UID and EUID in Linux

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.

See also:

• User identifier
  
• Difference between UID and EUID (effective UID) in Linux/Unix

NOTE: EUID is not available in standard POSIX shell: /bin/sh. Therefore, $EUID can not be used in a shell script starts with #!/bin/sh.