coreutils: print nth line in file, block until it exists

Question

How would I print the nth line of a file or input, and block until it exists? I want to stick to coreutils.

sed 'NUMq;d' file will quickly give me the nth line, but doesn't block.

tail -f file will block, but doesn't do the other thing.

I should be able to pipe the line to something else eg with a file:

<block-until-line-20-exists> file | <process-line>

or, with input:

tail -n 0 -f file | <block-until-line-20-exists> | <process-line>

Re: tail -n 0 -f file | block-until-line-20-exists, think about { while (( i=0; i<20; i++)); do IFS= read -r line; done; printf '%s\n' "$line"; } as something that could fill in block-until-line-20-exists. Assuming your shell is actually bash; for /bin/sh the loop will be uglier. — Charles Duffy
– Charles Duffy, Commented Apr 29, 2022 at 21:06
...on which point: Do you need POSIX sh compatibility here, or can a shell with ksh extensions (as adopted by both bash and zsh) be assumed? — Charles Duffy
– Charles Duffy, Commented Apr 29, 2022 at 21:08

crinklywrappr · Accepted Answer · 2022-04-29 21:49:30Z

0

Nevermind. I think I answered my own question. tail & sed is the solution after all.

tail -n 0 -f file | sed '20q;d'

Turns out that tail stops blocking after sed completes.

EDIT: depending on how you use this, there is a potential for a time-of-check/time-of-use bug.

answered Apr 29, 2022 at 21:18

crinklywrappr

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