Re: grep | sed do not function inline?

[Mike Bird <mgb-debian@yosemite.net>]

On Sunday 22 April 2007 16:13, bilbo wrote:
> Recently I faced a strange behaviour of the Linux commands. I have a
> log file which contains a lot of information. I wanted to extract some
> informations out of it. But when I tried to
>
> tail -f mylog | grep 'smtg' | sed -n 's/end/notend/p'
>
> no output could be found. Which was quite surprising since the
> following tests did their job without any problem:
>
> tail -f mylog | grep 'smtg'
> tail -f mylog | sed -n 's/end/notend/p'
> tail -200 mylog | grep 'smtg' | sed -n 's/end/notend/p'
>
> So, of course, to achieve this result I simply went for perl. But can
> anyone tell me what is the reason why this didn't work?

You did not wait for grep's output buffer to fill and send the
first output to sed.  Add the --line-buffered switch to the grep
command, or use:

tail -f mylog | sed -n '/smtg/s/end/notend/p'

--Mike Bird