How do I replace certain text with the respective line number using sed?

Question

I have a file where certain (not all) lines look like so:

Sequence: n

I wish to replace n with the respective line number.

How can I do it with sed?

By "do it with sed" do you mean "do it with sed" or "do it with any common tool"? Because people say the former and mean the latter about 98% of the time — Michael Mrozek, Commented Aug 25, 2011 at 16:48

enzotib · Accepted Answer · 2011-08-26 07:35:13Z

5

It is very simple with awk (thanks to @glenn jackman):

awk '/Sequence: n/ { $2 = NR } { print }' input-file

Regarding sed, I've only been able to do this with a two-pass elaboration (tested with GNU sed and with ssed:):

sed '/^Sequence: n$/{s/n$//p;=;d;}' | sed '/^Sequence: $/{N;s/\n//;}'

The first inserts the line number in the row after "Sequence: ", the second joins the two lines.

answered Aug 25, 2011 at 16:42

enzotib

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2

I'd write your awk command as: awk '/Sequence: n/ {$2 = NR} {print}'
– glenn jackman
Commented Aug 25, 2011 at 21:22
@glenn jackman: well done, my bad.
– enzotib
Commented Aug 26, 2011 at 7:22

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