Real, Effective and Saved UserID in Linux

Every user in Unix like operating system is identified by different integer number, this unique number is called as UserID.

There are three types of UID defined for a process, which can be dynamically changed as per the privilege of task.

The three different types of UIDs defined are :
1. Real UserID
2. Effective UserID
3. Saved UserID

1. Real UserID : It is account of owner of this process. It defines which files that this process has access to.

2. Effective UserID : It is normally same as Real UserID, but sometimes it is changed to enable a non-privileged user to access files that can only be accessed by root.

3. Saved UserID : It is used when a process is running with elevated privileges (generally root) needs to do some under-privileged work, this can be achieved by temporarily switching to non-privileged account.

While performing under-privileged work, the effective UID is changed to some lower privilege value, and the euid is saved to saved userID(suid), so that it can be used for switching back to privileged account when task is completed.

You can print UID by simply typing id on terminal :

# id

Output :

uid=1000(mandeep) gid=1000(mandeep) 
groups=1000(mandeep), 4(adm), 24(cdrom), 
27(sudo), 30(dip), 46(plugdev), 113(lpadmin), 

id command can be used to print real and effective user and group IDs

Different options of id :

-g, --group : print only effective group id
-G, --groups : print all group IDs
-r, --real : print only real user id
-u, --user : print only effective user id

For example :

id -g

Output :


Note : While you use id command with -r option, you will get error like

id: cannot print only names or real IDs in default format

To deal with this, use -r option in conjunction with other option, for example, id -rg
Now, for setting up real user ID, the effective user ID, and the saved set-user-ID of the calling process, we use setresuid() and setresgid()

Syntax :

int setresuid(uid_t ruid, uid_t euid, uid_t suid); # for specific user
int setresgid(gid_t rgid, gid_t egid, gid_t sgid); # for specific group

Return Value :
On success, 0 is returned.
On error, -1 is returned.

For more details : Use linux mannual page (man user id).

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