linux uid/euid/suid

Each UNIX process has 3 UIDs associated to it. Superuser/root is UID=0. 

• UID
• Read UID. It is of the user/process that created THIS process. It can be changed only if the running process has EUID=0.

• EUID
• Effective UID. It is used to evaluate privileges of the process to perform a particular action. EUID can be change either to RUID, or SUID if EUID!=0. If EUID=0, it can be changed to anything.

• SUID 
• If the binary image file, that was launched has a Set-UID bit on, SUID will be the UID of the owner of the file. Otherwise, SUID will be the RUID.

referer:

http://notepad2.blogspot.com/2012/03/uid-and-euid-in-linux.html