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May 2, 2025

Linux File Links and Permissions

Links

Soft Link (Symbolic Link)

ln -s
Link will be removed if the original file is removed or renamed.

Hard Link

ln
Deleting, renaming, or moving the original file will not affect the hard link.

Diagram

       INODE
        ↑
MY HARD LINK ← MYFILE.TXT → MY-SOFT-LINK

Command to check links:

ls -lTri

File Permissions

Permission Types

Symbol	Permission	Example (Mnemonic)
R	Read	Riri (4)
W	Write	Winnie (2)
X	Execute	Xena (1)

Each Permission (R, W, X) Can Be Controlled at Three Levels

U → User
G → Group
O → Others (Everyone)

Permission Strings

Example:

-rwxrwxrwx
 |   |  |
 |   |  └── Others
 |   └───── Group
 └───────── User

To remove write permission from all:

chmod a-w filename

To add read and write permission for user:

chmod u+rw filename

Directory Permissions

Example:

drwxrwxrwx
^   ^  ^
|   |  └── Others
|   └───── Group
└───────── User

Numeric Permission Mapping

Value	Permissions	Symbol	Mnemonic
0	No permission	---
1	Execute	--x	Xena
2	Write	-w-	Winnie
3	Execute + Write	-wx
4	Read	r--	Riri
5	Read + Execute	r-x
6	Read + Write	rw-
7	Read + Write + Exec	rwx

Example to set permissions:

chmod 764 file

Breakdown:

User (7) → rwx
Group (6) → rw-
Others (4) → r--

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Navigating File System / Linux filetypes

ls -l → List
pwd → Print working directory

dr-xr-xr-x
|
directories

-rw-r--r--
|
file

Linux File Types

Symbol	Type
-	Regular file
d	Directory
l	Link
c	Special file or device file
s	Socket → Network communication / process data between external processes
p	Named pipe → FIFO → First In First Out, data between internal processes
b	Block device

Inode = Pointer or number of a file on a hard disk.

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Linux filesystem

/boot -> Grub.cfg
/root -> home of root
/dev -> system device (mouse, keyboard)
/etc -> configuration files
/bin -> /usr/bin -> everyday user commands
/sbin -> /usr/sbin -> system/filesystem commands
/opt -> third party app (not part of os)
/proc -> running process (only in memory)
/lib -> /usr/lib -> C prog lib
/tmp -> temporary folder
/home -> user dir
/var -> system logs
/run -> system daemon -> store temporary runtime / PID file
/mnt -> mount external file system
/media -> CDrom

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