sed replace matching line which does not start with #

Question

I have one file named "test-file". It has just two following lines (one line with # sign in the front and second line with same content but without hash):

#contents of line
contents of line

I want to update/replace the contents of second line by using sed command. I know its simple syntax for example:

sed -i 's/.*contents of line.*/updated contents/' test-file

but the above command is replacing both of the lines whereas I don't want to update the first line with # sign in the front.

Answer 1

try

sed 's/^[^#]*contents of line.*/updated contents/' test-file

or with the start of line anchor ^, and you know that line start with content ..., you could do:

sed 's/^contents of line.*/updated contents/' test-file

If you know which line you are going to update, you can change directly with its address line number like.

sed '2s/.*/updated content/' test-file

add -i when you were happy with the result.

Answer 2

You could do the edit the following way:

sed -e '/^#/!s/.*/UPDATED_CONTENTS/' input-file.txt

or

sed -e '/^#/!c\
UPDATED_CONTENTS' input-file.txt

this is to read as: for any line that does not begin with a # , change that whole line to what is specified.

And in case you are concerned that the octothorpe can be preceded by blanks, then tweak the regex to /^[[:blank:]]*#/ while keeping the rest exactly as is.