当你新登录一个主机，过着管理一个主机，这时候你就需要这些命令来进行查看了：
1. w : Show who is logged on and what they are doing.
w [options] user [...]
常用命令：
w -h --no-header
w -u --no-current
w -s : Use the short format. Don't print the login time, JCPU or PCPU times.
w -i, --ip-addr : Display IP address instead of hostname for from field.
2. who : Show who is logged on
who -a, --all 展示所有的; 相当于 使用 -b -d --login -p -r -t -T -u
3. users: Print the user names of users currently logged in to the current host
4. whoami : Print effective userid
Print the user name associated with the current effective user ID. Same as id -un.
5. last : last, lastb - show a listing of last logged in users
last -a
last [options] [username...] [tty...]
last searches back through the /var/log/wtmp file (or the file designated by the -f option) and displays a list of all users logged in
(and out) since that file was created. One or more usernames and/or ttys can be given, in which case last will show only the entries
matching those arguments. Names of ttys can be abbreviated, thus last 0 is the same as last tty0. When catching a SIGINT signal (generated by the interrupt key, usually control-C) or a SIGQUIT signal, last will show how far it has
searched through the file; in the case of the SIGINT signal last will then terminate. The pseudo user reboot logs in each time the system is rebooted. Thus last reboot will show a log of all the reboots since the log file
was created. lastb is the same as last, except that by default it shows a log of the /var/log/btmp file, which contains all the bad login attempts.
last reboot 展示所有的机器重新启动的情况。
6. lastb：lastb is the same as last, except that by default it shows a log of the /var/log/btmp file, which contains all the bad login attempts.
在root 权限下，使用 lastb 命令，将会显示所有尝试登录失败的情况。
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