Overview of Special Permission Bits
In addition to standard Read, Write, and Execute bits, Linux supports three special permission modes. These bits occupy the leading 4th digit in 4-digit octal notation.
4000
Setuid (Set User ID)
When set on an executable file, the program executes with the privileges of the file owner, rather than the privileges of the user running the command.
Real-world Example:
Symbolic representation: /usr/bin/passwd has Setuid enabled so regular users can temporarily run as root to update their own encrypted password in /etc/shadow.rws-r-xr-x (lowercase s if execute bit is set; uppercase S if execute is missing).2000
Setgid (Set Group ID)
On executable files, the process runs with the file group's permissions. On directories, new files created inside automatically inherit the directory's group owner.
Real-world Example: Shared team project folders. Applying Setgid ensures all newly created sub-files remain accessible to group members.
Symbolic representation: rwxr-sr-x (lowercase s if execute bit is set).1000
Sticky Bit
When set on a directory, users can only delete or rename files that they own (or root), even if the directory is world-writable.
Real-world Example: The public
Symbolic representation: /tmp folder (permissions 1777 or drwxrwxrwt). Everyone can create temporary files, but no user can delete another user's temp file.rwxrwxrwt (lowercase t if execute bit is set).How to Apply Special Bits in Terminal
# Set Setuid (4000) + 755 = 4755
chmod 4755 /usr/local/bin/custom_toolchmod u+s /usr/local/bin/custom_tool# Set Setgid (2000) + 775 = 2775 on shared folderchmod 2775 /var/www/shared_projectchmod g+s /var/www/shared_project# Set Sticky Bit (1000) + 777 = 1777 on temp folderchmod 1777 /var/tmp/shared_uploadschmod +t /var/tmp/shared_uploads