What is this yellow thing with a pipe? [duplicate]

Question

I noticed that /var/spool/nullmailer directory contains a weird trigger entity which is displayed in yellow and has a “|” suffix.

It doesn't seem to be a directory (at least I can't cd into it), and when I try to open it with vi, the console freezes.

What is that thing?

That's a great clickbait title! :D +1 – Gallifreyan Feb 25 '17 at 15:51 — Gallifreyan, Feb 25 '17 at 15:51

Zanna · Accepted Answer · 2017-02-25 22:46:51Z

It is a named-pipe - one of the 7 types of file in Unix. The p in prw--w--w- confirms it is a named-pipe.

You can write to the pipe from one side and read the data from the other side:

$ mkfifo mypipe
$ cat mypipe

In another terminal

$ cat > mypipe
Hello from the other side
What's up?

Whatever you type in here will appear in the other terminal.

l is an alias to ls -CF and ll is alias to ls -alFh.
According to the man page

-F, --classify
    append indicator (one of */=>@|) to entries

So for a named-pipe | is appended to the filename.

djsmiley2k - CoW · Accepted Answer · 2017-02-26 02:04:51Z

3

You can identify any 'file' on a linux system by using the command file, followed by the file name.

file "trigger"

Would tell you it's a named pipe.

answered Feb 25 '17 at 23:37

djsmiley2k - CoW

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2

The filename doesn't include the pipe symbol, so it should be file trigger. This can be seen with mkfifo trigger; ll trigger; file "trigger|" – Chai T. Rex Feb 26 '17 at 1:42
@ChaiT.Rex good catch, i didn't notice that is just weird formating they have in ls ! – djsmiley2k - CoW Feb 26 '17 at 2:05

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