Force a shell script to fflush

Question

I was wondering if it was possible to tell bash that all calls to echo or printf should be followed up by a subsequent call to fflush() on stdout/stderr respectively?

A quick and dirty solution would be to write my own printf implementation that did this and use it in lieu of either built in, but it occurred to me that I might not need to.

I'm writing several build scripts that run at once, for debugging needs I really need to see messages that they write in order.

Answer 1

If comands use stdio and are connected to a terminal they'll be flushed per line. Otherwise you'll need to use something like stdbuf on commands in a pipe line http://www.pixelbeat.org/programming/stdio_buffering/

tl;dr: instead of printf ... try to put to the script stdbuf -o0 printf .., or stdbuf -oL printf ...

Answer 2

If you force the file to be read, it seems to cause the buffer to flush. These work for me.

Either read the data into a useless variable:

    x=$(<$logfile)

Or do a UUOC:

    cat $logfile > /dev/null