Imagine you're curious about a program's resource usage as you modify its parameters... Repeatably watch a process using top and pgrep.
Easy "Enter, q, up" repeat...
top -p $(pgrep i3status)Here, $(pgrep i3status) substitutes the pid of i3status into top's -p - for pid - flag argument.
* Also, at least in GNU top, when running top, you can press o and then enter COMMAND=i3status for the same effect