Linux命令：man

作用：查看Linux中指令的帮助、配置文件帮助、编程函数的帮助等信息。
语法：man [OPTION...] [SECTION] PAGE

man命令的所有参数如下：

[root@lena file]# man --help
Usage: man [OPTION...] [SECTION] PAGE...

  -C, --config-file=FILE     use this user configuration file
  -d, --debug                emit debugging messages
  -D, --default              reset all options to their default values
      --warnings[=WARNINGS]  enable warnings from groff

 Main modes of operation:
  -f, --whatis               equivalent to whatis	#等价于whatis命令 显示指定关键词的简单描述
  -k, --apropos              equivalent to apropos
  -K, --global-apropos       search for text in all pages
  -l, --local-file           interpret PAGE argument(s) as local filename(s)
  -w, --where, --path, --location
                             print physical location of man page(s)
  -W, --where-cat, --location-cat
                             print physical location of cat file(s)

  -c, --catman               used by catman to reformat out of date cat pages
  -R, --recode=ENCODING      output source page encoded in ENCODING

 Finding manual pages:
  -L, --locale=LOCALE        define the locale for this particular man search
  -m, --systems=SYSTEM       use manual pages from other systems
  -M, --manpath=PATH         set search path for manual pages to PATH	#指定man手册搜索的路径

  -S, -s, --sections=LIST    use colon separated section list

  -e, --extension=EXTENSION  limit search to extension type EXTENSION

  -i, --ignore-case          look for pages case-insensitively (default)
  -I, --match-case           look for pages case-sensitively

      --regex                show all pages matching regex
      --wildcard             show all pages matching wildcard

      --names-only           make --regex and --wildcard match page names only,
                             not descriptions

  -a, --all                  find all matching manual pages	#在所有的man帮助手册中搜索
  -u, --update               force a cache consistency check

      --no-subpages          don't try subpages, e.g. 'man foo bar' => 'man
                             foo-bar'

 Controlling formatted output:
  -P, --pager=PAGER          use program PAGER to display output	#分页显示
  -r, --prompt=STRING        provide the `less' pager with a prompt

  -7, --ascii                display ASCII translation of certain latin1 chars
  -E, --encoding=ENCODING    use selected output encoding
      --no-hyphenation, --nh turn off hyphenation
      --no-justification,                              --nj   turn off justification
  -p, --preprocessor=STRING  STRING indicates which preprocessors to run:
                             e - [n]eqn, p - pic, t - tbl,
g - grap, r - refer, v - vgrind

  -t, --troff                use groff to format pages
  -T, --troff-device[=DEVICE]   use groff with selected device

  -H, --html[=BROWSER]       use elinks or BROWSER to display HTML output
  -X, --gxditview[=RESOLUTION]   use groff and display through gxditview
                             (X11):
                             -X = -TX75, -X100 = -TX100, -X100-12 = -TX100-12
  -Z, --ditroff              use groff and force it to produce ditroff

  -?, --help                 give this help list
      --usage                give a short usage message
  -V, --version              print program version #查看版本

选项：

• 数字：指明从哪个man手册中搜索得到帮助

  1. 用户在shell环境可操作的命令、执行文件。
  2. 系统内核可调用的函数与工具等
  3. 常用函数function、库函数lib、大部分C库函数libc
  4. 设备文件说明，通常是/dev下的文件
  5. 配置文件或某些格式文件
  6. 游戏games
  7. 通用协议和规则
  8. 系统管理员可用的管理命令
  9. kernel相关文件

• 关键字：指名需要搜索帮助的关键词

如果使用指令man 2 关键词的时候出现以下提示：

[root@lena file]# man 2 open
No manual entry for open in section 2

这时候需要使用命令yum install -y man-pages安装，这时候重新查询即可看到以下信息：可以查看到该函数需要指定的库，该函数需要携带的参数等信息。

OPEN(2)                           Linux Programmer's Manual                          OPEN(2)

NAME
       open, creat - open and possibly create a file or device

SYNOPSIS
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fcntl.h>

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode);

       int creat(const char *pathname, mode_t mode);

DESCRIPTION
       Given  a  pathname for a file, open() returns a file descriptor, a small, nonnegative
       integer for use in subsequent system calls (read(2),  write(2),  lseek(2),  fcntl(2),
       etc.).  The file descriptor returned by a successful call will be the lowest-numbered
       file descriptor not currently open for the process.

       By default, the new file descriptor is set to remain open across an execve(2)  (i.e.,
       the  FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled; the
       O_CLOEXEC flag, described below, can be used to change this default).  The file  off‐
       set is set to the beginning of the file (see lseek(2)).

       A call to open() creates a new open file description, an entry in the system-wide ta‐
       ble of open files.  This entry records the file offset  and  the  file  status  flags
       (modifiable via the fcntl(2) F_SETFL operation).  A file descriptor is a reference to
 Manual page open(2) line 1 (press h for help or q to quit)

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参考文章：https://www.zhihu.com/tardis/sogou/art/417321490