Watch: Only print to screen if output has changed since last output

Question

I'm running a process and I'm counting the number of threads with

ps huH p <PID_OF_U_PROCESS> | wc -l

I can run this thread with watch like this;

watch -n 1 ps huH p <PID_OF_U_PROCESS> | wc -l

This will output the number of threads the process is running, but usually that number doesn't change.

How can I only print the new number to screen if it changed from the last time the command was run?

For example:

64 65 64 (a few minutes go by) 65

Etc.

Answer 1

You could just pipe to uniq:

while ps -o nlwp= -p "$pid"; do sleep 1; done | uniq

Answer 2

watch is not capable of doing this directly. Although you can highlight differences in the command output (via option -d) or exit when the output changes (via option -g) it is not possible to display the output of more than one run. But you can achieve this via other common tools.

One of many possible solutions:

last=""; while true; do cur="$(ps h -o nlwp -p <PID>)"; if ! [ "$cur" = "$last" ]; then last="$cur"; echo "$(date) $cur"; fi; sleep 1; done

Or more readable:

last=""
while true
do
    cur="$(ps h -o nlwp -p <PID>)"
    if ! [ "$cur" = "$last" ]
    then
        last="$cur"
        echo "$(date) $cur"
    fi
    sleep 1
done

Explanation: The ps option -o nlwp directly prints the number of threads, so you don't have to call wc -l. You can also use $(pidof programname) instead of <PID> in order to determine the process ID automatically. I also added the current date via $(date) which seems useful to me. If you don't like it then just remove it.