pipe bug question

[paratrap]

Hello!

I'm trying to read from 'xinput test 14' to variable and got very strange behaviour. Can anyone explain why:

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xinput test 14 | awk '{print $3}'

outputs (when pressed ABC on keyboard):

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38
56
54

and:

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xinput test 14 | awk '{print $3}' | cat

outputs nothing. Yet at the same time:

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echo ok | awk '{print $1}' | cat

works as expected. BTW 'xinput test 14' - 14 - main keyboard.

[LE_746F6D617A7A69]

Strange. For Me it works like this: (pressed a, b, c, then ctrl-c)

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$> xinput test 10 | awk '{print $3}' | cat
abc^C

and this:

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$> xinput test 10 | awk '{print $1}' | cat
abc^C

It looks like xinput is printing directly to terminal somehow, not to stdout -> awk gets nothing, and so nothing is passed to cat
The echo is printing to stdout, so the first pipe works as expected.

As you can see, on my system xinput is printing the letters, not keycodes. (Debian10 amd64, xfce-terminal)
So the question is: what system You're using and which terminal emulator? Do You use standard xorg config?

[paratrap]

I'm using Debian 10 + lightdm + gnome-terminal.

To be exact first output of 'xinput test 14 | awk '{print $3}'' is:

a38
56
b56
54
c54

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$ xinput test 14 | awk '{print $3}' | cat 
abc

and second output is 'abc' because I add 'cat' to the end. Which is very wired behavior. Like someone changing IFS or other environment variables (witch I checked - everything is ok) or deliberately changing output pipe number. I do not understand what is going on.

And full output of 'xinput test 14' is:

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key press   38 
akey release 38 
key press   56 
bkey release 56 
key press   54 
ckey release 54 

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xinput list
⎡ Virtual core pointer                    	id=2	[master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer              	id=4	[slave  pointer  (2)]
⎜   ↳ KYE SYSTEMS CORP. Wired Mouse           	id=9	[slave  pointer  (2)]
⎜   ↳ ELAN1301:00 04F3:3115 Mouse             	id=11	[slave  pointer  (2)]
⎜   ↳ ELAN1301:00 04F3:3115 Touchpad          	id=12	[slave  pointer  (2)]
⎣ Virtual core keyboard                   	id=3	[master keyboard (2)]
    ↳ Virtual core XTEST keyboard             	id=5	[slave  keyboard (3)]
    ↳ Power Button                            	id=6	[slave  keyboard (3)]
    ↳ Video Bus                               	id=7	[slave  keyboard (3)]
    ↳ Power Button                            	id=8	[slave  keyboard (3)]
    ↳ USB2.0 HD UVC WebCam: USB2.0 HD         	id=10	[slave  keyboard (3)]
    ↳ Asus WMI hotkeys                        	id=13	[slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard            	id=14	[slave  keyboard (3)]

[LE_746F6D617A7A69]

I don't have the "AT Translated Set 2 keyboard" on the list, and for other keyboard devices xinput just prints the letters, not events.
However, it works with the mouse device:

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xinput --test 8
motion a[0]=447 
motion a[1]=932 
motion a[0]=447 
^C

I have checked that the redirection works at the awk program level:

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xinput --test 8 | awk '{print $2 | "cat -"}'
a[0]=607
a[0]=606
a[1]=898
a[1]=899
a[0]=605

So, it seems that the problem is related to awk

But anyway, this is just another example of UUOC

[paratrap]

Nah. It wasn't about 'cat award' actual code is:

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while read -r LINE; do echo $LINE; done < <(xinput test 14 | awk '{print $3}')

I have same issue with '< <()' or ' | cat'. You saying it is awk issue, but I gave an example which works:

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echo ok | awk '{print $1}' | cat

So, I still have no idea why I can't ' < <()' xinput test + awk

[Head_on_a_Stick]

actual code is

You really should have put that in the OP...

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while read -r line ; do echo ${line##* } ; done < <(xinput test 14)

I think your "pipe bug" is due to the fact that bash runs the last element of the pipe in a subshell: http://mywiki.wooledge.org/BashFAQ/024

[paratrap]

I do not know how to explain this better then I already did. This line works:

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xinput test 14 | awk '{print $3}'

This line gives no output (except bash echo's symbols your type):

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xinput test 14 | awk '{print $3}' | cat

sub shell is not an issue here. Just regular bash commands. If you could confirm those lines works on your machine or they don't it would help a lot (do not forget to update keyboard id == 14) !

[LE_746F6D617A7A69]

Using only bash built-ins:

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while read -a line; do echo ${line[1]}; unset line; done < <(xinput test 8)

What it does:
1. Read line from xinput into a string array, using default IFS
2. Print 2nd word from the array (indexes are zero-based)
3. Clear the array
4. goto 1

[Head_on_a_Stick]

sub shell is not an issue here

Yes, I was wrong about that. My apologies.

Tomazzi is right, it's an awk issue:

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xinput test 14 | awk '{print $3;fflush(stdout)}' | cat

See also https://superuser.com/questions/379122/ ... utput-file

[paratrap]

Wow! Thank you very match. This is actually a solution. Indeed awk does not fflush! How did I never run into this issue yet!!!

[Head_on_a_Stick]

This is actually a solution

Well not really, awk isn't needed at all — just use either a bash array or parameter expansion (which can be used with any Bourne shell) to extract the information.

Anyway, please mark the thread [SOLVED] for the benefit of others. Thanks.