在Linux中man的使用频率应该是很高的，灵活运用它可以让自己快速的掌握一个不熟悉命令的使用方法。下面来介绍下man
环境CentOS6.8
man - format and display the on-line manual pages 在线使用手册格式及展示
通俗点来说它就是命令的使用手册。
它共分九个章节
1、用户命令
2、系统调用
3、C库调用
4、设备文件及特殊文件
5、配置文件格式
6、游戏
7、杂项
8、管理命令及守护进程
9、Linux内核API（CentOS7以后出现）
之所以分这九个章节是应为有的命令分属不同类，查看命令具体属于man中的哪个章节需要使用命令
whatis COMMAND
下面就命令echo为例来具体说明man的手册内容
#whatis echo
echo (1) - display a line of text
echo (1p) - write arguments to standard output
echo [builtins] (1) - bash built-in commands, see bash(1)
从上面可以看出来echo属于man的第一章节
下面从具体的man 1 echo中解释具体每项的意义
#man 1 echo
NAME 命令名称及功能简要说明
echo - display a line of text
SYNOPSIS 用法说明，包括可用的选项
echo [SHORT-OPTION]... [STRING]...
echo LONG-OPTION
<>:此符号内为必给内容
[]:此符号内为可选内容
|：此符号表示多选一
{}：此符号表示为分组
...:同一内容出现多次
DESCRIPTION 命令功能的详尽说明，可能包括每一项的意义
Echo the STRING(s) to standard output.
-n do not output the trailing newline
-e enable interpretation of backslash escapes
-E disable interpretation of backslash escapes (default)
--help display this help and exit
--version
output version information and exit
.
.
.
AUTHOR 作者
Written by Brian Fox and Chet Ramey.
REPORTING BUGS 报告程序bug的方式
Report echo bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
Report echo translation bugs to <http://translationproject.org/team/>
COPYRIGHT 版权声明
Copyright 2010 Free Software Foundation, Inc. License GPLv3+: GNU
GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
SEE ALSO 另外参照
The full documentation for echo is maintained as a Texinfo manual. If
the info and echo programs are properly installed at your site, the
command
info coreutils 'echo invocation'
should give you access to the complete manual.
有的命令中还有OPTIONS（说明每一个选项的意义）、FILES（此命令相关的配置文件）、ARGUMENTS（命令的作用对象），大致有上面这几项。
man命令中的快捷键，便于查看文本信息
enter return键：向后翻一行 k：向前翻一行
ctrl+d :向文件尾部翻半屏 ctrl+u:向文件首部翻半屏
G:跳至最后一行 #G:跳转至指定行，#表示行号
man查找，用于在man手册中快速定位需要的信息
/keyword:以keyword指定的字符串为关键字，从当前界面所在位置向文件尾部搜索；不区分字符大小写
n:下一个 N:上一个
？keyword:以keyword指定的字符串为关键字，从当前界面所在位置向文件首部搜索；不区分字符大小写
n:跟收缩命令同方向，下一个 N:跟搜索命令反向，上一个
当不看man手册时q为退出
man的实际应用
下面通过一个实际的问题，来说明man的作用
例子：字符终端登陆时，显示当前登陆终端号，主机名和当前时间。
要求使用man查找实现上述要求，提示:修改/etc/issue文件。怎么修改可以通过man查看issue求助。
我们先用cat看下/etc/issue文件
# cat /etc/issue
CentOS release 6.8 (Final)
Kernel on an m
之后我们
man issue
Formatting page, please wait...
ISSUE(5) Linux Programmer’s Manual ISSUE(5)
NAME
issue - pre-login message and identification file
DESCRIPTION
The file /etc/issue is a text file which contains a message or system
identification to be printed before the login prompt. It may contain
various @char and char sequences, if supported by the getty-type pro-
gram employed on the system.
FILES
/etc/issue
SEE ALSO
motd(5), agetty(8), mingetty(8)
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 1993-07-24 ISSUE(5)
查看之后发现找不到一点有价值的信息，不过在最后提示我们可以查看motd、agetty、mingetty这三个文件。
man motd
MOTD(5) Linux Programmer’s Manual MOTD(5)
NAME
motd - message of the day
DESCRIPTION
The contents of /etc/motd are displayed by login(1) after a successful
login but just before it executes the login shell.
The abbreviation "motd" stands for "message of the day", and this file
has been traditionally used for exactly that (it requires much less
disk space than mail to all users).
FILES
/etc/motd
SEE ALSO
login(1), issue(5)
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 1992-12-29 MOTD(5)
motd里面也没有值得我们参考的信息，继续下一个
man agentty
AGETTY(8) AGETTY(8)
NAME
agetty - alternative Linux getty
SYNOPSIS
agetty [-8ihLmnUw] [-f issue_file] [-l login_program] [-I init] [-t
timeout] [-H login_host] port baud_rate,... [term]
agetty [-8ihLmnw] [-f issue_file] [-l login_program] [-I init] [-t
timeout] [-H login_host] baud_rate,... port [term]
DESCRIPTION
agetty opens a tty port, prompts for a login name and invokes the
/bin/login command. It is normally invoked by init(8).
agetty has several non-standard features that are useful for hard-wired
and for dial-in lines:
o Adapts the tty settings to parity bits and to erase, kill, end-
of-line and uppercase characters when it reads a login name.
The program can handle 7-bit characters with even, odd, none or
space parity, and 8-bit characters with no parity. The following
special characters are recognized: @ and Control-U (kill); #,
DEL and back space (erase); carriage return and line feed (end
of line). See also --erase-chars and --kill-chars options.
o Optionally deduces the baud rate from the CONNECT messages pro-
duced by Hayes(tm)-compatible modems.
...
..
.
由于这个手册内容很多，所以我们使用/issue快速找到关于/etc/issue的相关内容作为参考。
connection and turn on auto-answer after 1 ring.)
/sbin/agetty -w -I ’ATE0Q1&D2&C1S0=1