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