Permissions

Understanding Permissions

In Unix-like operating systems, permissions control who can read, modify, or execute files and directories. This is crucial for managing security and access on your system. The way permissions are handled in Unix is somewhat similar to Windows, but the specifics differ. In Unix-like systems, permissions can be set for the file's owner (u), the group associated with the file (g), and all other users (o). Below are various ways to modify these permissions using the chmod command.

Basic Concepts

• Ownership: Every file and directory has an owner and a group associated with it. The operating system tracks these using numeric IDs.
• Categories: Each user falls into one of three categories for any file:
  • Owner: The user who owns the file.
  • Group: Users who are part of the file’s associated group.
  • Others: Everyone else on the system.

Permission Types

• Read (r): Allows viewing the contents of the file.
• Write (w): Allows modifying the file.
• Execute (x): Allows running the file if it is a program or script.

Viewing Permissions

Use the ls -l command to view detailed file permissions. The output includes a string like -rwxr-xr-x that shows permissions.

• The first character indicates the type (e.g., - for files, d for directories).
• The next three characters are the owner's permissions (aka the "user").
• The following three are the permissions for the group the user belongs to.
• The last three are the permissions for all other users.

For example, -rwxr-xr-x means:
• Owner: Can read, write, and execute.
• Group: Can read and execute, but not write.
• Others: Can read and execute, but not write.

Changing Permissions

Grant Read Permission

  $ chmod u+r filename.txt

Grant Write Permission

This command adds write permission for the owner. The file's permissions will then be --w-------.

$ chmod u+w filename.txt

Grant Execute Permission

This command allows the owner to execute the file. The permissions would be updated to ---x------.

$ chmod u+x filename.txt

Grant Read, Write, and Execute Permissions

This command provides full access (read, write, and execute) to the file's owner. The file permissions will be -rwx------.

$ chmod u+rwx filename.txt

Applying Permissions to the Group and Others

For the Group

This grants the group read, write, and execute permissions, resulting in ----rwx---.

$ chmod g+rwx filename.txt

For Others

This command allows all other users to read, write, and execute the file, setting permissions to -------rwx.

$ chmod o+rwx filename.txt

Combine Permission Changes

This updates permissions to -rwx-rw-r--, providing different levels of access to the owner, group, and others.

$ chmod u+rwx,g+rw,o+r filename.txt

Setting Exact Permissions

To set permissions precisely, use the equal sign (=) to define permissions explicitly. For example:

$ chmod u=rw,g=rw,o=r filename.txt
							      
This command configures permissions to -rw-rw-r--, ensuring read and write access for the owner and group, and read-only access for others.
							      

Using Numerical Permission Codes

Numerical codes offer a compact way to set file permissions. Each digit represents permissions for the owner, group, and others, respectively.

Read Permission Only

$ chmod 400 filename.txt

This sets permissions to -r--------, allowing only the owner to read the file.

Read Permission for Owner and Group

$ chmod 440 filename.txt

This results in -r--r-----, enabling read access for both the owner and the group.

Read Permission for Everyone

$ chmod 444 filename.txt

This configuration, -r--r--r--, allows all users to read the file. Here, the number 4 signifies read permission.

Read and Write Permissions

$ chmod 664 filename.txt

This sets permissions to -rw-rw-r--, giving both the owner and the group read and write access.

Read, Write, and Execute Permissions

$ chmod 774 filename.txt

$ chmod 700 filename.txt

$ chmod 755 MyDir