Search files with grep and find
Find files in or below the current folder containing a certain search term
grep -R "search_term" *
Find files with a certain extension, where these files contain a certain search term
find . -type f -name \*.php -exec grep -il "search_term" {} \;
Find files with certain extensions
find . -regex ".*\(php\|html\|tpl\)$"
As an alternative you could also use
find . -iname "*.php" -or -iname "*.tpl" -or -iname "*.html"
Find files larger than 1 GB
find . -size +1G -exec ls -lh {} \;
Count the number of send emails in /var/log/maillog
cat /var/log/maillog | grep -v "relay=local" | grep "relay=" | grep "status=sent" | grep "May 10" | wc -l