unbuffer Man page

UNBUFFER(1) General Commands Manual UNBUFFER(1)


unbuffer – unbuffer output


unbuffer program [ args ]

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

od -c /tmp/fifo | more

You will not see anything until a full page of output has been pro‐

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 impossi‐
ble 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

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 pro‐
cessing 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

$ 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 Pro‐
grams” by Don Libes, O’Reilly and Associates, January 1995.


Don Libes, National Institute of Standards and Technology

1 June 1994 UNBUFFER(1)

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