What is the watch command in UNIX?

The watch command runs a given command repeatedly and displays the output, refreshing the screen with each interval. This allows a command to be watched and produces functionality similar to the top command. The command will run until terminated, usually with CTRL-C. The watch command is very useful for generating a real-time view of events that are happening on an operating system.

Watching a file download

In this example an large file is being downloaded via a browser. Using watch the download folder can be monitored to see if the download has completed.

watch -n 0.5 ls -a /isos

The -n option sets the interval at which the screen refreshes in seconds. In this case it is set to half a second.

Of course for downloading isos you would be much better off using a torrent if you can!

Watching a network interface come up

In this example a wireless network connection is enabled and a connection is established. Using watch a real-time of the connection being established and an ip address being assigned can be seen. The following command will refresh every 0.5 seconds.

watch -n 0.5 ip a

The command will refresh as the and update connection is established and eventually it shows the connection is up and the ip address is assigned.

2: wlp3s0:  mtu 1500 qdisc fq_codel state DOWN group default qlen 1000
  link/ether fe80::fe80:fe80:fe80:fe80 brd ff:ff:ff:ff:ff:ff

2: wlp3s0:  mtu 1500 qdisc fq_codel state DORMANT group default qlen 1000
  link/ether fe80::fe80:fe80:fe80:fe80 brd ff:ff:ff:ff:ff:ff
  inet6 fe80::fe80:fe80:fe80:fe80/64 scope link tentative
  valid_lft forever preferred_lft forever

2: wlp3s0:  mtu 1500 qdisc fq_codel state UP group default qlen 1000
  link/ether fe80::fe80:fe80 brd ff:ff:ff:ff:ff:ff
  inet6 fe80::fe80:fe80:fe80:fe80/64 scope link tentative
  valid_lft forever preferred_lft forever

2: wlp3s0:  mtu 1500 qdisc fq_codel state UP group default qlen 1000
  link/ether fe80::fe80:fe80:fe80:fe80 brd ff:ff:ff:ff:ff:ff
  inet 192.168.43.53/24 brd 192.168.43.255 scope global wlp3s0
  valid_lft forever preferred_lft forever
  inet6 fe80::fe80:fe80:fe80:fe80/64 scope link
  valid_lft forever preferred_lft forever

In this example using watch is a much more efficient way of seeing whether a connection is up than repeatedly running ip a or pinging some external site.

Showing the most CPU intensive processes

The ps can show detailed information on running processes. When combined with the sort and head commands it can provide a snapshot of processes at a single point in time.

The following provides a snapshot of the most intensive CPU processes.

ps aux | sort -nrk 3,3 | head -n 5

This is a good example of a where the watch command makes something much more useful. By using the watch command the command can be turned in a real-time top like view of CPU intensive. If you are running Google Chrome you will probably see all five processes coming from Chrome!

watch "ps aux | sort -nrk 3,3 | head -n 5"

Further reading

• watch man page
• How to use the watch command