Favorite Unix Commands

I am by no means a unix expert and don't use it often enough to have these commands at my fingertips, so I collecting them in one place:

Restart Server:
shutdown -r now

What's running:
ps aux

What using up disk space
du -k | sort -rn | head -20

Check for latest package:
rpm -qa | grep "mysql"

To replace all instances of oranges with bananas in the file mytext.txt

sed -e 's/oranges/bananas/g' mytext.txt

This will display the file with changes. I havn't found a way of making the changes in situ, but to write the output to a new file:

sed -e 's/oranges/bananas/g' mytext.txt > newfile.txt

20 most recently updated files:
find . -type f -printf '%TY-%Tm-%Td %TT %p\n' | sort -r | head -20

What's using memory
top

Value of environment variables set
export

List of all files recursively, containing text

grep -lir "no todo" *

Links
Sed tutorial

Serious Manual