There many GNU/Linux command line tools that provide continuously updated statistics or information like iotop, nload, and top. However, it is often necessary to see the output of another simple command or complex chain of tools, updated in real-time.
watch is a Linux command that does just that, and by doing so, is incredibly useful for continuously monitoring the output of any Linux terminal command. Usage of the watch command is incredibly simple, as the following example monitoring the currently running tsp task in the task-spooler queue demonstrates.
watch "tsp | head"
By default, the output is usually updated every 2 seconds, but this can be configured with the -n/--interval argument (provided in seconds).
# Here return output every 30 minutes
watch -n 1800 "tsp | head"
To highlight any changes in the output, provide the -d/--differences argument.
# Highlight if the running task has changed
watch -d "tsp | head"
In addition to providing a terminal based ad-hoc dashboard solution, it can be used control the flow of a command string or shell script if so required. The -g/--chgexit will exit watch when the output of the command changes.
# Run second command (ls) when task-spooler started the next job.
watch -g "tsp | head" && tsp -t