A detailed explanation of how to set special permissions for Linux

Introduction

By usingsetuid、setgid 、sticky, they areLinuxSpecial permissions in can provide additional control over how files and directories are accessed and executed.

Order	Octal numbers	Function
setuid	4	When executing a file, it runs with the permissions of the file owner, not the permissions of the user who executed it.
setgid	2	When executing a file, it will run with permissions of the file group. For directories, it will ensure that the file inherits the group of the directory.
sticky	1	For directories, it ensures that only the file owner can delete or rename the file, even if others have write permissions.

setuid（Set User ID）

Typically used for executable binary files that require elevated permissions

Add setuid to the file

chmod u+s <filename>

Verify setuid

ls -l filename

# The example output is as follows:-rwsr-xr-x 1 root root 12345 Nov 29 12:00 filename

The s in the owner's execution location (rws) represents setuid

Remove setuid

chmod u-s <filename>

setgid（Set Group ID）

When used on a file, make sure that the file runs with the file's group permissions, rather than the user's primary group permissions

When used on a directory, make sure that all files created within the directory inherit the group ownership of the directory, rather than the user's main group

Add setgid to a file or directory

chmod g+s <filename>/<directory>

Verify setgid

ls -ld &lt;filename&gt;/&lt;directory&gt;

# The example output is as follows:drwxr-sr-x 2 user group 4096 Nov 29 12:00 directory_name

s in the group execution position (r-s) represents setgid

Remove setgid

chmod g-s <filename>/<directory>

sticky

Usually used for directories to prevent users from deleting or renaming files that are not their own, even if the directory has write permissions to them, and applies to shared directories such as /tmp

Add sticky bit

chmod +t <directory_name>

Verify sticky bit

ls -ld &lt;directory_name&gt;

# The example output is as follows:drwxrwxrwt 2 user group 4096 Nov 29 12:00 directory_name

The t in the execution position of others (rwt) represents sticky (sticky bit)

Remove the sticky bit

chmod -t <directory_name>

Settings using octal numbers

chmod 6755 &lt;filename&gt;

# First 6 = setuid + setgid (4 + 2)# The second 7 = owner's permissions rwx# The last two 5 = permissions of group and others r-x

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