Sometimes you need to monitor something without creating an elaborate solution. In many cases, shell may be all you need.

Option 1: watch

The shortest command is watch. It is great if you want to just visually inspect the output.

watch -n  

For example:

watch -n 1 cat /proc/stat

It will use the entire terminal and redraw the output with each update. For this reason, it is not well-suited for saving the output.

Option 2: while

while loop is an alternative. It is not as accurate in timing as watch, but for simple tasks, it is not a problem. It uses shell built-in command to define an infinite loop in which the command is called, with sleep in each iteration.

while true; do
	<command>
	sleep <interval>
done

For example:

while true; do
	cat /proc/stat
	sleep 1
done

With this approach, you can easily redirect the output to file.

Good to know

• watch is one of the standard utilities, but is not always available in embedded systems.
• !! is a history expansion syntax available in Bash. It is replaced by the last called command, so you can use it in a one-liner, like this:

  cat /proc/stat
  while true; do !!; sleep 1; done

• true is a command that does nothing and always succeeds, so it is great as an infinite loop condition.
• timeout is useful when you need to stop execution after given period of time. The basic syntax is timeout <time>. So, if you run timeout 2 sleep 5, it will stop after 2 seconds. It can be combined with the while loop, but there is a caveat. Since while is a built-in shell command, not an executable, you cannot simply pass it to timeout. Instead, you need to call the interpreter (like sh or bash) with the -c switch and insert the loop after that.