Basic Linux Directory Permissions and How to Check them

Linux permissions directory are a fundamental aspect of system security and management. They determine which users and processes can

• Read

• Write

• Execute files and directories. 

Thereby ensuring proper access control. 

Understanding Linux Directory Permissions

In Linux, each file and directory is associated with several permissions. They determine what actions can be performed on them. These permissions are divided into:

Owner: The user who owns the file or directory.

Group: A set of users who share certain permissions.

Others: All other users on the system.

Permissions for these categories are represented by a combination of

 Read (r): Permission to read the contents of the file or directory.

Write (w): Permission to change the contents of the file or directory.

Execute (x): Permission to execute the file as a program or to access the directory's contents.

Permission Representation

Permissions are displayed using the `ls -l` command. It lists files and directories and their permissions in a specific format. For example:

drwxr-xr--

This string can be broken down as:

The first character indicates the file type (`d` for the directory, `-` for a regular file, `l` for symbolic link, etc.).

The next nine characters represent the permissions, grouped in threes:

  The first set of three (after the type character) is for the owner.

  The second set is for the group.

  The third set is for others.

In the example above:

 `d` indicates it's a directory.

`rwx` means the owner has read, write, and execute permissions.

`r-x` implies the group has read and executed permissions but not written.

 `r--` means others have only read permission.

Changing Permissions

Permissions can be modified using the `chmod` command. It  stands for "change mode." There are two ways to use `chmod`: 

Symbolic Method

The symbolic method uses letters to represent permissions:

 `u` for user (owner)

 `g` for group

 `o` for others

-` a` for all (user, group, and others)

And the following symbols to set or remove permissions:

`+` to add a permission

`-` to remove a permission

`=` to set a specific permission

For instance, to add execute permission for the group on a directory:

Numeric Method

The numeric method uses octal numbers to represent permissions. Each type of permission is assigned a number:

 Read (`r`) is 4

Write (`w`) is 2

Execute (`x`) is 1

These numbers are summed to get a numeric representation of the permissions. For example:

- `rwx` (read, write, execute) is 4 + 2 + 1 = 7

- `r-x` (read, execute) is 4 + 0 + 1 = 5

- `r--` (read only) is 4 + 0 + 0 = 4

To set permissions using the numeric method, use `chmod` followed by a three-digit number representing owner, group, and others. For example:

chmod 755 directory name

This command sets the permissions to `rwxr-xr-x.` Thus, the owner has full access and read/execute access to the group and others.

 Checking Directory Permissions

The `ls -ld` command is useful for checking directory permissions. For example:

ls -ld /path/to/directory

This command displays detailed information about the specified directory, including its permissions.

Additionally, the `stat` command provides comprehensive details:

stat /path/to/directory

This outputs information, including permissions in symbolic and numeric formats, ownership, and timestamps.

 Practical Examples

Suppose you have a directory `/home/user/docs` and you want to check its permissions:

ls -ld /home/user/docs

Output might be:

drwxr-xr--

This indicates that its a directory in which 

The owner has full permissions

The group can read and execute

Others can only read.

To change its permissions so that everyone can read and write but not execute:

chmod 666 /home/user/docs

This sets the permissions to `rw-rw-rw-`.

To Sum It Up!

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