（一）

whoami  是一个相当简单的命令

（二）
 `who‘: Print who is currently logged in
============================================

`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.
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`-a‘
`--all‘
     Same as `-b -d --login -p -r -t -T -u‘.

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`-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

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`-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

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`-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

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`-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

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`-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

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`-H‘
`--heading‘
     Print a line of column headings.