Unix File Permissions Explained

To chmod or not to chmod 777, that is the question.
— A wise sysadmin at 3 AM

Welcome to the wild world of Unix file permissions — where three letters decide whether your script runs like a charm or ends up throwing a permission denied error that makes you question your career.

In this guide, we break down those cryptic rwxr-xr-- strings, explain who gets to do what with your files, and sprinkle in some terminal humor.

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Meet the Trio: Owner, Group, and Others

Every file in Unix is protected by three types of users:

Who	Description
Owner	The person who created the file
Group	A team or set of users
Others	Everyone else on the system

Each of these roles can be assigned specific powers — the Unix equivalent of magical abilities — using r, w, and x.

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The Three Permissions

Symbol	Name	What It Allows You To Do
r	Read	View the file contents
w	Write	Modify or delete the file
x	Execute	Run the file as a program or script

So rwx means full control. rw- is like “edit but don’t run”, and r-- is “you can look, but don’t touch.”

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The Famous Nine: Understanding rwxr-xr–

Here is how a typical permission string looks:

-rwxr-xr–

Here’s what each part means:

Position	Characters	Who?	Meaning
1st	-	File type	- = file, d = directory, l = symlink
2nd–4th	rwx	Owner	Read, write, execute
5th–7th	r-x	Group	Read, no write, execute
8th–10th	r--	Others	Read only

So, -rwxr-xr-- means:

• It’s a file (-)
• The owner can read, write, and execute
• The group can read and execute
• Others can only read

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Octal Permissions Cheat Code

Unix uses numbers to represent these permissions:

Permission	Binary	Octal
r	100	4
w	010	2
x	001	1
rwx	111	7
rw-	110	6
r--	100	4

Examples:

• chmod 755 means rwxr-xr-x
• chmod 700 means rwx------
• chmod 644 means rw-r--r--

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Common chmod Use Cases

Octal	Symbolic	Description
777	rwxrwxrwx	Everyone can do everything (Nope)
755	rwxr-xr-x	Owner full, others can run
700	rwx------	Owner-only access
644	rw-r--r--	Common for config or HTML files
600	rw-------	Private data files

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Real Life Permission Scenarios (Cheat Table)

Command	Purpose	Access Level
chmod 700 secret.sh	Lock it down — private script or file	Only owner: read, write, execute
chmod 644 index.html	Public read access — for web files	Owner can edit, others can view
chmod 755 public/	Public directory with no write access	Owner full, others can run/view
chmod 777 party.txt	The Wild West — full access to all	Everyone: read, write, execute
chmod 664 report.txt	Group collaboration — shared editing	Owner/group edit, others read
chmod 750 deploy.sh	Executable script for your team	Group can run, only you edit
chmod 644 terms.pdf	Readable document served publicly	Owner writes, world reads
chmod 600 diary.txt	Your eyes only — maximum privacy	Only owner can read/write
chmod 555 run_me.sh	No edits, just execution	All can run, none can edit
chmod 666 config.yml	Read/write by all, but not executable	Everyone can edit (handle with care)

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Essential Commands Recap

Command	What It Does
ls -l	View file permissions
chmod	Change file permissions
chown	Change the file owner
chgrp	Change the group associated with file

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Visual Reference

flowchart TD
    A[Unix File Permissions] --> B[rwxr-xr--]
    
    B --> C1[Owner: rwx]
    C1 --> D1[Read (r)]
    C1 --> D2[Write (w)]
    C1 --> D3[Execute (x)]
    
    B --> C2[Group: r-x]
    C2 --> D4[Read (r)]
    C2 --> D5[No Write (-)]
    C2 --> D6[Execute (x)]
    
    B --> C3[Others: r--]
    C3 --> D7[Read (r)]
    C3 --> D8[No Write (-)]
    C3 --> D9[No Execute (-)]

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Pro Tips

• Do not casually use chmod 777. It’s the nuclear option. • Use groups to manage shared access. • Always verify your umask settings for default file permissions.

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References

• GNU Coreutils Manual – chmod
  Dive into the official documentation for chmod, straight from the source:
  https://www.gnu.org/software/coreutils/manual/html_node/chmod-invocation.html

• The Linux Documentation Project – File Permissions
  A beginner-friendly explanation of Unix permissions from the classic TLDP guides:
  https://tldp.org/LDP/intro-linux/html/sect_03_04.html

• Ubuntu Wiki – File Permissions
  Another great reference for understanding how permissions work in a real-world Linux distro:
  https://help.ubuntu.com/community/FilePermissions

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