033-Unit 10 Advanced Topics in Users ,Groups and Permissions

User and Group ID Numbers

/etc/passwd :User database

/etc/shadow : Password database

/etc/group : Group database

System User and Group : 1-499 for system

changing your indentity : chang password : passwd command

passwd --status user1

su user1 切换账号

su - user1 切换账号和环境变量(run login shell)

whoami command

groups command

id command

users , who , w command to display users

last command show login history

default permission for file is 666 ; for directory is 777

umask command : default root 022 , other 002 ; so for root file is 644 , directory is 755 ;

and other user file is 664 , directory is 755 

Special Permissions : suid 4(u+s), sgid 2(g+s), sticky 1(0+t) 小写有x权限，大写没有x权限

chmod u+s xxx.txt or chmod 4755 xxx.txt

suid : run as command owner user (ex. /bin/ping)   

sticky bit : only owner and root cat remove files in the directory

sgid : only members of the group can build files in the directory