help 帮助命令，用于查看Linux中其他命令的使用方法。
格式：
help <命令名称>
<命令名称> –help
内部命令用：help <命令名称>
外部命令用：<命令名称> –help
两种格式是严格区分的，不能同时支持，也就是不能混用。
例如，使用man查看type命令：man type
报错：No manual entry for type
使用外部帮助命令： type –help
显示如下：
type: type [-afptP] name [name ...]
Display information about command type.
For each NAME, indicate how it would be interpreted if used as a
command name.
Options:
-a display all locations containing an executable named NAME;
includes aliases, builtins, and functions, if and only if
the `-p' option is not also used
-f suppress shell function lookup
-P force a PATH search for each NAME, even if it is an alias,
builtin, or function, and returns the name of the disk file
that would be executed
-p returns either the name of the disk file that would be executed,
or nothing if `type -t NAME' would not return `file'
-t output a single word which is one of `alias', `keyword',
`function', `builtin', `file' or `', if NAME is an alias,
shell reserved word, shell function, shell builtin, disk file,
or not found, respectively
Arguments:
NAME Command name to be interpreted.
Exit Status:
Returns success if all of the NAMEs are found; fails if any are not found.