前言
大部分Linux发行版的默认账户是普通用户,而更改系统文件或者执行某些命令,需要root身份才能进行,这就需要从当前用户切换到root用户。
切换用户身份有两个命令 su [-] username 和 sudo
su和su -区别

su只是切换了root身份,但Shell环境仍然是普通用户的Shell;此时pwd,发现工作目录仍然是普通用户的工作目录

su -连用户和Shell环境一起切换成root身份了。只有切换了Shell环境才不会出现PATH环境变量错误。此时pwd,工作目录变成root的工作目录了。用echo $PATH命令看一下su和su -以后的环境变量有何不同。

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

以此类推,要从当前用户切换到其它用户也一样,应该使用su -命令。

su和sudo的区别

su 或者 su -可以切换用户身份,而且每个用户都能切换到root用户,只要他知道root密码即可。这样很不安全,万一哪个用户不消息泄露了root密码就GG了。

于是引出了改进版命令sudo

使用sudo执行一个只有root才能执行的命令是可以办到的,但是需要密码。注意。这里的密码不在是root的密码,而是用户自己的密码。默认情况下只有root用户才能执行sudo命令,普通用户要想执行sudo,需要root预先设定。通过visudo命令编辑/etc/sudoers来实现。(好像直接vi  /etc/sudoers也行)

/etc/sudoers文件内容如下

## 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

重点介绍下面两处

①此处可以单独设置某个用户。3列分别代表 用户名   我也不知道(反正都这么写)  指定可以使用sudo的命令有哪些

②如果用户有几百几千个,总不能一条一条写吧。。。加入wheel组就可以实现所有用户都有sudo权力。

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