Implement suggestions from shellcheck

Script was run through https://www.shellcheck.net/

Some parts were not quoted or could have unwanted side effects like "rm -r /".

Remaining suggestions can be ignored IMO:

Line 106:
    declare -rg date=$(date +"%Y%m%d-%H%M%S")
                ^-- SC2155: Declare and assign separately to avoid masking return values.
 
Line 123:
        for dir in $(ls -t1 "$BACKUP_PATH" | tail -n +"$NUMBACKUPS"); do
                     ^-- SC2012: Use find instead of ls to better handle non-alphanumeric filenames.
 
Line 137:
    if [[ $? -ne 0 ]]; then
          ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.
 
Line 153:
        dbs=$(ls ./*.sql | cut -d "." -f 1 | cut -d "-" -f 1 | sort | uniq)
              ^-- SC2012: Use find instead of ls to better handle non-alphanumeric filenames.
 
Line 188:
            [[ $? -ne 0 ]] \
               ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.