Bash

Encode and decode base64

echo "id" | base64
# aWQK
echo "aWQK" | base64 -d | bash
uid=1000(user)

Check if a file exists

if [ -f /etc/passwd ]; then
    echo "File exists"
else
    echo "File does not exist"
fi

Find files

find / -name "passwd"

Find files by extension

find / -name "*.txt"