watch — underrated Linux command

Many Linux users write small scripts to monitor system state — CPU usage, disk space, open connections, and more.
But there is a much simpler tool built into most Linux systems: watch.
It allows you to run any command repeatedly and see the output update in real time.

Basic usage

The syntax is simple:

watch command

By default, the command runs every 2 seconds.

Example: monitoring the mail queue

watch mailq

This refreshes the list of outgoing messages every two seconds so you can see the mail queue shrink or grow in real time.

Example: watching network connections

watch ss -tuln

This is useful when debugging network services or checking if a port becomes active.

Example: monitoring processes

watch "ps aux | grep nginx"

This lets you observe whether a process starts, stops, or changes.

Changing the interval

You can change the refresh interval with -n.

watch -n 1 mailq

This runs the command every 1 second.

Highlighting differences

One of the most useful options is -d, which highlights changes between updates.

watch -d netstat -tuln

This makes it much easier to see what changed since the previous refresh.

Conclusion

watch is a small but powerful tool.
It can turn almost any command into a simple real-time monitoring dashboard without writing scripts.
Sometimes the best tools in Linux are also the simplest.