SUID SGID and Sticky Bit

​

What Are Special Permission Bits?

• SUID (Set User ID)
  Runs an executable with the file owner’s user ID.
• SGID (Set Group ID)
  Runs an executable with the owning group’s privileges.
• Sticky Bit
  Restricts deletion of files in shared directories to the file owner or root.

​

1. Set User ID (SUID)

​

Step-by-Step

1. Create a test file and view its default permissions:
   touch suidfile
   ls -l suidfile
   # -rw-rw-r--. 1 aaron aaron 0 Apr 26 05:08 suidfile
   
2. Enable SUID without execute for the owner (octal 4664):
   chmod 4664 suidfile
   ls -l suidfile
   # -rwSrw-r--. 1 aaron aaron 0 Apr 26 05:08 suidfile
   

3. Grant both execute and SUID for the owner (octal 4764):
   chmod 4764 suidfile
   ls -l suidfile
   # -rwsrwxr--. 1 aaron aaron 0 Apr 26 05:08 suidfile
   
   The lowercase s shows both SUID and execute bits are active.

​

2. Set Group ID (SGID)

​

Step-by-Step

1. Create a test file and inspect permissions:
   touch sgidfile
   ls -l sgidfile
   # -rw-rw-r--. 1 aaron aaron 0 Apr 26 05:11 sgidfile
   
2. Set SGID without group execute (octal 2664):
   chmod 2664 sgidfile
   ls -l sgidfile
   # -rw-rwSr--. 1 aaron aaron 0 Apr 26 05:11 sgidfile
   
   • Uppercase S in the group’s execute position shows SGID is set but no execute.
3. Add both group execute and SGID (octal 2764):
   chmod 2764 sgidfile
   ls -l sgidfile
   # -rw-rwsr--. 1 aaron aaron 0 Apr 26 05:11 sgidfile
   
   • Lowercase s indicates SGID and execute bits are set for the group.

​

3. Finding SUID/SGID Files

# Find all SUID files
find / -perm /4000

# Find all SGID files
find / -perm /2000

# Find files with either SUID or SGID (or both)
find / -perm /6000

​

4. Sticky Bit on Directories

​

Step-by-Step

1. Create a directory and view its default permissions:
   mkdir stickydir
   ls -ld stickydir
   # drwxrwxr-x. 2 aaron aaron 6 Apr 26 05:14 stickydir
   
2. Set the sticky bit with execute (octal 1777):
   chmod 1777 stickydir
   ls -ld stickydir
   # drwxrwxr-t. 2 aaron aaron 6 Apr 26 05:14 stickydir
   
   • The lowercase t shows both execute and sticky bits are set.
3. Demonstrate sticky without execute (octal 1666):
   chmod 1666 stickydir
   ls -ld stickydir
   # drw-rw-rwT. 2 aaron aaron 6 Apr 26 05:14 stickydir
   
   • Uppercase T indicates sticky is set but execute is not.

​

Links and References

• Linux File Permissions
• chmod Manual Page
• Understanding Linux File System Permissions