uid vs euid | Chapter09's space

当一个用户登陆系统时，系统会将UID和EUID都赋值为/etc/passwd文件中的UID，一般情况下2个ID是相同的，但是某些情况下会出现2个ID不同的情况。

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.
```
  $ ls -l shadow
  -rw-r----- 1 root shadow 978 2009-02-22 21:25 shadow
	
   ls -l passwd
  -rwsr-xr-x 1 root root 32988 2008-06-10 02:10 passwd
```

setuid，setgid，sticky的八进制位分别是4, 2, 1，助记法表示为u+s，g+s，o+t，(删除标记位是u-s，g-s，o-t)。

可以用 ls -l 来查看权限. 如果有这些标志, 则会在原来的执行标志位置上显示. 如

rwsrw-r--       表示有setuid标志 
rwxrwsrw-       表示有setgid标志 
rwxrw-rwt       表示有sticky标志 