watch Command: Tutorial & Examples

Run a program in regular intervals

The watch command is a powerful tool in the Linux system. It allows you to continuously execute a program or command at regular intervals and monitor the output in your terminal. This is particularly useful when you want to keep an eye on a process, monitor system resources, or watch for specific changes in output.

How does `watch` Work?

The watch command works by executing a specified command or script at regular intervals, the default being every 2 seconds. It then displays the output of the command to the terminal, clearing the screen and refreshing the output each time.

It does this by making use of the shell to execute the command, then displays the output. This process is repeated at the specified interval, giving you a live look at the output of your command.

Uses of `watch` Command

The watch command can be used in a wide variety of scenarios. Here are a few examples:

Monitor system resources: You can use watch in combination with other commands like top or free to keep a live watch on your CPU usage or memory consumption.
Track changes in a directory: You can use watch with the ls command to monitor changes in a directory's content.
Monitor network activity: The watch command can be paired with commands like netstat to observe live network activity on your server.

How to Use `watch` Command

Using watch is fairly straightforward. It follows this basic syntax:

watch [options] command

Here are a few examples of how to use watch:

To watch the contents of a directory, you could use:
```
watch ls /path/to/directory
```
To monitor free memory on your system, you can use:
```
watch free -m
```
To monitor active network connections, you can use:
```
watch netstat -tup
```

Each of these commands will execute the specified command every 2 seconds, and display the output to your terminal.

Common Command Parameters

Here are some common parameters you might use with watch:

-n or --interval: This allows you to specify the interval at which watch will execute your command. For example, watch -n 5 free -m will execute free -m every 5 seconds.
-d or --differences: This tells watch to highlight differences between successive updates. watch -d ls /path/to/directory will show you when changes are made to the specified directory.
-h or --help: Displays the help message and exits.

Potential Problems and Pitfalls

While watch is a powerful tool, it's important to be aware of a few potential issues:

Overuse: Executing a command too frequently can cause unnecessary load on your system. Be sure to use an appropriate interval.
Terminal size: If the output of your command is too large for your terminal, it can make the output hard to read. You may need to adjust your terminal size or the command you are using.
Long running commands: If the command you are watching takes longer to execute than the interval you've set, watch will start another instance of the command before the first has finished. This can lead to unpredictable results or increased system load.

Conclusion

The watch command is a powerful tool for monitoring and debugging on your Linux server. By understanding how it works and how to use it effectively, you can gain greater insight into your system and solve common problems more effectively.