How to Use the Command 'chkconfig' (with Examples)

chkconfig is a command-line utility in Linux distributions like CentOS 6 that allows users to manage system services and their runlevels. Runlevels determine the state of your machine, such as which services are running or stopped. With chkconfig, you can list services, enable or disable them at boot time, and configure what services should start based on the current runlevel.

Use Case 1: Listing All Services with Their Runlevels

Code:

chkconfig --list

Motivation:

Knowing which services are set to start at different runlevels can help you optimize system resources and ensure that unnecessary services are not started. Listing them provides a snapshot of service configurations and their corresponding runlevels, which is crucial for debugging and system audits.

Explanation:

• chkconfig: Invokes the main program to manage runlevels.
• --list: This option instructs chkconfig to list all services and their current runlevels. It outputs a comprehensive list showing each service’s state across the seven runlevels.

Example Output:

sshd          0:off  1:off  2:on   3:on  4:on  5:on  6:off
ntpd          0:off  1:off  2:off  3:on  4:on  5:on  6:off

Use Case 2: Showing a Specific Service’s Runlevels

Code:

chkconfig --list ntpd

Motivation:

It’s often necessary to check the runlevel configuration of a specific service—for instance, when troubleshooting issues or optimizing startup processes. By focusing on one service, you can quickly assess its runlevel setup without sifting through all services.

Explanation:

• chkconfig: Calls the utility to access service runlevel configurations.
• --list: Directs the command to display current runlevel settings for services.
• ntpd: Specifies the Network Time Protocol Daemon as the target service, thus narrowing down the output to show only its runlevel states.

Example Output:

ntpd          0:off  1:off  2:off  3:on  4:on  5:on  6:off

Use Case 3: Enabling a Service at Boot

Code:

chkconfig sshd on

Motivation:

Enabling a service to start at boot is essential for services you need every time the system starts, such as sshd for remote login capabilities. This ensures that necessary services run automatically, improving system efficiency and reliability.

Explanation:

• chkconfig: Invokes the primary command.
• sshd: Specifies the Secure Shell Daemon, which allows secure remote logins.
• on: Instructs the system to enable sshd in applicable runlevels (by default 3, 4, and 5) to start at boot.

Example Output:

sshd 0:off  1:off  2:on  3:on  4:on  5:on  6:off

Use Case 4: Enabling a Service for Specific Runlevels

Code:

chkconfig --level 2345 sshd on

Motivation:

In some cases, you might only want a service active during specific runlevels that suit your operational needs, such as multi-user operations or graphical interfaces. This command provides granularity in service management, allowing you to tailor service availability systematically.

Explanation:

• chkconfig: Triggers the runlevel configuration utility.
• --level 2345: Specifies that sshd should be enabled for runlevels 2, 3, 4, and 5. This customization lets you choose specific runlevels rather than default ones.
• sshd: Refers to the Secure Shell Daemon.
• on: Signals that the sshd service should start automatically for the specified runlevels at boot.

Example Output:

sshd          0:off  1:off  2:on  3:on  4:on  5:on  6:off

Use Case 5: Disabling a Service at Boot

Code:

chkconfig ntpd off

Motivation:

Disabling a service that you don’t need can free up system resources and reduce boot time. If a service like ntpd is not required for synchronization tasks, turning it off at boot helps maintain a lean and focused server environment.

Explanation:

• chkconfig: The command used for managing runlevels.
• ntpd: Designates the Network Time Protocol Daemon.
• off: Specifies that the ntpd service should not start on any runlevel at boot time, thereby disabling it entirely.

Example Output:

ntpd          0:off  1:off  2:off  3:off  4:off  5:off  6:off

Use Case 6: Disabling a Service for a Specific Runlevel

Code:

chkconfig --level 3 ntpd off

Motivation:

In specialized scenarios where certain services should only run in particular states, disabling them for specific runlevels enhances control. For instance, you might only need ntpd in a specific multi-user environment and want it off in others to conserve resources or improve security.

Explanation:

• chkconfig: Executes the command-line utility.
• --level 3: Targets runlevel 3, a typical multi-user mode without graphical display services.
• ntpd: Identifies the Network Time Protocol Daemon.
• off: Commands that ntpd should be disabled, meaning it will not start automatically at boot for runlevel 3.

Example Output:

ntpd          0:off  1:off  2:off  3:off  4:on  5:on  6:off

Conclusion:

The chkconfig command is a powerful tool for managing services and their runlevel configurations in CentOS 6. Through clear understanding and utilization of its options and functionalities, system administrators can ensure optimal system performance, efficient resource use, and proper service readiness across different system states. From listing services to customizing their runlevels, chkconfig plays a crucial role in the maintenance of a robust and flexible Linux environment.