Work out Linux file permissions as a number like 755 or as rwx letters.
Each of the three digits covers one group of people: the file's owner, its group, and everyone else. Read is worth 4, write 2 and execute 1, and you add up what's allowed. 7 is 4+2+1 (everything), 6 is read and write, 5 is read and execute, 4 is read only and 0 is nothing. So 754 means the owner can do everything, the group can read and run it, and others can only read it.
The symbolic form says the same thing with letters: -rwxr-xr-- is 754. The first character shows the type, and - means a regular file.
644 for normal files such as web pages and images. 755 for folders and scripts that need to run. 600 for private files like SSH keys, which ssh refuses to use if others can read them. 700 for private folders. 777 lets anyone change the file and is almost never the right fix, even if it makes an error go away.
The calculator covers the standard nine permission bits. The special setuid, setgid and sticky bits, written as a fourth digit in front (as in 4755), aren't included.
The owner can read, write and execute; group and others can read and execute. It's the usual setting for folders and scripts.
The owner can read and write; everyone else can only read. It's the usual setting for regular files.
On a folder, execute means permission to enter it and reach the files inside. Without it, even readable files can't be opened.
Use chmod -R, but carefully: files and folders usually need different values, such as 644 and 755.
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