will a named pipe always be size 0 on filesystem?

UNIX for Advanced & Expert Users
1

I did

cat < myFile >> myPipe

I was hoping that if I did ls -l, myPipe would now be holding the contents of myFile, and would be the same size. But it was 0.

Also strange was that when I did the command above, cat did not return control back to the shell. Why?

thanks

2

Your command doesn't return back control to the shell until some process reads the content of the named pipe.

After issuing that command, open another shell and type "cat myPipe": at this point the content of myPipe (that is, the content of myFile) is displayed and the first shell will return to the prompt.

About the zero size, yes, a named pipe doesn't use space on disk. Otherwise what would be the difference between a named pipe and a temporary file?

To be honest, on this point I am not totally sure, maybe googling around about this argument may clear our ideas. Or maybe some more experienced user could put light on this doubt :slight_smile:

3

thanks for your answer, it gives me all the information I needed

4

There are several different implementations of named pipes and OS's often switch to move to a faster implementation. With some implementations, a non-zero size is possible but it's been about 25 years since I saw an example of that (HP-UX 3.1). You should ignore the size field for a named pipe.

5

thanks for your answer