Jenkins console output not in realtime

Pretty new to Jenkins and I have simple yet annoying problem. When I run job (Build) on Jenkins I am triggering ruby command to execute my test script.

Problem is Jenkins is not displaying output in real time from console. Here is trigger log.

Building in workspace /var/lib/jenkins/workspace/foo_bar
No emails were triggered.
[foo_bar] $ /bin/sh -xe /tmp/hudson4042436272524123595.sh
+ ruby /var/lib/jenkins/test-script.rb

Basically it hangs on this output until build is complete than it just shows full output. Funny thing is this is not consistent behavior, sometimes it works as it should. But most of the time there is no real time console output.

Jenkins version: 1.461

To clarify some of the answers.

ruby or python or any sensible scripting language will buffer the output; this is in order to minimize the IO; writing to disk is slow, writing to a console is slow...
usually the data gets flush()'ed automatically after you have enough data in the buffer with special handling for newlines. e.g. writing a string without newline then sleep() would not write anything until after the sleep() is complete (I'm only using sleep as an example, feel free to substitute with any other expensive system call).

e.g. this would wait 8 seconds, print one line, wait 5 more seconds, print a second line.

from time import sleep

def test():
    print "ok",
    time.sleep(3)
    print "now",
    time.sleep(5)
    print "done"
    time.sleep(5)
    print "again"

test()

for ruby, STDOUT.sync = true, turns the autoflush on; all writes to STDOUT are followed by flush(). This would solve your problem but result in more IO.
```
STDOUT.sync = true
```
for python, you can use python -u or the environment variable PYTHONUNBUFFERED to make stdin/stdout/stout not buffered, but there are other solutions that do not change stdin or stderr
```
export PYTHONUNBUFFERED=1
```
for perl, you have autoflush
```
autoflush STDOUT 1;
```

Make sure your script is flushing its stdout and stderr. In my case I had a buffering issue similar to what you describe but I was using python. The following python code fixed it for me:

import sys
sys.stdout.flush()

I'm not a Ruby coder, but Google reveals the following:

$stdout.flush

It seems to me that python -u works as well.

E.g. In batch command

python -u foo.py

Jenkins console output not in realtime

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