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

Don't use sed to tell if it has changed a file; instead, use grep to tell if it is going to change a file, then use sed to actually change the file. Notice the single line of sed usage at the very end of the Bash function below:

# Usage: `gs_replace_str "regex_search_pattern" "replacement_string" "file_path"`
gs_replace_str() {
    REGEX_SEARCH="$1"
    REPLACEMENT_STR="$2"
    FILENAME="$3"

    num_lines_matched=$(grep -c -E "$REGEX_SEARCH" "$FILENAME")
    # Count number of matches, NOT lines (`grep -c` counts lines), 
    # in case there are multiple matches per line; see: 
    # https://superuser.com/questions/339522/counting-total-number-of-matches-with-grep-instead-of-just-how-many-lines-match/339523#339523
    num_matches=$(grep -o -E "$REGEX_SEARCH" "$FILENAME" | wc -l)

    # If num_matches > 0
    if [ "$num_matches" -gt 0 ]; then
        echo -e "\n${num_matches} matches found on ${num_lines_matched} lines in file"\
                "\"${FILENAME}\":"
        # Now show these exact matches with their corresponding line 'n'umbers in the file
        grep -n --color=always -E "$REGEX_SEARCH" "$FILENAME"
        # Now actually DO the string replacing on the files 'i'n place using the `sed` 
        # 's'tream 'ed'itor!
        sed -i "s|${REGEX_SEARCH}|${REPLACEMENT_STR}|g" "$FILENAME"
    fi
}

Place that in your ~/.bashrc file, for instance. Close and reopen your terminal and then use it.

Usage:

gs_replace_str "regex_search_pattern" "replacement_string" "file_path"

Example: replace do with bo so that "doing" becomes "boing" (I know, we should be fixing spelling errors not creating them :) ):

$ gs_replace_str "do" "bo" test_folder/test2.txt 

9 matches found on 6 lines in file "test_folder/test2.txt":
1:hey how are you doing today
2:hey how are you doing today
3:hey how are you doing today
4:hey how are you doing today  hey how are you doing today  hey how are you doing today  hey how are you doing today
5:hey how are you doing today
6:hey how are you doing today?
$SHLVL:3 

Screenshot of the output:

References:

1. https://superuser.com/questions/339522/counting-total-number-of-matches-with-grep-instead-of-just-how-many-lines-match/339523#339523
2. https://unix.stackexchange.com/questions/112023/how-can-i-replace-a-string-in-a-files/580328#580328