（一）
whoami 是一个相当简单的命令
（二）
`who‘: Print who is currently logged in
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`who‘ prints information about users who are currently logged on.
Synopsis:
`who‘ [OPTION] [FILE] [am i]
If given no non-option arguments, `who‘ prints the following
information for each user currently logged on: login name, terminal
line, login time, and remote hostname or X display.
If given one non-option argument, `who‘ uses that instead of a
default system-maintained file (often `/var/run/utmp‘ or `/etc/utmp‘)
as the name of the file containing the record of users logged on.
`/var/log/wtmp‘ is commonly given as an argument to `who‘ to look at
who has previously logged on.
If given two non-option arguments, `who‘ prints only the entry for
the user running it (determined from its standard input), preceded by
the hostname. Traditionally, the two arguments given are `am i‘, as in
`who am i‘.
Time stamps are listed according to the time zone rules specified by
the `TZ‘ environment variable, or by the system default rules if `TZ‘
is not set. *Note Specifying the Time Zone with `TZ‘: (libc)TZ
Variable.
#记住用户的概念非常广泛，不仅仅指实际存在的用户，还包括各种特殊用户，一般id为 0 ～ 500
#参考/etc/passwd文件更深入理解用户的概念。
The program accepts the following options. Also see *note Common
options::.
I.
****************************************************************************************
`-a‘
`--all‘
Same as `-b -d --login -p -r -t -T -u‘.
*************************************************************************************
`-b‘
`--boot‘
Print the date and time of last system boot. #最后一次系统启动的时间
#who -a 所显示的第一行
**************************************************************************************
`-r‘
`--runlevel‘
Print the current (and maybe previous) run-level of the init
process.
#who -a 所显示的第二行，运行级别
****************************************************************************************
`-l‘
`--login‘
List only the entries that correspond to processes via which the
system is waiting for a user to login. The user name is always
`LOGIN‘.
#who -a 正文的前六行，等待登录的用户端口
************************************************************************************************
`-u‘
After the login time, print the number of hours and minutes that
the user has been idle. `.‘ means the user was active in the last
minute. `old‘ means the user has been idle for more than 24 hours.
#正文who -a 中的已登录的用户，给出 “登录地址” “登录时间” 还有其他一些数据
*************************************************************************************
`-w‘
`-T‘
`--mesg‘
`--message‘
`--writable‘
After each login name print a character indicating the user‘s
message status:
`+‘ allowing `write‘ messages
`-‘ disallowing `write‘ messages
`?‘ cannot find terminal device
#显示是否可以用write命令去发送信息
***************************************************************************************
`-H‘
`--heading‘
Print a line of column headings.
#这是个比较好的辅助命令，能提高显示结果的可读性
************************************************************************************
II
其他一些参数
1、显示dead processes 和 active processes
`-d‘
`--dead‘
Print information corresponding to dead processes.
`-p‘
`--process‘
List active processes spawned by init.
`-p‘
`--process‘
List active processes spawned by init.
2、比whoami 稍微详细的结果
`-m‘
Same as `who am i‘.
3、显示登录的用户总数
`-q‘
`--count‘
Print only the login names and the number of users logged on.
Overrides all other options.
4、其他
`--lookup‘
Attempt to canonicalize hostnames found in utmp through a DNS
lookup. This is not the default because it can cause significant
delays on systems with automatic dial-up internet access.
`-s‘
Ignored; for compatibility with other versions of `who‘.
`-t‘
`--time‘
Print last system clock change.
An exit status of zero indicates success, and a nonzero value
indicates failure.