How to check if sed has changed a file

Question

I am trying to find a clever way to figure out if the file passed to sed has been altered successfully or not.

Basically, I want to know if the file has been changed

Answer 1

You can diff the original file with the sed output to see if it changed:

sed -i.bak s:$pattern:$new_pattern: "$filename"
if ! diff "$filename" "$filename.bak" &> /dev/null; then
  echo "changed"
else
  echo "not changed"
fi
rm "$filename.bak"

Answer 2

In macos I just do it as follows:

changes=""
changes+=$(sed -i '' "s/$to_replace/$replacement/g w /dev/stdout" "$f")
if [ "$changes" != "" ]; then
  echo "CHANGED!"
fi

I checked, and this is faster than md5, cksum and sha comparisons

Answer 3

A bit late to the party but for the benefit of others, I found the 'w' flag to be exactly what I was looking for.

sed -i "s/$pattern/$new_pattern/w changelog.txt" "$filename"
if [ -s changelog.txt ]; then
    # CHANGES MADE, DO SOME STUFF HERE
else
    # NO CHANGES MADE, DO SOME OTHER STUFF HERE
fi

changelog.txt will contain each change (ie the changed text) on it's own line. If there were no changes, changelog.txt will be zero bytes.

A really helpful sed resource (and where I found this info) is http://www.grymoire.com/Unix/Sed.html.