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Limit on number of pipes after long-running command?


 
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# 1  
Limit on number of pipes after long-running command? [SOLVED]

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

Last edited by DevuanFan; 05-16-2019 at 01:10 PM..
# 2  
grep filters lines and shows only lines which contain "press" (case sensitive).

What's the output of xinput test 6 in all cases?
# 3  
And so?
Code:
$ xinput test 6 | grep press | awk '{print $0}'

# 4  
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.
# 5  
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}'

# 6  
@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.

Last edited by DevuanFan; 05-16-2019 at 12:20 PM..
# 7  
Quote:
Originally Posted by DevuanFan
@vgersh: 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.
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

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