Table of Contents

1. Linux Special Permissions – One Page Cheat Sheet
   1. Special Permission Bits
   2. Permission Positions
   3. Common Examples
   4. Setting Permissions
      1. Symbolic Mode
      2. Numeric Mode
   5. Numeric Breakdown
   6. Uppercase vs Lowercase Permission Bits
   7. Verification Commands
   8. RHCSA Exam Reminders
   9. References

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Linux Special Permissions – One Page Cheat Sheet

Linux supports special permission bits that extend standard read (r), write (w), and execute (x) permissions. These are frequently tested on exams and commonly used in real systems.

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Special Permission Bits

Bit	Name	Numeric	Symbol	Applies To	Effect
SUID	Set User ID	4	s	Files	Runs program as file owner
SGID	Set Group ID	2	s	Files, Directories	Runs as group / forces group inheritance
Sticky	Sticky Bit	1	t	Directories	Only owner can delete files

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Permission Positions

-rwsr-sr-t
 ||| ||| |||
 ||| ||| ||└─ others
 ||| ||└──── group
 ||└──────── user

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Common Examples

Path	Permissions	Meaning
/usr/bin/passwd	-rwsr-xr-x	SUID root binary
/tmp	drwxrwxrwt	Sticky bit directory
project/	drwxrwsr-x	SGID group inheritance

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Setting Permissions

Symbolic Mode

chmod u+s file      # SUID
chmod g+s dir       # SGID
chmod +t dir        # Sticky

Numeric Mode

chmod 4755 file     # SUID
chmod 2755 dir      # SGID
chmod 1777 /tmp     # Sticky
chmod 3775 shared/  # SGID + Sticky

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Numeric Breakdown

Digit	Meaning
4xxx	SUID
2xxx	SGID
1xxx	Sticky
7xxx	All special bits

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Uppercase vs Lowercase Permission Bits

Symbol	Meaning
s	Special bit + execute set
S	Special bit, execute not set
t	Sticky bit + execute
T	Sticky bit, execute not set

Example:

-rwSr--r--  # SUID set, but file is not executable

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Verification Commands

ls -l
stat file
find / -perm /6000     # Find SUID and SGID files
find / -perm /1000     # Find sticky directories

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RHCSA Exam Reminders

• Sticky bit is almost always associated with /tmp
• SGID on directories enforces group collaboration
• SUID binaries should be rare and audited
• Know numeric vs symbolic permission setting cold

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References

• man chmod
• man ls
• man stat