Question

I’d like to create a script to comment out lines of my Mac OS X hosts file that contain .com. And also one to reverse it.

So this:

127.0.0.1    foo.com
127.0.0.1    bar.com
127.0.0.1    baz
127.0.0.1    qux

would become:

#127.0.0.1   foo.com
#127.0.0.1   bar.com
127.0.0.1    baz
127.0.0.1    qux

I looked around on Google and the sed man page and tried a few things with bash and sed, but I haven’t come close.

sed 's/^/^#/' | grep '.com' < hosts

grep '.com' | sed 's/^/^#/' < hosts

Thank you for any help!

Answer 1

sed '/\.com/s/^/#/' < hosts

Interpretation:

• /\.com/ – only perform the rest of the command on lines matching this regex
• s/^/#/ – insert # at the beginning of the line

If you want to replace the original file, use sed’s -i option:

sed -i.bak '/\.com/s/^/#/' hosts

This will rename hosts to hosts.bak and create a new hosts with the updated contents.

To undo it, use:

sed -i.bak '/^#.*\.com/s/^#//' hosts