Why does a named pipe not get deleted after system restart

fifopipe

As far as I understand, named pipes are not written to disk but are stored in memory. Here's how I created a named pipe –

$ mkfifo pipe21
$ grep "simple" SimpleDoc.txt > pipe21 &
[1] 2775
$ cat pipe21
Very simple doc that contains plaintext. 
[1]+  Done                    grep --color=auto "simple" SimpleDoc.txt > pipe21

After this, I restarted my system. However, even after the restart, I see pipe21 in my current directory. Why is that?

Best Answer

No they're written to disk. The command mkfifo pipe21 creates the corresponding device on your filesystem. Often times these devices are kept under /dev but named pipes (aka. FIFOS) don't necessarily have to be kept in this directory.

excerpt from wikipedia article

The named pipe can be deleted just like any file:
$ rm my_pipe

Example

Make a FIFO:

$ pwd
/home/saml

$ mkfifo pipe21

Check out the FIFO device:

$ ls -l | grep pipe
prw-rw-r--   1 saml saml        0 Jul 24 12:22 pipe21

$ file pipe21 
pipe21: fifo (named pipe)

Delete the device:

$ rm pipe21 

$ ls -l | grep pipe

References

Named Pipes - Wikipedia

Related Solutions

Does named pipe modify the filesystem

The file object itself is created in the filesystem, but no data is stored in a file system. From the mkpipe(3) manpage:

   A  FIFO special file is similar to a pipe, except that it is created in
   a different way.  Instead of being an anonymous communications channel,
   a  FIFO  special  file  is  entered  into  the  file  system by calling
   mkfifo().

About the only time the data might be stored on disk is during hibernation when memory is written to the swap space, including buffers - however this an corner case.

Problem with pipes. Pipe terminates when reader done

As for the cause, use strace.

tail -f | strace bash >> foo

The second echo echo hello > pToB gives me then this:

rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
read(0, "e", 1)                         = 1
read(0, "c", 1)                         = 1
read(0, "h", 1)                         = 1
read(0, "o", 1)                         = 1
read(0, " ", 1)                         = 1
read(0, "h", 1)                         = 1
read(0, "e", 1)                         = 1
read(0, "l", 1)                         = 1
read(0, "l", 1)                         = 1
read(0, "o", 1)                         = 1
read(0, "\n", 1)                        = 1
write(1, "hello\n", 6)                  = -1 EPIPE (Broken pipe)
--- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=3299, si_uid=1000} ---
+++ killed by SIGPIPE +++

So, the second time it tries to write hello\n, it gets a broken pipe error; that's why you can't read hello (it was never written), and bash quits so that's the end of it.

You'd have to use something that keeps the pipe open, I guess.

How about this?

(while read myline; do echo $myline; done) < pToP

For more background information, man 7 pipe may be relevant, it describes the various error cases around pipes.