[TriLUG] How to debug piping issues

[Brian via TriLUG]

Hi there!

I thought I'd be clever and use zbarcam (a webcam barcode reader) and 
xdotool (a keyboard/mouse event generator) to create a 
keyboard-wedge-style barcode reader.  Here's the command I use:

$ zbarcam --raw | xdotool type --file -

The --raw option tells zbarcam to write out only the data in the barcode 
it read, plus a newline.

The type command to xdotool means generate some typing events
The --file - option to that command means read standard input for what 
to type

Only thing is, it doesn't work.  xdotool doesn't type anything.  But I 
can do this:

$ zbarcam --raw | tee afile
[scan a code][press ^C]
$ cat afile | xdotool type --file -

... and it works, typing out the barcode I scanned.

So..  Why does a direct pipe not work?  Piping out of zbarcam clearly 
works (tee writes stuff to the file) and piping in to xdotool clearly 
works (xdotool types the stuff cat-piped into it).  What could be going 
wrong here?

I got around the problem by writing a perl script that reads from stdin 
and system()s a call to xdotool, but I'd still love to understand what's 
going on with the simple pipe...

Thanks,
-B