Problem with line buffered IO when no tty

[Steven W. Orr]

I have a problem with whether things should or should not be line buffered vs. 
fully buffered. Here's the deal:


#! /bin/bash

sleep 6
cat <<EOF
Here are two
lines to stdout.
EOF
sleep 6
cat <<EOF 1>&2
And two more
to stderr
EOF
exit 22

When I run this program, it does what it's supposed to do. I get two lines of 
output going to stdout (after 6 seconds) and then I get two more lines to 
stderr (after another 6 seconds).

Now the fun begins: I run the same program using ssh. Because I'm using ssh, 
the program is not running from a console, and so, should be setting its 
output buffers to fully buffered (i.e., not line buffered). In fact, if I just 
run the tty command using ssh, the answer I get back is "not a tty".

But, I actually do get line buffered IO. I get the stdout stuff as it happens, 
and the stderr stuff when that happens.

Why do I care you ask? I care because the remote command I'm running was 
previously written in bash and now it got translated to python. It takes about 
20 minutes to run. Now that it's in python, it runs the same way when run 
locally, but when I run it through ssh, I see *no* output or err channel info 
till completion. (I used to see lots of progress messages.) I can demonstrate 
the same result with a small python program:

[swagent@kodiak ~]$ cat xxx.py
#! /usr/bin/python

import time
import sys

time.sleep(6)
print """Here are two
lines to stdout."""
time.sleep(6)
print >> sys.stderr, """And two more
to stderr"""
sys.exit(22)

If you run this program locally, it does the same thing. BUT! if you run it 
remotely, the two channels all block up till program exit and it all comes out 
in one squirt.

GNU bash, version 4.0.35(1)-release (x86_64-redhat-linux-gnu)

So, my question is this: Is this a bug in bash and python is doing it right? 
Or is this some enhancement that bash did to always get line buffering? Or is 
there another explanation?

TIA

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