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Consider a text file users.txt:

#alice
#bob
charlie
dotan
eric

I need to comment everything from (exclusive) the last commented line until (inclusive) dotan. This is the result:

#alice
#bob
#charlie
#dotan
eric

Is there a nice sed oneliner to do this? I'll be happy with any tool, not just sed, really.

Currently I am getting the line number of the last commented line as so:

$ cat -n users.txt | grep '#' | tail -n1
  2 #bob

I then add one and comment with sed:

$ sed -i'' '3,/dotan/ s/^/#/' users.txt

I know that I could be clever and put this all together with some bc into an ugly one-liner. Surely there must be a cleaner way?

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2 Answers 2

up vote 3 down vote accepted

How about

perl -pe '$a=1 if s/^dotan/#$&/; s/^[^#]/#$&/ unless $a==1;' file

or, the same idea in awk:

awk '(/^dotan/){a=1; sub(/^/,"#",$1)} (a!=1 && $1!~/^#/){sub(/^/,"#",$1);}1; ' file
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Perfect, thank you! –  dotancohen Jul 21 at 13:36

If the existing commented lines form a single contiguous block, then you could match from the first commented line instead, commenting-out only those lines up to and including your end pattern that are not already commented

sed '/^#/,/dotan/ s/^[^#]/#&/' file

If the existing comments are not contiguous, then due to the greedy nature of the sed range match I think you would need to do something like

tac file | sed '/dotan/,/^#/ s/^[^#]/#&/' | tac

i.e. match upwards from the end pattern to the 'first' comment - obviously that's not so convenient if you want an in-place solution though.

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Thank you, I've learned from this. –  dotancohen Jul 21 at 20:51

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