Effectively disables buffering of stdio on the child process.
This effect is removed if the input is proxied between unbuffer and the target program.
This works because eglibc disables buffering on stdin and stdout if they look like a TTY (console), but enable buffering if it's a pipe. This jumps through all of the hooks to create a virtual terminal (PTY) to do this fooling.
Normally, processes which detect that stdin is a pipe do processing in batch mode. In the example below, Python does not actually execute the print command until stdin closes.
$ (echo "print 'python'"; sleep 1; echo bash>&2; sleep 1;) | python
bash
pythonHere, it executes the print command immediately.
$ (echo "print 'python'"; sleep 1; echo bash>&2; sleep 1;) | ./unbuffer python
print 'python'
Python 2.7.9 (default, Jan 1 2015, 14:20:57)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> python
>>> bash