07-28-2011
From Bash cookbook:
Useless Use of cat
Certain Unix users take a positively giddy delight in pointing out inefficiencies in
other people's code. Most of the time this is constructive criticism gently given and
gratefully received.
Probably the most common case is the so-called “useless use of cat award” bestowed
when someone does something like cat file | grep foo instead of simply grep foo
file. In this case, cat is unnecessary and incurs some system overhead since it runs in
a subshell. Another common case would be cat file | tr '[A-Z]' '[a-z]' instead of
tr '[A-Z]' '[a-z]' < file. Sometimes using cat can even cause your script to fail (see
Recipe 19.8, “Forgetting That Pipelines Make Subshells”).
But... (you knew that was coming, didn't you?) sometimes unnecessarily using cat
actually does serve a purpose. It might be a placeholder to demonstrate the fragment
of a pipeline, with other commands later replacing it (perhaps even cat -n). Or it
might be that placing the file near the left side of the code draws the eye to it more
clearly than hiding it behind a < on the far right side of the page.
While we applaud efficiency and agree it is a goal to strive for, it isn't as critical as it
once was. We are not advocating carelessness and code-bloat, we're just saying that
processors aren't getting any slower any time soon. So if you like cat, use it.
The good book, btw.
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LEARN ABOUT DEBIAN
expect_unbuffer
UNBUFFER(1) General Commands Manual UNBUFFER(1)
NAME
unbuffer - unbuffer output
SYNOPSIS
unbuffer program [ args ]
INTRODUCTION
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
CAVEATS
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 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 place-
holder 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
$
BUGS
The man page is longer than the program.
SEE ALSO
"Exploring Expect: A Tcl-Based Toolkit for Automating Interactive Programs" by Don Libes, O'Reilly and Associates, January 1995.
AUTHOR
Don Libes, National Institute of Standards and Technology
1 June 1994 UNBUFFER(1)