Linux File Permissions

• Linux permissions work and how to manage them. It covers basics, notation, essential commands, special modes, and real-world practices.

Permission Basics

Types

• r (read) → View file contents
• w (write) → Modify file
• x (execute) → Run file

For directories:

• r → List names
• w → Create/remove files
• x → Enter (traverse)

Categories

• Owner (user) → File creator
• Group → Group members
• Others → Everyone else

Permissions are applied separately to each category.

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Reading Notation

Symbolic Example

rwxr-xr--

• Owner → rwx (read, write, execute)
• Group → r-x (read, execute)
• Others → r– (read only)

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Numeric (Octal)

Permission	Value
r	4
w	2
x	1

Example:

755 → (7)(5)(5)
7 = rwx (4+2+1)
5 = r-x (4+1)
5 = r-x (4+1)

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Common Patterns

Code	Meaning	Use Case
777	rwxrwxrwx	❌ Avoid (security risk)
755	rwxr-xr-x	Directories, executables
750	rwxr-x—	Team-shared scripts
644	rw-r–r–	Regular files
600	rw——-	Secrets (SSH keys)
700	rwx——	Private scripts/dirs

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Essential Commands

View Permissions

ls -l

Example:

-rwxr-xr-- 1 alice dev 5320 Mar 26 script.sh

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Change Permissions

Numeric Mode

chmod 755 file

Symbolic Mode

chmod u=rwx,go=rx file
chmod g+w file
chmod o-r file

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Change Ownership

chown newuser file
chown -R user:group dir

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Change Group

chgrp dev file

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Default Permissions (umask)

Check:

umask

Set:

umask 022

Umask	File Default	Dir Default
022	644	755
027	640	750

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Special Permissions

SUID (Set User ID)

• Executes file as owner
• Symbol: s (user position)

chmod u+s file

Example:

rwsr-xr-x

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SGID (Set Group ID)

• Executes as group
• Directories inherit group

chmod g+s dir

Example:

rwxr-sr-x

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Sticky Bit

• Only owner/root can delete files in directory

chmod +t dir

Example:

rwxrwxrwt

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🌍 Real-World Usage

• Web content

  • Directories: 755
  • Files: 644

• Team repository

  • Use group ownership
  • Enable SGID
  • Set umask 027

• Private keys

chmod 600 ~/.ssh/id_rsa

• Executable script

chmod 750 script.sh

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✅ Best Practices

• Follow least privilege principle

• Avoid 777 permissions

• Prefer group access over public access

• Use:

  • SGID → shared directories
  • Sticky bit → public writable dirs

• Avoid unnecessary SUID

• Audit SUID/SGID files regularly

• Set proper umask

  • 022 → general use
  • 027 → secure/shared environments

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🚀 Quick Cheat Sheet

Task	Command
View permissions	ls -l
Change permissions	chmod
Change owner	chown
Change group	chgrp
Set default permissions	umask

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