Send SSH public key to another server
ssh-copy-id -i <id_file> user@machine
Get public IP adress
curl ifconfig.me
Create self-signed certificate
openssl req -x509 -newkey rsa:4096 -keyout foo.key -out foo.crt -days 365
Get total size of a repository
du -sh .
Get total size of subdirectories
du -sh ./* | sort -hr
List the largest file of a repository
du -a /dir/ | sort -n -r | head -n 20
Test if a service port is open
nc -vz <ip_address> <port>
Print absolute path of a folder or a file
readlink -f <file/folder>