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I've written a command line that effectively extracts the highest CPU java PID thread process from top -H (example code):

top -H -n 1 | grep "java" | head -n 1 | cut -d' ' -f1

I want to inspect the PID in jstack. Due to how fast the threads appear and disappear, it's not possible to enter the PID manually, and I was hoping to pipe the result directly into jstack, however whenever one does, for example (simplest code example that reproduces the issue):

12345 | jstack

jstack merely throws up the usage help page, as if pipe isn't sending the variable to jstack.

How can I get the PID I've got with my command into jstack?

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jstack expects the process id to be provided as a parameter, so you should use command substitution:

jstack "$(top -H -n 1 | grep "java" | head -n 1 | cut -d' ' -f1)"

You can use ps to find the process instead of filtering top’s output:

jstack "$(ps -C java -o pid --sort %cpu --no-headers | head -n 1)"

This uses ps to find processes whose command matches java, outputs their PID only, sorted by CPU usage, with no headers, and keeps the first one; the result is given to jstack.

To find the thread ID using the most CPU, output tid instead, with the -L option to get ps to process threads:

ps -L -C java -o tid --sort %cpu --no-headers | head -n 2

(I’m extracting the first two because the first will always match the PID, which groups all the CPU usage for the process as a whole).

You can use printf to output that in hexadecimal:

printf "%x\n" $(ps -L -C java -o tid --sort %cpu --no-headers | head -n 2)
  • Using top -H to inspect individual thread processes (as ps lumps all java threads together) as I'm trying to track down individual Java processes that are hogging CPU. Wasn't aware variable substitution was a thing, so thank you for solving my problem! – SSight3 Oct 24 '18 at 13:37
  • I realise you were using top -H, however since you’re extracting the PID from top, not the TID, I didn’t think there would be any practical difference with the ps approach — do you have a scenario where there is? – Stephen Kitt Oct 24 '18 at 13:40
  • Unfortunately, I am not knowledgeable enough to comment. I was attempting to follow (somewhat) the steps detailed here: stackoverflow.com/a/15815829/889894 – SSight3 Oct 24 '18 at 14:02

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