How to unbuffer cut?

Question

I want to get just e-mail addresses that end in "@xyz.nl" from my mail logfile. To achieve this I do:

# tail -f /var/log/mail.log | grep --i --line-buffered "@xyz.nl" | cut -d '@' -f 1 | cut -d '<' -f 2

The --line-buffered with grep is necessary because it will otherwise buffer its output because the pipe is not considered a terminal. Grep will output lines like these:

Aug 29 11:56:01 localhost postfix/smtp[4124]: 05491500123: to=<[email protected]>, relay=123.456.123.456[123.456.123.456]:25, delay=2, delays=0.4/0/0.4/1.2, dsn=2.0.0, status=sent (250 2.0.0 u7T9twxN074009 Message accepted for delivery)

The first cut then makes:

Aug 29 11:56:01 localhost postfix/smtp[4124]: 05491500123: to=<someone

The second cut should generate:

someone

However it seems that cut is also buffering. If I start the command with cat instead of tail -f I get all relevant results (in the prefered format) from the log file. But I need the results from the log file in real time.

I tried using unbuffer for this:

# tail -f /var/log/mail.log | grep --i --line-buffered "@xyz.nl" | unbuffer cut -d '@' -f 1 | cut -d '<' -f 2

Also tried:

# unbuffer tail -f /var/log/mail.log | grep --i --line-buffered "@xyz.nl" | unbuffer cut -d '@' -f 1 | cut -d '<' -f 2

...which should remove the 4K buffering from the first cut. However, this doesn't work. I know it is buffering because if I grep for our local domain it gets a lot more hits, the buffer is filled sooner and output is generated earlier (in 4K batches).

So my question is: how do I unbuffer cut?

Related: I know sed and (g)awk can deliver the e-mail addresses to me. I have been trying but as yet without any result. Answers using sed or (g)awk are welcome and may solve my direct issue but I remain interested in the nominal answer of the question how to unbuffer the cut command. The cut command doesn't speak of (un)buffering.

Answer 1

If you're on a system using GNU Coreutils (almost any Linux), you can try stdbuf:

… | stdbuf -oL cut -d '@' -f 1 | …

-oL makes it line buffered, which seems like what you want.