Watch command to execute script/shell command repeatedly

Learn watch command to execute script or shell commands repeatedly every n seconds. Very much useful in automation or monitoring.

“watch” command is small utility using which you can execute shell command or script repetitively and after every n seconds. Its helpful in automation or monitoring. Once can design automation by monitoring some code/command output using watch to trigger next course of action e.g. notification.

watch command is part of procps package. Its bundled with OS still you can verify if package is installed on system. This utility can be used directly issuing watch followed by command/script name to execute.

For example, I created small script which writes junk data continuously in file placed under /. This will change utilization numbers in df -k output. In above GIF, you can see changes in “Used” and “Available” column of df -k output when monitored with watch command.

In output, you can see –

1. Default time interval is 2 seconds as shown n first line
2. Time duration followed by command which is being executed by watch
3. Current date, time of server on right hand side
4. Output of command being executed

Go though below watch command examples to understand how flexible watch is.

Different options of watch:

Now, to change default time interval use option -n followed by time interval of your choice. To execute command after 20 seconds you can use :

See above output, interval is changed to 20 seconds (highlighted row)

If you want to hide header in output i.e. time interval, command being executed and current server date, time, use -t option. It will strip off first line of output.

Highlighting difference in present and previous output is made easy with -d option. To understand this, watch below GIF –

In above output, I used same data writing script to fill /. You can observe only portion which is different from previous output is being highlighted by watch in white box!