date:
ntp:network time protocol 網絡時間協議
硬件時鐘
系統時鐘
獲得命令使用幫助
help 查看一個內部命令使用幫助,查看一個內部命令幫助一般使用該命令
help COMMOND
2.man 查看外部命令使用幫助
man COMMOND 查看使用手冊
COMMOND --help
man是分章節的，常見章節有8個：
[root@redhat ~]# man ls
LS(1) User Commands LS(1)
（1）用戶命令（/bin、/usr/bin、/usr/local/bin等路徑下的命令）
（2）系統調用
（3）庫用戶
（4）特殊文件（設備文件）
（5）文件格式（解釋某一個命令配置文件的語法 【例：passwd中以：隔開的每一段字符含義】）
（6）遊戲說明
（7）不便歸類的雜項
（8）管理命令（管理員纔有權限使用/sbin/、/usr/sbin、/usr/local/sbin等路徑下的命令）
whatis COMMOND:查看一個命令會出現在哪些章節中
[root@redhat ~]# man date
SYNOPSIS<>命令用法說明
date [OPTION]... [+FORMAT]
date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]]
[]:可省略
|:多選一
<>:必選
{}:分組
……:可多次出現
DESCRIPTION<>命令用法詳盡說明，可能包括每一個命令選項的意義
DESCRIPTION<>命令用法詳盡說明，可能包括每一個命令選項的意義
OPTIONS<>有的命令會把每一個選項的意義寫在此處
FILES<>此命令相關的配置文件
BUGS<>bug的反饋
EXAMPLES<>使用示例
SEE ALSO<>另外參照
翻屏：
向後翻一屏：SPACE
向前翻一屏：b
向後翻一行：enter
向前翻譯行：k
查找：
/string：向後查找
n：下一個
N：前一個
?string：向前查找
n:前一個
N：後一個
FORMAT controls the output. Interpreted sequences are:
%% a literal %
%a locale's abbreviated weekday name (e.g., Sun)<>星期幾縮寫
[root@redhat ~]# date +%a
Tue
%A locale's full weekday name (e.g., Sunday)<>星期幾完整的
[root@redhat ~]# date +%a
Tuesday
%b locale's abbreviated month name (e.g., Jan)
[root@redhat ~]# date +%b
Jun
%B locale's full month name (e.g., January)
[root@redhat ~]# 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號到現在經過時間,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 硬件時鐘到系統，同步以後系統時間爲硬件時間
-w 系統時鐘到迎檢，同步以後硬件時間爲系統時間
cal 日曆
echo
Options:
-n do not append a newline<>不換行輸出，默認換行輸出
-e enable interpretation of the following backslash escapes<>逃逸字符
-E explicitly suppress interpretation of backslash escapes
\b backspace<>退格
[root@redhat ~]# echo "You is a shadiao.\bnonono"
You is a shadiao.\bnonono
[root@redhat ~]# echo -e "You is a shadiao.\bnonono"
You is a shadiaononono
\v vertical tab<>縱向製表符
[root@redhat ~]# echo "You is a shadiao.nonono"
You is a shadiao.nonono
[root@redhat ~]# echo -e "You is a shadiao.\vnonono"
You is a shadiao.
nonono
\n new line<>換行
[root@redhat ~]# echo "You is a shadiao.nonono"
You is a shadiao.nonono
[root@redhat ~]# echo -e "You is a shadiao.\nnonono"
You is a shadiao.
nonono
\t horizontal tab<>橫向製表符
[root@redhat ~]# echo "You is a shadiao.nonono"
You is a shadiao.nonono
[root@redhat ~]# echo -e "You is a shadiao.\tnonono"
You is a shadiao. nonono
printf 格式化輸出