date:
ntp:network time protocol 网络时间协议
硬件时钟
系统时钟
获得命令使用帮助
help 查看一个内部命令使用帮助,查看一个内部命令帮助一般使用该命令
help COMMOND
2.man 查看外部命令使用帮助
man COMMOND 查看使用手册
COMMOND --help
man是分章节的，常见章节有8个：
[[email protected] ~]# man ls
LS(1) User Commands LS(1)
（1）用户命令（/bin、/usr/bin、/usr/local/bin等路径下的命令）
（2）系统调用
（3）库用户
（4）特殊文件（设备文件）
（5）文件格式（解释某一个命令配置文件的语法 【例：passwd中以：隔开的每一段字符含义】）
（6）游戏说明
（7）不便归类的杂项
（8）管理命令（管理员才有权限使用/sbin/、/usr/sbin、/usr/local/sbin等路径下的命令）
whatis COMMOND:查看一个命令会出现在哪些章节中
[[email protected] ~]# man date
SYNOPSIS<>命令用法说明
date [OPTION]... [+FORMAT]
date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]]
[]:可省略
|:多选一
<>:必选
{}:分组
……:可多次出现
DESCRIPTION<>命令用法详尽说明，可能包括每一个命令选项的意义
DESCRIPTION<>命令用法详尽说明，可能包括每一个命令选项的意义
OPTIONS<>有的命令会把每一个选项的意义写在此处
FILES<>此命令相关的配置文件
BUGS<>bug的反馈
EXAMPLES<>使用示例
SEE ALSO<>另外参照
翻屏：
向后翻一屏：SPACE
向前翻一屏：b
向后翻一行：enter
向前翻译行：k
查找：
/string：向后查找
n：下一个
N：前一个
?string：向前查找
n:前一个
N：后一个
FORMAT controls the output. Interpreted sequences are:
%% a literal %
%a locale‘s abbreviated weekday name (e.g., Sun)<>星期几缩写
[[email protected] ~]# date +%a
Tue
%A locale‘s full weekday name (e.g., Sunday)<>星期几完整的
[[email protected] ~]# date +%a
Tuesday
%b locale‘s abbreviated month name (e.g., Jan)
[[email protected] ~]# date +%b
Jun
%B locale‘s full month name (e.g., January)
[[email protected] ~]# date +%a
June
%d day of month (e.g, 01)
%D date; same as %m/%d/%y
%e day of month, space padded; same as %_d
%F full date; same as %Y-%m-%d
%g last two digits of year of ISO week number (see %G)
%G year of ISO week number (see %V); normally useful only with %V
%h same as %b
%H hour (00..23)
%I hour (01..12)
%j day of year (001..366)
%k hour ( 0..23)
%l hour ( 1..12)
%m month (01..12)
%M minute (00..59)
%n a newline<>新的一行
%N nanoseconds (000000000..999999999)
%p locale‘s equivalent of either AM or PM; blank if not known
%P like %p, but lower case
%r locale‘s 12-hour clock time (e.g., 11:11:04 PM)
%R 24-hour hour and minute; same as %H:%M
%s seconds since 1970-01-01 00:00:00 UTC<>1970年1月1号到现在经过时间,除以86400
%S second (00..60)
%t a tab
%T time; same as %H:%M:%S
%u day of week (1..7); 1 is Monday
%U week number of year, with Sunday as first day of week (00..53)
%V ISO week number, with Monday as first day of week (01..53)
%w day of week (0..6); 0 is Sunday
%W week number of year, with Monday as first day of week (00..53)
%x locale‘s date representation (e.g., 12/31/99)
%X locale‘s time representation (e.g., 23:13:48)
%y last two digits of year (00..99)
%Y year
hwclock
-s 硬件时钟到系统，同步以后系统时间为硬件时间
-w 系统时钟到迎检，同步以后硬件时间为系统时间
cal 日历
echo
Options:
-n do not append a newline<>不换行输出，默认换行输出
-e enable interpretation of the following backslash escapes<>逃逸字符
-E explicitly suppress interpretation of backslash escapes
\b backspace<>退格
[[email protected] ~]# echo "You is a shadiao.\bnonono"
You is a shadiao.\bnonono
[[email protected] ~]# echo -e "You is a shadiao.\bnonono"
You is a shadiaononono
\v vertical tab<>纵向制表符
[[email protected] ~]# echo "You is a shadiao.nonono"
You is a shadiao.nonono
[[email protected] ~]# echo -e "You is a shadiao.\vnonono"
You is a shadiao.
nonono
\n new line<>换行
[[email protected] ~]# echo "You is a shadiao.nonono"
You is a shadiao.nonono
[[email protected] ~]# echo -e "You is a shadiao.\nnonono"
You is a shadiao.
nonono
\t horizontal tab<>横向制表符
[[email protected] ~]# echo "You is a shadiao.nonono"
You is a shadiao.nonono
[[email protected] ~]# echo -e "You is a shadiao.\tnonono"
You is a shadiao. nonono
printf 格式化输出