How to check if the sed command replaced some string? [duplicate]

Question

This command replaces the old string with the new one if the one exists.

sed "s/$OLD/$NEW/g" "$source_filename" > $dest_filename

How can I check if the replacement happened ? (or how many times happened ?)

Answer 1

sed is not the right tool if you need to count the substitution, awk will fit better your needs :

awk -v OLD=foo -v NEW=bar '
    ($0 ~ OLD) {gsub(OLD, NEW); count++}1
    END{print count " substitutions occured."}
' "$source_filename"

This latest solution counts only the number of lines substituted. The next snippet counts all substitutions with perl. This one has the advantage to be clearer than awk and we keep the syntax of sed substitution :

OLD=foo NEW=bar perl -pe '
    $count += s/$ENV{OLD}/$ENV{NEW}/g;
    END{print "$count substitutions occured.\n"}
' "$source_filename"

Edit

Thanks to william who had found the $count += s///g trick to count the number of substitutions (even or not on the same line)