How to check if sed has changed a file

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.

This might work for you (GNU sed):

sed -i.bak '/'"$old_pattern"'/{s//'"$new_pattern"'/;h};${x;/./{x;q1};x}' file || echo changed

Explanation:

• /'"$old_pattern"'/{s//'"$new_pattern"'/;h} if the pattern space (PS) contains the old pattern, replace it by the new pattern and copy the PS to the hold space (HS).
• ${x;/./{x;q1};x} on encountering the last line, swap to the HS and test it for the presence of any string. If a string is found in the HS (i.e. a substitution has taken place) swap back to the original PS and exit using the exit code of 1, otherwise swap back to the original PS and exit with the exit code of 0 (the default).

I believe you may find these GNU sed extensions useful

t label

If a s/// has done a successful substitution since the last input line
was read and since the last t or T command, then branch to label; if
label is omitted, branch to end of script.

and

q [exit-code]

Immediately quit the sed script without processing any more input, except 
that if auto-print is not disabled the current pattern space will be printed. 
The exit code argument is a GNU extension.

It seems like exactly what are you looking for.

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"