Oneliners

MySQL

Get the "hit rate" percentage of Connections vs. Threads_created, helps you with finding the sweet spot for thread_cache_size (the closer to 100%, the better):

echo "scale=4; 100-`mysql -NB -e "SHOW STATUS LIKE 'Threads_created'; SHOW STATUS LIKE 'Connections';" | awk '{print $2}' | tr '\n' '/'`100" | bc

Filesystem

Get the size of the a folder and it's subdirectories, sorted by size:

du -hx --max-depth=1 | sort -h

(removed the convoluted old command, this is much simpler)

Postfix

Mass-Delete from mailqueue by sender address

mailq | awk ' /^[0-9A-F][0-9A-F]*.*user@example.com$/ {print $1}' | xargs -rn1 postsuper -d