Edit crontab of user where www-data is the user

crontab -u www-data -e

Sync two directories. This overwrites files with the same names

rsync -a /dir1/ /dir2/

Show information about a file or directory

stat file.txt

Show file or directory size

du -sh directory/

Zip up a directory

zip -r zip_filename.zip

Unzip file to current directory

unzip zip_filename.zip

List files and directories in a zip file

zipinfo zip_filename.zip

Search for “whatever” in file.txt

grep -i “whatever” file.txt

Run last command

!!

Run last command as root

sudo !!

Debug SSH connection

ssh -vvvvA user@host

Get information about a site’s SSL certificate

openssl s_client -connect example.com:443

Using UFW, allow from a source to a port

ufw allow from 172.25.0.0/21 to any port 53

Notes

One of the easiest tools to use if you don’t know the syntax of a command is TLDR. On Ubuntu you
can simply install it using apt, apt install tldr. Simply use tldr zip where zip is the
command you wish to know more about.