How to fix stdio buffering

Jun 23, 2011
tags: awk, grep, sed

It’s a common problem. You write some shell command like:

$ tail -f /var/log/foo | egrep -v 'some|stuff' | sed | awk

and wonder why nothing is printed, even though you know some text has matched. The problem is that stdio is being buffered, and there’s a very good write-up of the problem here so I won’t repeat the technical background.

What I will provide though is how to fix it for common cases.


stdbuf is part of GNU coreutils, and is essentially an LD_PRELOAD hack which calls setvbuf() for an application. Thus it is a generic solution to the problem and can be used to fix most applications. Usage looks like this:

$ tail -f /var/log/foo | stdbuf -o0 app ...

which will disable output buffering for app, assuming it does not do something itself to reverse the setvbuf() call. An example of a misbehaving application is mawk, below.


GNU awk needs no modifications, that is it does not buffer when there is no controlling tty.

mawk however (the default awk in Debian/Ubuntu and possibly others) buffers output, and also does not seem to work with stdbuf. It does however provide a -Winteractive option which will turn off buffering.

$ tail -f /var/log/foo | gawk


$ tail -f /var/log/foo | mawk -Winteractive


GNU sed provides the -u option which calls fflush(), thereby providing unbuffered output. You can also use stdbuf as above.

$ tail -f /var/log/foo | sed -u


$ tail -f /var/log/foo | stdbuf -o0 sed


Similar to sed, GNU grep provides a specific option, --line-buffered, to disable buffering, or again you can use stdbuf.

$ tail -f /var/log/foo | grep --line-buffered


$ tail -f /var/log/foo | stdbuf -o0 grep
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