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I usually use watch Linux utility to watch the output of a command repeatedly every n seconds, like in watch df -h /some_volume/.

But I seem not to be able to use watch with a piped series of command like:

$ watch ls -ltr|tail -n 1

If I do that, watch is really watching ls -ltr and the output is being passed to tail -n 1 which doesn't output anything.

If I try this:

$ watch (ls -ltr|tail -n 1)

I get

$ watch: syntax error near unexpected token `ls'

And any of the following fails some reason or another:

$ watch <(ls -ltr|tail -n 1)

$ watch < <(ls -ltr|tail -n 1)

$ watch $(ls -ltr|tail -n 1)

$ watch `ls -ltr|tail -n 1)`

And finally if do this:

$ watch echo $(ls -ltr|tail -n 1)

I see no change in the output at the given interval because the command inside $() is run just once and the resulting output string is always printed ("watched") as a literal.

So, how do I make the watch command work with a piped chain of commands [other that putting them inside a script]?

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  • 2
    run man watch and scroll down to examples... – don_crissti Oct 25 '16 at 17:57
185
watch 'command | othertool | yet-another-tool'
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  • 1
    Might be worth noting the generic approach as well watch sh -c 'command | etc' particularly looking at the approaches tried in the question. – sourcejedi Oct 25 '16 at 18:08
  • @sourcejedi I haven't really figured out why, but this won't always produce the same results. – Michael Mior Oct 16 '18 at 1:25
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watch -n 1 "ls -lrt | tail -n20; date"

let's you pipe and run in a row.

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  • what is -n 1 for? – Nam G VU Mar 8 '19 at 4:50
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    @NamGVU: Run the command every 1 second – Duc Tran Mar 13 '19 at 18:46
3

If you would like to list all files in subdirectories too, you can use find command with exec option.

watch will update every 30 seconds and find will search for all *.log files in current dir (subdirs included) and will print filenames and their last 10 line:

watch -n30 'find . -name "*.log" -print -exec tail -n10 {} \; '
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Use a combination of single quotes (') and double quotes ("). For example:

watch -n 1 "links -dump 127.0.0.1/server-status | grep -e '\S' -Fe 'www.'"
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The simplest way is using inbuilt option -x,

watch -n 5 -x tail -4 output.log.  
watch -n 2 -x ls

The first eg. will display the last 4 lines of output.log file every 5 sec., second eg list content every 2 sec.

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  • Neither watch -x 'ls -ltr | tail -n 10' nor watch -x ls -ltr | tail -n 10 work! – John Lawrence Aspden Nov 15 '20 at 10:30
  • Maybe you are right. -x can be useful to execute something instead of sh -c. For your purpose you can just go with watch 'ls -ltr | tail -n 10' – amol Nov 16 '20 at 11:21

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