Possible bug with grep/sed/tail?

[David Corlette]

Hello there,

I am not sure this is actually a bug as there may be reasons why it has to be 
this way, but certainly these tools work counter to my expectations.

I am trying to read lines from a log file and pass them through grep or sed, 
and then on to another tool (netcat, for example).  What I am seeing is that:

A) # tail trace.txt | grep "com"
- WORKS: produces output
B) # tail trace.txt | grep "com" | cat
- WORKS: produces output
C) # tail -f trace.txt | grep "com"
- WORKS: produces output, then waits and reports new lines
D) # tail -f trace.txt | grep "com" | cat
- FAILS: no output from existing lines, never gets new data

Similarly:
A) # tail trace.txt | sed s/com/newcom/
- WORKS: produces output
B) # tail trace.txt | sed s/com/newcom/ | cat
- WORKS: produces output
C) # tail -f trace.txt | sed s/com/newcom/
- WORKS: produces output, then waits and reports new lines
D) # tail -f trace.txt | sed s/com/newcom/ | cat
- FAILS: no output even from existing lines, no new data reported

To me, it seems completely counterintuitive that A, B, and C would work, but D 
does not.  Each line of input read by tail should be passed to STDOUT, which is 
then read as STDIN by grep/sed, then passed to STDOUT and read by cat.  It 
should not matter if tail is "done" reading the output or not, as clearly that 
works fine in case C.

Thoughts?  Is there some limitation of which I am not aware that prevents this 
from working?  Where does the data go in case D?

Details:
Linux moolap 2.6.16.60-0.30-smp #1 SMP Thu Aug 28 09:26:55 UTC 2008 x86_64 
x86_64 x86_64 GNU/Linux
tail (GNU coreutils) 5.93
GNU sed version 4.1.4
grep (GNU grep) 2.5.1