Linux File Permissions Basics

In this post, you explored the world of Linux file permissions — starting with the basic "who can do what" trio and the different types of access. You learned how to interpret permissions using both the symbolic (-rwxrwxrwx) and numeric (octal) formats, and how to instantly identify file types like directories or symlinks using ls -l. Finally, I covered dangerous permissions to watch out for and the essential commands used to modify them.

 Author:  @ Deepak Kandpal
Sat Jun 14 2025

🐧 Linux Permissions:

I've broken down Linux file permissions into simple, human terms, without any jargon. My goal is to make understanding Linux permissions as easy as possible for you.

:: Enjoy the content☕ ::

Permission Structure and Basics 📁

1. The Basic Trio

In Unix-based operating system (Linux / macOS) every file/folder has 3 permission sets.

pie title Permission Scope "User (Owner)" : 33 "Group" : 33 "Others" : 34
Symbol Meaning Files Directories
r Read View content List files (ls)
w Write Modify/delete Add/remove files
x Execute Run as program Enter (cd)

2. Permission Types

flowchart TD A[Permission] --> B[Read] A --> C[Write] A --> D[Execute] B -->|Files| E[View content] B -->|Folders| F[List files] C -->|Files| G[Modify] C -->|Folders| H[Add/Delete files] D -->|Files| I[Run program] D -->|Folders| J[Enter directory]

Permission Notation 📜

Symbolic View ( -rwxrwxrwx )

Permissions (-rwxr-xr--)
├── Type
│   ├── - : Regular file
│   ├── d : Directory
│   └── l : Symlink
├── User (rwx)
│   ├── r : Read
│   ├── w : Write
│   └── x : Execute
├── Group (r-x)
│   ├── r : Read
│   └── x : Execute
└── Others (r--)
    └── r : Read

Octal Notation 🔢

# Permission Symbolic Typical Use
7 rwx rwx Scripts, directories
6 rw- rw- Data files
5 r-x r-x Shared libraries
4 r-- r-- Config files
0 --- --- Locked files

Spotting File Types with ls -l 🔍

First Char File Type Example Output
- Regular file -rwxr-xr--
d Directory drwxr-xr-x
l Symlink lrwxrwxrwx
c Character device crw-rw----
b Block device brw-r-----

1. Regular File ( - )

Permission e.g. : -rw-r--r-- 1 alice devs 1024 Jun 15 report.txt

Breakdown:

  • - → Regular file ( text, scripts, binaries )

  • rw- → Owner ( alice ) can read/write

  • r-- → Group ( devs ) can only read

  • r-- → Others can only read

Why It Matters:

Common for config files ( e.g., /etc/hosts )

Risk: -rw-rw-rw- ( 666 ) lets anyone modify the file.

2. Directory ( d )

Permission e.g. : drwxr-x--- 2 bob devs 4096 Jun 15 projects/

Breakdown:

  • d → Directory

  • rwx → Owner ( bob ) can list/add/delete files

  • r-x → Group ( devs ) can list files but not add/delete

  • --- → Others blocked entirely

Key Insight:

Execute ( x ) = Enter the directory ( cd ).

Sticky Bit ( t ) ( e.g., /tmp ): Only owners can delete their files.

3. Symbolic Link ( l )

Permission e.g. : lrwxrwxrwx 1 root root 11 Jun 15 config -> /etc/config

Breakdown:

  • l → Symlink ( shortcut )

  • rwxrwxrwx → Permissions are ignored ( always shows full access )

  • Points to /etc/config ( actual permissions depend on the target )

Security Note:

Always verify where symlinks point ( readlink -f config ).

Broken links appear in red in some terminals.

4. Character Device ( c )

Permission e.g. : crw-rw---- 1 root tty 5, 1 Jun 15 /dev/tty1

Breakdown:

  • c → Character device ( streams data, like keyboards )

  • rw- → Owner ( root ) and group ( tty ) can read/write

  • 5, 1 → Major/minor device numbers

Use Case:

Serial ports, terminals ( /dev/tty* ).

Risk: World-writable ( crw-rw-rw- ) = Privilege escalation vector.

5. Block Device ( b )

Permission e.g. : brw-r----- 1 root disk 8, 0 Jun 15 /dev/sda

Breakdown:

  • b → Block device ( storage, like disks )

  • rw- → Owner ( root ) can read/write

  • r-- → Group ( disk ) can only read

  • 8, 0 → Major/minor numbers ( disk identifier )

Why It Matters:

Direct access to disks/partitions ( /dev/sda* ).

Critical: Write access = Raw disk modification ( e.g., dd if=/dev/sda ).

⚠️ Dangerous Permissions

graph TD A[Warning Signs] --> B[777] A --> C[SUID on writable files] A --> D[World-writable system files] B --> E[Anyone can modify] C --> F[Privilege escalation] D --> G[Unauthorized changes]

🛠️ Common Commands

Viewing Permissions

Command Description Example Output
ls -l Detailed view -rwxr-xr-- 1 user group 1024 Jan 1 file
stat -c "%a %n" * Octal values 644 file.txt

Modifying Permissions

Command Effect Secure Example
chmod u+x Add owner execute chmod u+x script.sh
chmod 644 rw-r--r-- chmod 644 config.txt
chown user:group Change ownership chown www-data:dev app.log

❤︎ Stay tuned – for more exciting content !

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