Explain Linux file permissions and what chmod 755, 644 and 600 mean. Which would you use for a script, a config file and an SSH private key?

Every file has an owner and a group. It has three sets of permissions:

• one for the owner
• one for the group
• one for others (everyone else)

Each set adds up three numbers: read (4), write (2) and execute (1). Each digit in 755 is one of these sums, written in octal (base 8).

The three common values:

• 755 is rwxr-xr-x. The owner can read, write and execute. Everyone else can read and execute. Use it for scripts, binaries and directories.
• 644 is rw-r--r--. The owner can read and write. Everyone else can only read. Use it for config files and static assets.
• 600 is rw-------. Only the owner can read or write. Use it for secrets like SSH private keys and .env files. ssh refuses to use a private key that other users can read.

On a directory, the letters mean different things:

• r lets you list the names inside.
• w lets you create and delete entries.
• x lets you enter the directory and reach the files inside.

Use chmod to change permissions. Use chown user:group file to change who owns the file.