系统帮助命令如何使用

以获得history命令使用方法为例：

help +内置命令：获得命令的使用方法，该命令只对内置命令有效

[root@FQDN ~]# help history
history: history [-c] [-d offset] [n] or history -anrw [filename] or history -ps arg [arg...]
    Display or manipulate the history list.
    
    Display the history list with line numbers, prefixing each modified
    entry with a `*'.  An argument of N lists only the last N entries.
    
    Options:
      -c	clear the history list by deleting all of the entries
      -d offset	delete the history entry at offset OFFSET.
    
      -a	append history lines from this session to the history file
      -n	read all history lines not already read from the history file
      -r	read the history file and append the contents to the history
    	list
      -w	write the current history to the history file
    	and append them to the history list
    
      -p	perform history expansion on each ARG and display the result
    	without storing it in the history list
      -s	append the ARGs to the history list as a single entry
    
    If FILENAME is given, it is used as the history file.  Otherwise,
    if $HISTFILE has a value, that is used, else ~/.bash_history.
    
    If the $HISTTIMEFORMAT variable is set and not null, its value is used
    as a format string for strftime(3) to print the time stamp associated
    with each displayed history entry.  No time stamps are printed otherwise.
    
    Exit Status:
    Returns success unless an invalid option is given or an error occurs.

man +命令：查看命令的手册，可以在man后面加数字来指定具体手册

[root@FQDN ~]# man history