权限与所有权：`chmod`、`chown`

In Linux, file permissions and ownership are vital for security and managing access to your data. They determine who can read, write, or execute files and directories.

Think of it like a set of rules for your digital property. Without proper permissions, anyone could access or modify your important files!

Linux permissions are categorized into three main types of users, often referred to as UGO:

• User (Owner): The person who created the file or directory.
• Group: A collection of users who share specific access rights to the file.
• Others: Everyone else on the system who is not the owner and not part of the file's group.

For each of the UGO categories, there are three basic types of actions they can perform:

• Read (r): Allows viewing the file's content or listing a directory's contents.
• Write (w): Allows modifying or deleting the file, or creating/deleting files within a directory.
• Execute (x): Allows running a file (if it's a program or script) or entering a directory.

You can see a file's permissions and ownership using the ls -l command. This command provides a detailed, 'long' listing of files.

The first column shows the file type and permissions in a symbolic format, like -rwxr-xr--.

The chmod (change mode) command is used to change file permissions. In symbolic mode, you specify who (u, g, o, a) gets what permission (r, w, x) using operators (+, -, =).

• u: user (owner)
• g: group
• o: others
• a: all (u, g, o)
• +: add permission
• -: remove permission
• =: set permission exactly

Let's make our example.txt file executable for the owner, readable for the group, and remove all permissions for others.

Notice how the permissions string changes after each command.

Octal mode is a numerical way to set permissions. Each permission (r, w, x) is assigned a numerical value:

• r (read) = 4
• w (write) = 2
• x (execute) = 1

You sum these values for each UGO category. For instance, rwx (4+2+1) is 7, and r-x (4+0+1) is 5.

Common octal permissions include:

• 755: Owner (rwx), Group (rx), Others (rx)
• 644: Owner (rw), Group (r), Others (r)
• 600: Owner (rw), Group (-), Others (-)

Let's set example.txt to rw-r--r-- (644).

The chown (change owner) command is used to change the user and/or group ownership of a file or directory.

Its basic syntax is chown [USER]:[GROUP] [FILE]. You can change just the user, just the group, or both. This command usually requires sudo privileges.

Let's create a file and then change its owner. For this example, we'll assume a user named 'coddy' exists. If not, replace 'coddy' with an existing user on your system.

Note: You'll likely need sudo to execute chown.

You have a file named report.txt. You want the owner to have full read, write, and execute permissions. The group should only have read and execute permissions. Others should have no permissions at all.

Which chmod command (octal mode) achieves this?

Great job! You've learned how to manage file permissions and ownership in Linux.

• ls -l shows permissions in symbolic format.
• chmod changes permissions using symbolic (u+r) or octal (755) modes.
• chown changes the owner and/or group of a file, often requiring sudo.

Understanding these commands is crucial for securing your system and collaborating effectively!