Linux Permissions
In Linux, permissions are like locks and keys for files and directories. These permissions determine who can open, modify, or execute a file or folder.
- Read (r): Allows viewing the content of a file.
- Write (w): Allows modifying the content of a file or directory.
- Execute (x): Allows running a file as a program or entering a directory.
Each file or directory has three types of “owners”:
- Owner: The person who created the file (usually).
- Group: A set of users who share common permissions.
- Others: Everyone else.
When you see something like this: -rwxr-xr—, it means:
Owner has read, write, and execute permissions (rwx).
Group has read and execute permissions (r-x).
Others have read-only permissions (r--).
Changing Permissions
You can change permissions using the chmod command. For example:
- Adding permissions: chmod a+r file gives read permissions to everyone.
- Removing permissions: chmod g-w file removes write permission for the group.
Permissions are also represented using numbers (called octal). For example:
7 = read (4) + write (2) + execute (1) → rwx
5 = read (4) + execute (1) → r-x
So, chmod 754 file will give:
- Owner: read, write, execute (rwx)
- Group: read, execute (r-x)
- Others: read (r—)
Changing Owner
You can also change the owner or group of a file using the chown command. For example:
chown root:admin file changes the owner to root and the group to admin.
Special Permissions
There are a few extra permissions for files:
- SUID (Set User ID): When set on a file, it allows the file to be run with the owner’s permissions, not the user’s.
- SGID (Set Group ID): Similar to SUID, but with group permissions.
- Sticky Bit: This is set on directories to prevent users from deleting or modifying files they don’t own, even if they have write access to the directory.
For example, when you see a sticky bit on a directory like this:
drw-rw-r-t it means the sticky bit is set. This protects files inside the directory.