The filesystem, files and permissions

Linux keeps every file in one tree that starts at /. Configuration lives under /etc, logs under /var/log, programs under /usr/bin and your own files under /home. You move around with cd, ls -la and find, and read files with less, head and tail.

File permissions decide who may do what with each file. Every file has an owner and a group. It also has three switches, read, write and execute, set separately for the owner, the group and everyone else.

So chmod 640 app.env means the owner can read and write, the group can only read, and everyone else gets nothing. Learn this first, because many later errors are really permission problems. A service that cannot read its config and SSH refusing a key are two common examples.