hwclock Command in Linux: Read and Sync the Hardware Clock

The hwclock command on Linux can be used to read the hardware clock or update the hardware clock with the system clock and vice-versa.

As mentioned at the beginning of the chapter, computers keep two clocks. One called the hardware clock, is connected when the system is idle, and is maintained by a quartz crystal and a battery when the computer is off. This battery is capable of keeping the clock running even when disconnected from the power grid for a long time.

When the computer is turned on, Linux reads the hardware clock to update its system clock. It's common for there to be a small difference between these two clocks.

The hwclock command has the following options:

• -r: displays the hardware clock time with the local time zone;
• -s: sets the system clock with the time from the hardware clock;
• -w: sets the hardware clock with the time from the system clock;
• -c: compares the hardware clock and system clock times every 10 seconds;
• --adjust: adds or subtracts time from the hardware clock to compensate for frequency drift;

Examples:

# hwclock -chw-time      system-time         freq-offset-ppm   tick1571594417   1571594417.0114231571594427   1571594427.012649               123      1

To set the hardware clock with the system clock:

# hwclock -w

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