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Systemd service

Viewing the Full Content of a Systemd Service with systemctl cat

The systemctl cat command is a powerful and useful feature of systemctl that allows you to view the full content of a systemd service unit file, including any drop-in configuration files that override or extend the default settings.

Purpose of systemctl cat

When you run systemctl cat <service_name>, it displays the contents of the main unit file along with any additional configuration files located in the /etc/systemd/system/<service_name>.service.d/ directory. This command is particularly useful for debugging or understanding how a service is configured, especially when changes have been made using drop-in files.

Example Usage

Suppose you want to inspect the puppet service unit file, including any modifications or overrides that might be in place. You would run the following command:

sudo systemctl cat puppet

Sample Output Explanation

sudo systemctl cat puppet

Output:

# /lib/systemd/system/puppet.service
#
# Local settings can be configured without being overwritten by package upgrades, for example
# if you want to increase puppet open-files-limit to 10000,
# you need to increase systemd's LimitNOFILE setting, so create a file named
# "/etc/systemd/system/puppet.service.d/limits.conf" containing:
# [Service]
# LimitNOFILE=10000
# You can confirm it worked by running systemctl daemon-reload
# then running systemctl show puppet | grep LimitNOFILE
#
[Unit]
Description=Puppet agent
Wants=basic.target
After=basic.target network.target

[Service]
EnvironmentFile=-/etc/sysconfig/puppetagent
EnvironmentFile=-/etc/sysconfig/puppet
EnvironmentFile=-/etc/default/puppet
ExecStart=/opt/puppetlabs/puppet/bin/puppet agent $PUPPET_EXTRA_OPTS --no-daemonize
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process

[Install]
WantedBy=multi-user.target

Explanation:

  • Main Unit File:
    The output shows the contents of the main unit file located at /lib/systemd/system/puppet.service. This file is provided by the package and contains the default configuration for the puppet service.

  • Configuration File Locations:

  • EnvironmentFile: The EnvironmentFile entries reference external files (/etc/sysconfig/puppetagent, /etc/sysconfig/puppet, and /etc/default/puppet) that can set environment variables for the service. The - prefix means that these files are optional; the service will still start if they are not found.
  • ExecStart: Specifies the command to start the service. In this case, it runs the Puppet agent with any additional options defined by $PUPPET_EXTRA_OPTS.
  • ExecReload: Specifies the command to reload the service without stopping it, typically sending a signal to the main process.

  • KillMode: Defines how processes are terminated when the service is stopped.

  • Custom Configuration:

  • The commented section explains how to create a custom configuration file, /etc/systemd/system/puppet.service.d/limits.conf, to increase the LimitNOFILE setting (which controls the maximum number of open files). This custom configuration will not be overwritten during package upgrades, as it's stored in the /etc directory.

Modifying and Extending Service Configurations

To modify or extend the configuration of a systemd service without altering the original unit file, you can create drop-in configuration files in the /etc/systemd/system/<service_name>.service.d/ directory.

Example:

To increase the LimitNOFILE for the puppet service:

  1. Create a Drop-In Directory:

bash sudo mkdir -p /etc/systemd/system/puppet.service.d/

  1. Create a Drop-In Configuration File:

bash sudo nano /etc/systemd/system/puppet.service.d/limits.conf

  1. Add the Configuration:
   [Service]
   LimitNOFILE=10000
  1. Reload systemd Configuration:

After saving the file, reload the systemd configuration to apply the changes:

bash sudo systemctl daemon-reload

  1. Verify the Change:

You can verify that the change took effect by running:

bash systemctl show puppet | grep LimitNOFILE

This command will show the current LimitNOFILE setting for the puppet service, confirming that your configuration change has been applied.

Summary

The systemctl cat command is an excellent tool for inspecting service unit files, including any custom modifications made through drop-in configurations. This approach is crucial for managing services in a way that ensures your custom configurations are preserved across system updates and service changes. By using systemctl cat, you gain full visibility into how a service is configured, making it easier to diagnose issues, make adjustments, and understand the service's behavior on your system.