Linux basics: File permissions

My notes while learning about Linux file permissions

I. Linux file permissions

When you type in ls -l (listing all files in current directory with details) you usually see something like this:

The first column represents the file type and permissions. The second column is the number of hard links (there’s another section towards the end of this post explaining hard links), third column is the creator of the file (in this case root user, next to that is the group that owns the file (also called root). Then, the size of the file in bytes (can do ls -lh for human readable storage size) is the 5th column. The last 2 columns are date last modified and file name.

We will focus on the first column, which is the file permissions.

Source

Eg: - rwx rw- r—
-: type - file (directory would be 'd')
rwx: permission for user who created the file - read, write, execute
rw-: permission for the group the user belongs to - only read, write
r--: permission for others - only read

The permissions of read, write, execute also have numeric values.

Source

Looking from a binary perspective, each group is 1 byte or 8 bits — each permission is either being enabled or disabled (1 or 0) and there are 3 kinds of permission (read, write, execute), so in total there are 8 combinations — the octal method. When the value is 0 in decimal, or 000 in binary, no permission is granted.

In the photo above, execute permission, when enabled, will have a decimal value of 1 (001), for write permission it is 2 (010)and then 4 (100) for read permission. When all permissions are enabled, you get number 7 (111).

Note: for a directory, read/write/execute have slightly different meaning:

• If read is not set, you cannot list (ls ) what’s inside the directory.
• If write is not set, you cannot delete or create the directory or its subdirectories.
• If execute is not set, you can’t run a command with it — such as cd or ls or find.

In order to change the permissions, we use the command chmod . There are 2 ways that you can modify the permissions — using letters (r,w,x) or numbers.

Using letters — usually used when you want to add or remove permissions, not set or reset:

chmod [ugo]+-r/w/x [file]

• [ugo] represents 3 groups — user who creates the file, group that owns the file, and others (everyone else). You could set multiple groups at a time i.e ug or go or uo . If none of [ugo] is give, the permission is applied to all user, group, and others.
• +- : plus symbol means adding permission(s), minus symbol means removing one(s).
• r/w/x : permission(s) to be added or removed — could be multiple permissions, not just single.
• [file] : the targeted file.

Example:

chmod ug+rw mylist.txt

This would add read and write permissions to the user and the group that own the mylist.txt file.

Using numbers — usually used when you want to set or reset permissions:

chmod [number][number][number] [file]

This syntax is quite straightforward, each number is for each group.

Example:

chmod 777 mylist.txt

This gives full permissions (read, write, execute) to all 3 groups for the mylist.txt file.

chmod 604 mylist.txt

This gives read (4) and write (2) permissions (4 + 2 = 6) to user who creates the file, giving no permission (0) to group who owns the file and giving only read (4) permission to everyone else.

II. Soft & Hard links

What is Index Node (or inode)

• Every file in the system has an inode.
• It is like the ID of a file. It has all information (space, permission, etc) about the file except file name and content of the file.

What are symbolic/soft link and hard link?

Hard link:

Source

• Refers to the physical location of the file.
• Cannot refer to directory.
• Cannot refer to file outside of current filesystem.
• Remain linked if the linked file is moved somewhere else in the current system — because the original file and the link file have the same inode.
• All hard links are like a copy of the file. If you delete the original file, the copy still remains.

Soft link or symbolic link:

Source

• A symbolic path indicating the location of a file — basically it’s just a string of the path to the target (hence, small file size)
• Act like a shortcut — a pointer to the file
• Can link between different filesystem
• Do not get updated when the target file is moved because the original file and the shortcuts have different inode
• If original file is deleted, shortcuts are useless

III. Resources

Linux file types and file permissions

Soft and Hard links in Linux

Beginner’s Guide to Bash Terminal (highly recommend this one)

Until next time. Happy reading!