unbuffer − unbuffer output

unbuffer program [ args ]

unbuffer disables the output buffering that occurs when
program output is redirected.  For example, suppose you are
watching the output from a fifo by running it through od and
then more.

     od ‐c /tmp/fifo | more

You will not see anything until a full page of output has
been produced.

You can disable this automatic buffering as follows:

     unbuffer od ‐c /tmp/fifo | more

Normally, unbuffer does not read from stdin.  This
simplifies use of unbuffer in some situations.  To use
unbuffer in a pipeline, use the ‐p flag.  Example:

        process1 | unbuffer ‐p process2 | process3

unbuffer ‐p may appear to work incorrectly if a process
feeding input to unbuffer exits.  Consider:
        process1 | unbuffer ‐p process2 | process3

If process1 exits, process2 may not yet have finished.  It
is impossible for unbuffer to know long to wait for process2
and process2 may not ever finish, for example, if it is a
filter.  For expediency, unbuffer simply exits when it
encounters an EOF from either its input or process2.

In order to have a version of unbuffer that worked in all
situations, an oracle would be necessary.  If you want an
application‐specific solution, workarounds or hand‐coded
Expect may be more suitable.  For example, the following
example shows how to allow grep to finish processing when
the cat before it finishes first.  Using cat to feed grep
would never require unbuffer in real life.  It is merely a
placeholder for some imaginary process that may or may not
finish.  Similarly, the final cat at the end of the pipeline
is also a placeholder for another process.

$ cat /tmp/abcdef.log | grep abc | cat
xxxabc defxxx
$ cat /tmp/abcdef.log | unbuffer grep abc | cat
$ (cat /tmp/abcdef.log ; sleep 1) | unbuffer grep abc | cat


xxxabc defxxx

The man page is longer than the program.

"Exploring Expect: A Tcl‐Based Toolkit for Automating
Interactive Programs" by Don Libes, O’Reilly and Associates,
January 1995.

Don Libes, National Institute of Standards and Technology