Unix & Linux Asked by Tulains Córdova on October 10, 2020
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]?
watch 'command | othertool | yet-another-tool'
Correct answer by DopeGhoti on October 10, 2020
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.
Answered by amol on October 10, 2020
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.'"
Answered by Dimitar Velev on October 10, 2020
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 {} ; '
Answered by yoyomanYo on October 10, 2020
watch -n 1 "ls -lrt | tail -n20; date"
let's you pipe and run in a row.
Answered by Soft Dot IE on October 10, 2020
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