcast. With this we can also make sure that the script is not killed by systemd if it continues to run for 5 minutes.

Provide executable permission to the script

[root@centos-8 ~]# chmod u+x /tmp/startup_script.sh

Create a new systemd service unit file with following content:

[root@centos-8 ~]# cat /etc/systemd/system/run-at-startup.service
[Unit]
Description=Run script at startup after network becomes reachable

[Service]
Type=simple
RemainAfterExit=yes
ExecStart=/tmp/startup_script.sh
TimeoutStartSec=0

[Install]
WantedBy=default.target

Next to activate the changes, execute following command:

~]# systemctl daemon-reload

Enable the service to make sure this is called automatically after reboot

[root@centos-8 ~]# systemctl enable run-at-startup.service
Created symlink /etc/systemd/system/default.target.wants/run-at-startup.service → /etc/systemd/system/run-at-startup.service.

Step 3: Create systemd unit file for different scenario

Step 3.1: Run script at startup with systemd after network becomes reachable

In this scenario we will execute our script as soon as the network becomes reachable i.e. when network.target starts running during bootup stage by using After=network.target. The unit file should be located in either /usr/lib/systemd/system or /etc/systemd/system. I will place this under /etc/systemd/system as this is is used to place custom unit file.

[root@centos-8 ~]# cat /etc/systemd/system/run-at-startup.service
[Unit]
Description=Run script at startup after network becomes reachable
After=network.target

[Service]
Type=simple
RemainAfterExit=yes
ExecStart=/tmp/startup_script.sh
TimeoutStartSec=0

[Install]
WantedBy=default.target

Step 3.2: Run script at startup with systemd after all systemd services are loaded

Now in this senario we must make sure all the systemd services for the respective target has loaded before starting the script. So to run script at startup with systemd after all systemd services are loaded we must defined our After= directive accordingly. Since the default.target will vary based on environment so rather than specifying specific target name we will use After=default.target so systemd will decide on it's own the default.target and will call the script at startup in Linux.

[root@centos-8 ~]# cat /etc/systemd/system/run-at-startup.service
[Unit]
Description=Run script at startup after all systemd services are loaded
After=default.target

[Service]
Type=simple
RemainAfterExit=yes
ExecStart=/tmp/startup_script.sh
TimeoutStartSec=0

[Install]
WantedBy=default.target

Step 3.3: Run script at startup with systemd after login prompt appears

Now in this scenario also we will only play around After= directive of the unit file. Now since the requirement is to run script at startup with systemd after login prompt appears then we will use After=getty.target

[root@centos-8 ~]# cat /etc/systemd/system/run-at-startup.service
[Unit]
Description=Run script at startup after all systemd services are loaded
After=getty.target

[Service]
Type=simple
RemainAfterExit=yes
ExecStart=/tmp/startup_script.sh
TimeoutStartSec=0

[Install]
WantedBy=default.target

Here,

After=        If the script needs any other system facilities (networking, etc), modify the [Unit] section to include appropriate 
		   After=, Wants=, or Requires= directives, as described in: man systemd.unit

Type=              Switch Type=simple for Type=idle in the [Service] section to delay execution of the script until all other 
		   jobs are dispatched (see man systemd.service for even more choices -- e.g., Type=oneshot can be useful in concert with other services).

TimeoutStartSec=   When a service doesn't signal start-up completion within TimeoutStartSec, systemd considers the service failed;
		   for long-running shell scripts it is essential to modify TimeoutStartSec or disable the timeout logic altogether 
		   as above, with TimeoutStartSec=0. See man systemd.service for more details.

Here if you observe we have defined After=network.target to make sure the script

Refresh the systemd configuration files and enable the service

[root@centos-8 ~]# systemctl daemon-reload
[root@centos-8 ~]# systemctl enable run-at-startup.service

Step 4: Verify the systemd unit file configuration

Now our configuration is in place to run script at startup in CentOS/RHEL 7/8 Linux.

Now post reboot of the Linux node we can see that our startup_script.sh is running in the background

[root@centos-8 ~]# ps -ef | grep startup
root       805     1  0 11:38 ?        00:00:00 /bin/bash /tmp/startup_script.sh
root      1198  1147  0 11:39 pts/0    00:00:00 grep --color=auto startup

Also in some time we start getting the broadcast messages from root user.

Broadcast message from root@centos-8.example.com (somewhere) (Thu Jan 16 11:39:

startup_script.sh: finished minute 1


Broadcast message from root@centos-8.example.com (somewhere) (Thu Jan 16 11:40:

startup_script.sh: finished minute 2


Broadcast message from root@centos-8.example.com (somewhere) (Thu Jan 16 11:41:

startup_script.sh: finished minute 3


Broadcast message from root@centos-8.example.com (somewhere) (Thu Jan 16 11:42:

startup_script.sh: finished minute 4


Broadcast message from root@centos-8.example.com (somewhere) (Thu Jan 16 11:43:

startup_script.sh: finished minute 5


Broadcast message from root@centos-8.example.com (somewhere) (Thu Jan 16 11:43:

startup_script.sh: COMPLETELY FINISHED

I have verified the script for both scenarios to run script at startup with systemd (witout crontab) in Linux but I am not putting output from both scenarios as the output is same in both case.

Lastly I hope the steps from the article to run script at startup with systemd without using crontab in CentOS/RHEL 7/8 Linux was helpful. So, let me know your suggestions and feedback using the comment section.

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