find logs dating back 3 weeks, if they are gzipped use zgrep, if they are a regular text log use grep, if they aren’t a log do nothing, search for the string in the found log file

# find /mnt/toaster1/logs/app_logs/application1/2014 -type f -mtime -21 -exec bash -c 'if [[ $0 == *.log ]]; then g=grep; elif [[ $0 == *.gz ]]; then g=zgrep; else g=:; fi; $g "foostring" $0' {} \;