su只是切換了root身份，但Shell環境仍然是普通用戶的Shell；此時pwd，發現工作目錄仍然是普通用戶的工作目錄
su -連用戶和Shell環境一起切換成root身份了。只有切換了Shell環境才不會出現PATH環境變量錯誤。此時pwd，工作目錄變成root的工作目錄了。用echo $PATH命令看一下su和su -以后的環境變量有何不同。
bash-4.2$ echo $PATH /usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin [root@localhost ~]# echo $PATH /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin
以此類推，要從當前用戶切換到其它用戶也一樣，應該使用su -命令。
su和sudo的區別
su 或者 su -可以切換用戶身份，而且每個用戶都能切換到root用戶，只要他知道root密碼即可。這樣很不安全，萬一哪個用戶不消息泄露了root密碼就GG了。
於是引出了改進版命令sudo
使用sudo執行一個只有root才能執行的命令是可以辦到的，但是需要密碼。注意。這里的密碼不在是root的密碼，而是用戶自己的密碼。默認情況下只有root用戶才能執行sudo命令，普通用戶要想執行sudo，需要root預先設定。通過visudo命令編輯/etc/sudoers來實現。(好像直接vi /etc/sudoers也行)
/etc/sudoers文件內容如下
## Sudoers allows particular users to run various commands as ## the root user, without needing the root password. ## ## Examples are provided at the bottom of the file for collections ## of related commands, which can then be delegated out to particular ## users or groups. ## ## This file must be edited with the 'visudo' command. ## Host Aliases ## Groups of machines. You may prefer to use hostnames (perhaps using ## wildcards for entire domains) or IP addresses instead. # Host_Alias FILESERVERS = fs1, fs2 # Host_Alias MAILSERVERS = smtp, smtp2 ## User Aliases ## These aren't often necessary, as you can use regular groups ## (ie, from files, LDAP, NIS, etc) in this file - just use %groupname ## rather than USERALIAS # User_Alias ADMINS = jsmith, mikem ## Command Aliases ## These are groups of related commands... ## Networking # Cmnd_Alias NETWORKING = /sbin/route, /sbin/ifconfig, /bin/ping, /sbin/dhclient, /usr/bin/net, /sbin/iptables, /usr/bin/rfcomm, /usr/bin/wvdial, /sbin/iwconfig, /sbin/mii-tool ## Installation and management of software # Cmnd_Alias SOFTWARE = /bin/rpm, /usr/bin/up2date, /usr/bin/yum ## Services # Cmnd_Alias SERVICES = /sbin/service, /sbin/chkconfig, /usr/bin/systemctl start, /usr/bin/systemctl stop, /usr/bin/systemctl reload, /usr/bin/systemctl restart, /usr/bin/systemctl status, /usr/bin/systemctl enable, /usr/bin/systemctl disable ## Updating the locate database # Cmnd_Alias LOCATE = /usr/bin/updatedb ## Storage # Cmnd_Alias STORAGE = /sbin/fdisk, /sbin/sfdisk, /sbin/parted, /sbin/partprobe, /bin/mount, /bin/umount ## Delegating permissions # Cmnd_Alias DELEGATING = /usr/sbin/visudo, /bin/chown, /bin/chmod, /bin/chgrp ## Processes # Cmnd_Alias PROCESSES = /bin/nice, /bin/kill, /usr/bin/kill, /usr/bin/killall ## Drivers # Cmnd_Alias DRIVERS = /sbin/modprobe # Defaults specification # # Refuse to run if unable to disable echo on the tty. # Defaults !visiblepw # # Preserving HOME has security implications since many programs # use it when searching for configuration files. Note that HOME # is already set when the the env_reset option is enabled, so # this option is only effective for configurations where either # env_reset is disabled or HOME is present in the env_keep list. # Defaults always_set_home Defaults env_reset Defaults env_keep = "COLORS DISPLAY HOSTNAME HISTSIZE KDEDIR LS_COLORS" Defaults env_keep += "MAIL PS1 PS2 QTDIR USERNAME LANG LC_ADDRESS LC_CTYPE" Defaults env_keep += "LC_COLLATE LC_IDENTIFICATION LC_MEASUREMENT LC_MESSAGES" Defaults env_keep += "LC_MONETARY LC_NAME LC_NUMERIC LC_PAPER LC_TELEPHONE" Defaults env_keep += "LC_TIME LC_ALL LANGUAGE LINGUAS _XKB_CHARSET XAUTHORITY" # # Adding HOME to env_keep may enable a user to run unrestricted # commands via sudo. # # Defaults env_keep += "HOME" Defaults secure_path = /sbin:/bin:/usr/sbin:/usr/bin ## Next comes the main part: which users can run what software on ## which machines (the sudoers file can be shared between multiple ## systems). ## Syntax: ## ## user MACHINE=COMMANDS ## ## The COMMANDS section may have other options added to it. ## ## Allow root to run any commands anywhere root ALL=(ALL) ALL ## Allows members of the 'sys' group to run networking, software, ## service management apps and more. # %sys ALL = NETWORKING, SOFTWARE, SERVICES, STORAGE, DELEGATING, PROCESSES, LOCATE, DRIVERS ## Allows people in group wheel to run all commands %wheel ALL=(ALL) ALL ## Same thing without a password # %wheel ALL=(ALL) NOPASSWD: ALL ## Allows members of the users group to mount and unmount the ## cdrom as root # %users ALL=/sbin/mount /mnt/cdrom, /sbin/umount /mnt/cdrom ## Allows members of the users group to shutdown this system # %users localhost=/sbin/shutdown -h now ## Read drop-in files from /etc/sudoers.d (the # here does not mean a comment) #includedir /etc/sudoers.d
重點介紹下面兩處
①此處可以單獨設置某個用戶。3列分別代表 用戶名 我也不知道（反正都這么寫） 指定可以使用sudo的命令有哪些
②如果用戶有幾百幾千個，總不能一條一條寫吧。。。加入wheel組就可以實現所有用戶都有sudo權力。