How to use watch command with a piped chain of commands/programs
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 $() is run just once and the resulting output string is always printed ("watched") as a literal.
So, how do I make the watch command work with a piped chain of commands [other that putting them inside a script]?
watch 'command | othertool | yet-another-tool'
watch -n 1 "ls -lrt | tail -n20; date"
let’s you pipe and run in a row.
If you would like to list all files in subdirectories too, you can use find command with exec option.
watch will update every 30 seconds and find will search for all *.log files in current dir (subdirs included) and will print filenames and their last 10 line:
watch -n30 'find . -name "*.log" -print -exec tail -n10 {} ; '
Use a combination of single quotes (') and double quotes ("). For example:
watch -n 1 "links -dump 127.0.0.1/server-status | grep -e 'S' -Fe 'www.'"
The simplest way is using inbuilt option -x,
watch -n 5 -x tail -4 output.log
watch -n 2 -x ls
The first eg. will display the last 4 lines of output.log file every 5 sec., second eg list content every 2 sec.
I have just made a little bit more complex pipe chain for watching ElasticSearch nodes’ performance. It requires handling variables as well as variable escaping. Hope it may be helpful to others.
watch -dn1 'kubectl exec elasticsearch-master-0 -c elasticsearch --
curl -s XGET "localhost:9200/_cat/nodes?h=hp,hc,hm,rp,r,rm,cpu,dup,du,dt,m,n&v" |
(read -r; printf "%sn" "$REPLY"; awk "{print $NF,$0}" |
sort | cut -f2- -d" ")'
Let’s say that you run a command and then want to watch it; use:
ls -lrt | tail -n20; date
watch "!!"