Commands

Find a string inside folder and subfolder
$grep -rl 'stringToFind' /opt/lampp/htdocs/foldername/

Find a string inside folder and subfolder (ignore case sensitive)
$grep -rl 'stringToFind' /opt/lampp/htdocs/foldername/

Find a string inside folder and subfolder (with line number)
$grep -r -n 'stringToFind' /opt/lampp/htdocs/foldername/

Find entered command (using head or tail )
$history | head -10