Question

When I do this:

find . -name 'pattern' | grep 'another-pattern' 

Are the processes, find and grep, spawned together? My guess is yes. If so, then how does this work?:

yes | command_that_prompts_for_confirmations 

If yes is continuously sending ‘y’ to stdout and command_that_prompts_for_confirmations reads ‘y’ whenever it’s reading its stdin, how does yes know when to terminate? Because if I run yes alone without piping its output to some other command, it never ends.

But if piping commands don’t spawn all the processes simultaneously, then how does yes know how many ‘y’s to output? It’s catch-22 for me here. Can someone explain me how this piping works in *NIX?

Answer 1

From the wikipedia page: ‘By itself, the yes command outputs ‘y’ or whatever is specified as an argument, followed by a newline, until stopped by the user or otherwise killed; when piped into a command, it will continue until the pipe breaks (i.e., the program completes its execution).’

yes does not ‘know’ when to terminate. However, at some point outputting ‘y’ to stdout will cause an error because the other process has finished, that will cause a broken pipe, and yes will terminate.

The sequence is:

1. other program terminates
2. operating system closes pipe
3. yes tries to output character
4. error happens (broken pipe)
5. yes terminates