Shell Programming and Scripting

I'm trying to create a minimal, crude keylogger for X using only a shell script. I was quickly stumped: Why do these two commands entered in a terminal emulator produce output when I type...

Code:

$ xinput test 6 | grep press

Code:

$ xinput test 6 | awk '{print $3}'

...but this command produces no output:

Code:

$ xinput test 6 | grep press | awk '{print $3}'

?

This observation is really challenging my understanding of stdout, stdin, and pipes.

BTW, 6 is the id of my keyboard, which I found by running xinput list. My shell is bash version 5.0.3

grep filters lines and shows only lines which contain "press" (case sensitive).

What's the output of xinput test 6 in all cases?

And so?

Code:

$ xinput test 6 | grep press | awk '{print $0}'

Here is the output in the terminal when I type unix after executing the command:

Code:

$ xinput test 6
key press   30 
key release 30 
key press   57 
key release 57 
key press   31 
key release 31 
key press   53 
key release 53

This is the output I want (you'd think this would be the output of the third command in my original post when, in fact, that command outputs nothing):

Code:

30
57
31
53

--- Post updated at 03:00 PM ---

@nezabudka: The command you suggested generates no output, either.

I don't know what you mean by 6 is the id of my keyboard, which I found by running xinput list, but how about:

Code:

xinput test 6 6>&1 | awk '/press/ {print $3}'

@vgersh99: X input devices have id's, which vary from machine to machine (on my machine, X sees the keyboard as input id=6 and mouse as input id=7, for instance).

Yes, the command you suggested gives the desired output when I type unix:

Code:

$ xinput test 6 | awk '/press/ {print $3}'
30
57
31
53

But it still leaves unanswered why we can only use one pipe after xinput test 6. For instance, this command produces no output when I type unix, although I'd expect grep to generate some ouput when u is pressed:

Code:

$ xinput test 6 | awk '/press/ {print $3}' | grep 30

What I'm trying to understand is why one pipe after xinput test 6 works fine, but two pipes never produce any output even when output is expected. That's the point of the thread.

well, it's not the "number" of pipes, but the output redirection of xinput:

Code:

xinput test 6 6>&1 | awk '/press/ {print $3}'

seems that xinput test 6 put its output not to stdout stream, but to stream with fileDescriptor 6, but the following awk reads its input from stdin (where stdin has the fd=1).
Hence, we need to map fd=6 of xinput to fd=1: 6>&1 for the awk to read its input from fd=1