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When I use the -p or --processes option, I get the output like this:

LISTEN   0   511   ...   ...   users:(("nginx",4209,255),("nginx",18819,255))

I know ("nginx",4209,255) means the nginx process with pid 4209 is using the socket, but what is the meaning of 255, what does it stand for?

I looked up in manuals but didn't find the answer.

2

That's file number descriptor. You can read these line from source code to see it.

On Debian system, you will get a better output:

tcp    ESTAB      0      0   ...      ...    users:(("skype",pid=3074,fd=67))

Base on your output, I guess you're on a Redhat or Ubuntu base systems.

  • Cuong, Ubuntu also does not show fd in front.. – heemayl Oct 30 '15 at 10:11
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    @heemayl: Yes, I also checked with Linux Mint. It seems that Redhat base and Ubuntu base don't show it. – cuonglm Oct 30 '15 at 10:14

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