Skillfully use the watch command to perform loop operations to free our hands

Sometimes we need to execute a command repeatedly to see how a file and a result change. You can write scripts to implement these requirements, but there are simpler ways to do so. This document describes the watch command.

1. Repeat a command for a fixed time

root@jaking-virtual-machine:~# watch -n 1 cat hello.txt

Every 1.0s: cat hello.txt

jaking-virtual-machine: Tue Mar 19 19:13:33 2019

Hello World!

Hello Jaking!

2. Highlight Change Content

root@jaking-virtual-machine:~# watch -d uptime #To highlight changes, use the-d (difference) argument.

Every 2.0s: uptime

jaking-virtual-machine: Tue Mar 19 19:14:01 2019

19:14:01 up 3 days, 12:53, 2 users, load average: 0.01, 0.01, 0.00

(Omitted here, the change content will be highlighted, very easy to observe)

3. Exit on execution error

root@jaking-virtual-machine:~# watch -n 1 -e cat hello.txt #Run a command that ends when the exit code is not 0, that is, when the command execution error occurs, you can use the-e (errexit) parameter.

Every 1.0s: cat hello.txt

jaking-virtual-machine: Tue Mar 19 19:16:49 2019

Open another terminal, perform mv operation, you can see the effect:

root@jaking-virtual-machine:~# mv hello.txt /tmp

#new terminal

root@jaking-virtual-machine:~# watch -n 1 -e cat hello.txt

#Old Terminal

Every 1.0s: cat hello.txt

jaking-virtual-machine: Tue Mar 19 19:16:49 2019

cat: hello.txt: No such file or directory

4. Exit when execution results change

root@jaking-virtual-machine:~# watch -n 1 -g 'du -b hello.txt'

Every 1.0s: du -b hello.txt

jaking-virtual-machine: Tue Mar 19 19:23:41 2019

27 hello.txt

Open another terminal to perform echo operation, you can see the effect:

root@jaking-virtual-machine:~# echo "watch -n -l -g command" >> hello.txt

#new terminal

root@jaking-virtual-machine:~# watch -n 1 -g 'du -b hello.txt'

#Old Terminal

Every 1.0s: du -b hello.txt

jaking-virtual-machine: Tue Mar 19 19:21:55 2019

50 hello.txt

#watch -n 1 -g 'du -b hello.txt' ends

root@jaking-virtual-machine:~#