How does an operating system manage file permissions?

1 File permissions types

There are three basic types of file permissions: read, write, and execute. Read permission allows you to view the contents of a file or directory. Write permission allows you to modify or delete a file or directory. Execute permission allows you to run a file as a program or script, or enter a directory. Depending on the operating system, there may be additional types of file permissions, such as append, which allows you to add data to a file but not modify or delete it, or sticky, which prevents users from deleting files that they do not own in a shared directory.

3 File permissions commands

You can check and change file permissions using commands in a terminal or shell. The commands may vary depending on the operating system and the file system, but some common ones are:

- ls -l : This command lists the files and directories in the current or specified directory, along with their permissions, owner, group, size, and modification date. The permissions are displayed as a sequence of 10 characters, such as -rw-r--r-- , where the first character indicates the file type (e.g., - for regular file, d for directory, l for symbolic link), the next three characters indicate the permissions for the owner (e.g., rw- for read and write), the next three characters indicate the permissions for the group (e.g., r-- for read only), and the last three characters indicate the permissions for other users (e.g., r-- for read only).

- chmod : This command changes the permissions of files and directories. You can use either symbolic or numeric notation to specify the permissions. For example, chmod u+x file adds execute permission to the owner of the file, while chmod 755 file sets the permissions to read, write, and execute for the owner, and read and execute for the group and other users.

- chown : This command changes the owner and group of files and directories. You can use either the user name or the user ID to specify the owner, and either the group name or the group ID to specify the group. For example, chown alice file changes the owner of the file to alice, while chown alice:staff file changes the owner to alice and the group to staff.

4 File permissions graphical interfaces

You can also check and change file permissions using graphical interfaces, such as file managers or property dialogs. Depending on the operating system and file system, you may be able to access the permissions, owner, or group settings by right-clicking on a file or directory and selecting properties or info. This should open a dialog that displays the relevant attributes. Alternatively, some file managers may have a menu or toolbar that allows you to quickly change the permissions, owner, or group of selected files or directories. For example, in Windows you can use the Security tab in the Properties dialog, or the Advanced Security Settings option in the right-click menu, to access these settings.

5 File permissions best practices

File permissions are essential for protecting your data and system from unauthorized access or modification, so it's important to follow some best practices when managing them. The principle of least privilege is key, which means granting only the minimum permissions necessary for a user or a program to perform a task. This reduces the risk of accidental or malicious changes. Additionally, using groups to manage permissions for multiple users who share a common role or function simplifies the administration and maintenance of file permissions. It’s also important to review and audit your file permissions regularly, and update them as needed. Doing this ensures that your file permissions reflect the current state and needs of your system and users, and prevents potential security issues or conflicts.