sha1sum

SHA1sum is a way of making sure files (like pictures or documents) are safe and have not changed when they are sent over the internet or stored on your computer. It works by creating a very long string of numbers and letters (called a 'checksum') that can be used to check if a file is safe or not. Imagine you were sending a friend a picture you took - you could use SHA1sum to make sure the picture isn't changed or damaged along the way and will arrive safely.