tail -f, multi sed

Question

I can do

tail -vf -c5 thefile    \
    | cat -n            \   
    | sed -E 's/a/b/g'  \
    ;

But the following gives no output.

tail -vf -c5 thefile    \
    | cat -n            \   
    | sed -E 's/a/b/g'  \
    | sed -E 's/f/F/g'  \
    ;

Why?

Ok. Maybe phrasing a question in a few different ways will make it more findable on the internet. Or we can just close it. — john-jones, Nov 23 '23 at 12:01
Don't worry, closing as a dupe is precisely the best way to make the question more findable as you say. Now that the question is closed as a dupe, anyone who finds this will be redirected to the dupe. Closing isn't deleting, we actually want duplicates since they can act as signposts directing you to the right place to the the answer. See When merging questions, leave the duplicate one as a placeholder — terdon, Nov 23 '23 at 12:38

choroba · Accepted Answer · 2023-11-23T11:03:13.997

1

sed is buffering its output. Use the -u option (if present, e.g. GNU sed) to flush the buffers more aggressively.

You can also use the stdbuf tool which modifies the buffering behaviour:

tail -vf -c5  thefile | cat -n | stdbuf -oL sed -E 's/a/b/g' | sed -E 's/f/F/g'

edited Nov 23 '23 at 11:03

answered Nov 23 '23 at 10:08

choroba

Which sed implementations have this? Is it GNU-specific? It isn't POSIX and it's not in BSD sed. – terdon Nov 23 '23 at 10:41
@terdon: Yes, it's present in GNU sed. I added an alternative solution using stdbuf. – choroba Nov 23 '23 at 11:06

1 Answers1