How to use the command 'hwclock' (with examples)

The hwclock command is a powerful tool used in Unix-like operating systems to interact with the hardware clock of your computer. This hardware clock, often referred to as the real-time clock (RTC), is a precise and consistent timekeeper for your system, maintaining the time regardless of whether the computer is powered on or off. The hwclock command allows system administrators to read the current time from the hardware clock, as well as synchronize it with the software clock. Understanding and correctly using this command is crucial, especially for tasks such as system setup, troubleshooting, and ensuring that time-sensitive applications run effectively.

Use case 1: Display the current time as reported by the hardware clock

Code:

hwclock

Motivation: In a computing environment, there are often two clocks at play: the hardware clock and the software clock. The hardware clock is found on the motherboard and runs even when the computer is turned off, thanks to a small onboard battery. Unlike the software clock, which operates within the operating system, the hardware clock is independent and therefore less susceptible to errors caused by system crashes or restarts. Being able to display the time from the hardware clock using the hwclock command allows system administrators and users alike to verify if the hardware time is accurate and aligned with the software clock.

Explanation: The command hwclock without any additional arguments reads and displays the current time set on the hardware clock in a human-readable format. By executing this simple variant of the command, users can quickly ascertain the current hardware time and evaluate its accuracy without making any changes to the clock settings. This basic use is effective in general time-checking scenarios.

Example Output:

Mon 09 Oct 2023 10:21:34 AM UTC  .467657 seconds

This output provides the current date and time as per the hardware clock. It includes the time zone and a measurement of the time in seconds.

Use case 2: Write the current software clock time to the hardware clock

Code:

hwclock --systohc

Motivation: The software clock, which is maintained by the operating system, provides time to applications and services. This clock is usually accurate and can be regularly synchronized with time servers on the internet. However, changes in the system time do not automatically update the hardware clock. By using hwclock --systohc, users can ensure that any adjustments to the software clock are reflected in the hardware clock. This is particularly useful during system setup, after daylight saving time changes, or if the software clock has been synchronized with an authoritative time source.

Explanation: When using the hwclock --systohc command, you are instructing the system to copy the current time from the software clock to the hardware clock. The --systohc (system to hardware clock) flag is a direct command to overwrite the hardware clock time with the software clock time. This action ensures both clocks are synchronized, minimizing discrepancies after reboots or power cycles.

Example Output:

There is typically no output for this command when it executes successfully. The absence of an error message indicates that the hardware clock has been successfully updated to match the software clock time.

Use case 3: Write the current hardware clock time to the software clock

Code:

hwclock --hctosys

Motivation: There are occasions when a system’s software clock might become inaccurate due to system crashes, improper shutdowns, or software malfunctions. In such situations, the hardware clock, considered a reliable time source, can be used to correct the software clock. By using the hwclock --hctosys command, users can ensure that the time within the operating system is accurate, which is critical for time-stamping files, coordinating tasks, and synchronizing with external servers or devices.

Explanation: The --hctosys (hardware clock to system) option in the hwclock command updates the software clock to align it with the hardware clock. This operation effectively reads the current hardware clock setting and applies it to the system’s operating time settings. By doing so, users can reset any inaccuracies that might have crept into the software clock due to various issues at the system level.

Example Output:

As with the --systohc operation, there is generally no output upon successful completion of this command. It is designed to run silently unless an error occurs, in which case an error message will be displayed.

Conclusion:

The hwclock command is an essential tool for managing time on Unix-like systems, offering functionality to both observe and manipulate the interaction between hardware and software clocks. By familiarizing yourself with its options and appropriate use cases, you can ensure that system timekeeping is precise and reliable, facilitating improved system administration and use of time-dependent applications.