sed comment several lines

Question

I have a text file test.txt with something like the following:

foo
configure
top
start
fun
bar
hello

I want to comment out line numbers 2, 5 and 7, so that the output would look like:

foo
#configure
top
start
#fun
bar
#hello

When I use sed -i "2,7 {s/^/#/}" test.txt lines 2, 3, 4, 5, 6 and 7 get commented. How to ensure only specific line numbers are commented? Appreciate any help.

Answer 1

With 2,7 you specify a range of lines to apply the expression on. In this case, you just want to pick three specific lines.

sed -e '2s/^/#/' -e '5s/^/#/' -e '7s/^/#/'  file

or with the actual commenting-out operation separated out into a single place in he sed editing code (only really useful if you need to do something that would be more complicated than just adding a # to the start of the line):

sed -e 2bc -e 5bc -e 7bc -e b -e :c -e 's/^/#/' file

The b command branches to a label (here c, defined with :c) or to the end of the script if no label is given.

or with awk:

awk 'NR == 2 || NR == 5 || NR == 7 { print "#" $0; next } 1'  file