linux - 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 or not without having to look at the file modification date.

The reason why I need this is because I need to do some extra stuff if sed has successfully replaced a pattern.

I currently have:

    grep -q $pattern $filename
    if [ $? -eq 0 ]
    then
        sed -i s:$pattern:$new_pattern: $filename
                # DO SOME OTHER STUFF HERE
    else
        # DO SOME OTHER STUFF HERE
    fi

The above code is a bit expensive and I would love to be able to use some hacks here.

Answer 1

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.