How do I get sed to quit on the first match and not after?

Question

A background process is writing to a log and I want to output all text from this log to the terminal, up until the word "Do".

I'm using this command:

tail -f service.log | sed '/Do/ q'

For some reason, this command will output the word "Do" and wait until the next line is written to the log before it quits.

To verify, I ran my command and then manually appended to the file using the following commands in another window:

echo A >> service.log
# outputs 'A' in the sed window
echo B >> service.log
# outputs 'B' in the sed window
echo C >> service.log
# outputs 'C' in the sed window
echo Do >> service.log
# outputs 'Do' in the sed window, but the tail and sed command is still running.
echo E >> service.log
# now the tail and sed command finishes.

sed seems to quit only after the next line is written to the file, not when it was outputted.

How do I make my sed command quit on the first output of the word 'Do'?

Which sed is this? I can't reproduce the lingering with GNU sed 4.7 — oguz ismail, Mar 26 '20 at 09:27
As a work-around, you could try a Perl equivalent `perl -pe '/Do/ && exit'` — Mark Setchell, Mar 26 '20 at 09:36
Thank you Mark, but the perl command has the same behavior as sed. — rodentry, Mar 26 '20 at 09:40
Which sed is this? You say it is sed 4.2.2, but is it GNU sed? Check it out with `sed --version`. — Quasímodo, Mar 26 '20 at 09:56
Aha, the linked duplicate question tells us it's the fault of `bash`. — Mark Setchell, Mar 26 '20 at 09:56
For future code-archeologists, the selected and most popular answer in the linked duplicate question will also kill your wrapping command script. Instead, https://stackoverflow.com/a/10142209/1204024 somehow fixed my issue without killing my script. I'm not sure so if it does by working around `bash` — rodentry, Mar 26 '20 at 17:26
Any ideas on why `tail -f service.log | sed '/Do/ q'` will wait until after a new line is written to quit, versus `sed '/Do/q' <(tail -f service.log)`? They seem semantically equivalent. — rodentry, Mar 26 '20 at 17:32

Pero · Answer 1 · 2021-06-11T08:00:50.577

-1

Try with capital Q

sed '/Compiled/Q'

@rodentry, you're right. How about

sed 's/Do/\n/' | head -1

Or the solution must be sed only?

edited Jun 11 '21 at 08:00

answered Mar 26 '20 at 09:28

Pero

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Thank you but the only change is that "Do" does not output at all. The sed command still waits until after a new line is appended to the log to quit. – rodentry Mar 26 '20 at 09:34
That will do just the same, except it won't print the last line. – Mark Setchell Mar 26 '20 at 09:35
This works for me: `sed '/Do/,$d'` – tkiwi Dec 07 '21 at 16:38

How do I get sed to quit on the first match and not after?

1 Answers1