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?

Answer 1

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.

Answer 2

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.