I use the sed
command on macOS with the following text:
$ cat pets.txt
This is my cat
my cat's name is betty
This is your dog
your dog's name is frank
This is your fish
your fish's name is george
This is my goat
my goat's name is adam
When I run BSD sed
it shows an error:
$ sed '/dog/,+3s/^/# /g' pets.txt
sed: 1: "/dog/,+3s/^/# /g": expected context address
What's wrong with it? When I use gsed
(GNU sed), it works well:
This is my cat
my cat's name is betty
# This is your dog
# your dog's name is frank
# This is your fish
# your fish's name is george
This is my goat
my goat's name is adam
Mac OS X uses BSD sed
and not GNU sed
.
When you use a GNU sed
extension with Mac OS X sed
, you get different results, or failures.
Classically, sed
does not support numeric offsets, forwards or backwards. You'll need to revise your script to work on Mac OS X.
This would work on the given data, but the use of fish
is suspect:
sed '/dog/,/your fish/s/^/# /g' pets.txt
It works with GNU sed because you're taking advantage of a feature added to sed by the GNU project, which didn't previously exist in the program (and still doesn't in non-GNU versions).
You can achieve the same results in non-GNU sed with something like this:
sed -E '/dog/{N;N;N;s/(^|\n)/&# /g;}' pets.txt
That is, once we see "dog", pull in the next three lines, too. Then stick a #
+ space after the beginning of the line (^
) and all the newlines (\n
). In order to be able to do that search and replace in a single regex, we need to enable extended regular expressions, which is what the -E
does. Without it, we could do it with two s
commands, one for the beginning of the line and one for the newlines:
sed '/dog/{N;N;N;s/^/# /;s/\n/&# /g;}' pets.txt
If you're looking for another way to do this on a stock Mac without installing GNU coreutils, you might want to reach for a different utility, for example awk:
awk '/dog/ {l=3}; (l-- > 0) {$0="# "$0} 1' pets.txt
or perl (same idea as the awk version, just different syntax):
perl -pe '$l=3 if /dog/; s/^/# / if $l-- > 0;' pets.txt
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