noise@noise:~$ type nano
nano is /bin/nano #这个是外部命令，需要自行安装 (yum -y install nano)
noise@noise:~$

noise@noise:~$ type pwd
pwd is a shell builtin #这是shell内部命令

noise@noise:~$ type egrep 
egrep is aliased to `egrep --color=auto‘ #这是alias别名，可行定义，可用alias查看所有别名

  1 # date 本身有很多参数，比如%F,%T
  2 noise@noise:~$ date --help
  3 Usage: date [OPTION]... [+FORMAT]
  4   or:  date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]]
  5 Display the current time in the given FORMAT, or set the system date.
  6 
  7 Mandatory arguments to long options are mandatory for short options too.
  8   -d, --date=STRING          display time described by STRING, not ‘now‘
  9       --debug                annotate the parsed date,
 10                               and warn about questionable usage to stderr
 11   -f, --file=DATEFILE        like --date; once for each line of DATEFILE
 12   -I[FMT], --iso-8601[=FMT]  output date/time in ISO 8601 format.
 13                                FMT=‘date‘ for date only (the default),
 14                                ‘hours‘, ‘minutes‘, ‘seconds‘, or ‘ns‘
 15                                for date and time to the indicated precision.
 16                                Example: 2006-08-14T02:34:56-06:00
 17   -R, --rfc-email            output date and time in RFC 5322 format.
 18                                Example: Mon, 14 Aug 2006 02:34:56 -0600
 19       --rfc-3339=FMT         output date/time in RFC 3339 format.
 20                                FMT=‘date‘, ‘seconds‘, or ‘ns‘
 21                                for date and time to the indicated precision.
 22                                Example: 2006-08-14 02:34:56-06:00
 23   -r, --reference=FILE       display the last modification time of FILE
 24   -s, --set=STRING           set time described by STRING
 25   -u, --utc, --universal     print or set Coordinated Universal Time (UTC)
 26       --help     display this help and exit
 27       --version  output version information and exit
 28 
 29 FORMAT controls the output.  Interpreted sequences are:
 30 
 31   %%   a literal %
 32   %a   locale‘s abbreviated weekday name (e.g., Sun)
 33   %A   locale‘s full weekday name (e.g., Sunday)
 34   %b   locale‘s abbreviated month name (e.g., Jan)
 35   %B   locale‘s full month name (e.g., January)
 36   %c   locale‘s date and time (e.g., Thu Mar  3 23:05:25 2005)
 37   %C   century; like %Y, except omit last two digits (e.g., 20)
 38   %d   day of month (e.g., 01)
 39   %D   date; same as %m/%d/%y
 40   %e   day of month, space padded; same as %_d
 41   %F   full date; same as %Y-%m-%d
 42   %g   last two digits of year of ISO week number (see %G)
 43   %G   year of ISO week number (see %V); normally useful only with %V
 44   %h   same as %b
 45   %H   hour (00..23)
 46   %I   hour (01..12)
 47   %j   day of year (001..366)
 48   %k   hour, space padded ( 0..23); same as %_H
 49   %l   hour, space padded ( 1..12); same as %_I
 50   %m   month (01..12)
 51   %M   minute (00..59)
 52   %n   a newline
 53   %N   nanoseconds (000000000..999999999)
 54   %p   locale‘s equivalent of either AM or PM; blank if not known
 55   %P   like %p, but lower case
 56   %q   quarter of year (1..4)
 57   %r   locale‘s 12-hour clock time (e.g., 11:11:04 PM)
 58   %R   24-hour hour and minute; same as %H:%M
 59   %s   seconds since 1970-01-01 00:00:00 UTC
 60   %S   second (00..60)
 61   %t   a tab
 62   %T   time; same as %H:%M:%S
 63   %u   day of week (1..7); 1 is Monday
 64   %U   week number of year, with Sunday as first day of week (00..53)
 65   %V   ISO week number, with Monday as first day of week (01..53)
 66   %w   day of week (0..6); 0 is Sunday
 67   %W   week number of year, with Monday as first day of week (00..53)
 68   %x   locale‘s date representation (e.g., 12/31/99)
 69   %X   locale‘s time representation (e.g., 23:13:48)
 70   %y   last two digits of year (00..99)
 71   %Y   year
 72   %z   +hhmm numeric time zone (e.g., -0400)
 73   %:z  +hh:mm numeric time zone (e.g., -04:00)
 74   %::z  +hh:mm:ss numeric time zone (e.g., -04:00:00)
 75   %:::z  numeric time zone with : to necessary precision (e.g., -04, +05:30)
 76   %Z   alphabetic time zone abbreviation (e.g., EDT)
 77 
 78 By default, date pads numeric fields with zeroes.
 79 The following optional flags may follow ‘%‘:
 80 
 81   -  (hyphen) do not pad the field
 82   _  (underscore) pad with spaces
 83   0  (zero) pad with zeros
 84   ^  use upper case if possible
 85   #  use opposite case if possible
 86 
 87 After any flags comes an optional field width, as a decimal number;
 88 then an optional modifier, which is either
 89 E to use the locale‘s alternate representations if available, or
 90 O to use the locale‘s alternate numeric symbols if available.
 91 
 92 Examples:
 93 Convert seconds since the epoch (1970-01-01 UTC) to a date
 94   $ date --date=‘@2147483647‘
 95 
 96 Show the time on the west coast of the US (use tzselect(1) to find TZ)
 97   $ TZ=‘America/Los_Angeles‘ date
 98 
 99 Show the local time for 9AM next Friday on the west coast of the US
100   $ date --date=‘TZ="America/Los_Angeles" 09:00 next Fri‘
101 
102 GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
103 Full documentation at: <http://www.gnu.org/software/coreutils/date>
104 or available locally via: info ‘(coreutils) date invocation‘
105 
106 noise@noise:~$ date
107 Fri Apr 16 14:16:10 CST 2021
108 noise@noise:~$ echo `date +%F`
109 2021-04-16
110 noise@noise:~$ touch `date +%F-%T`.LOG
111 noise@noise:~$ ls
112 2021-04-16-14:19:23.LOG

 1 [root@centos8 ~]# clock
 2 2021-04-16 14:23:24.633755+08:00
 3 [root@centos8 ~]# clock --help
 4 
 5 Usage:
 6  clock [function] [option...]
 7 
 8 Time clocks utility.
 9 
10 Functions:
11  -r, --show           display the RTC time
12      --get            display drift corrected RTC time
13      --set            set the RTC according to --date
14  -s, --hctosys        set the system time from the RTC
15  -w, --systohc        set the RTC from the system time
16      --systz          send timescale configurations to the kernel
17  -a, --adjust         adjust the RTC to account for systematic drift
18      --predict        predict the drifted RTC time according to --date
19 
20 Options:
21  -u, --utc            the RTC timescale is UTC
22  -l, --localtime      the RTC timescale is Local
23  -f, --rtc <file>     use an alternate file to /dev/rtc0
24      --directisa      use the ISA bus instead of /dev/rtc0 access
25      --date <time>    date/time input for --set and --predict
26      --update-drift   update the RTC drift factor
27      --noadjfile      do not use /etc/adjtime
28      --adjfile <file> use an alternate file to /etc/adjtime
29      --test           dry run; implies --verbose
30  -v, --verbose        display more details
31 
32  -h, --help           display this help
33  -V, --version        display version
34 
35 For more details see hwclock(8).

 1 [root@Centos8 ~]# hostnamectl --help
 2 hostnamectl [OPTIONS...] COMMAND ...
 3 
 4 Query or change system hostname.
 5 
 6   -h --help              Show this help
 7      --version           Show package version
 8      --no-ask-password   Do not prompt for password
 9   -H --host=[USER@]HOST  Operate on remote host
10   -M --machine=CONTAINER Operate on local container
11      --transient         Only set transient hostname
12      --static            Only set static hostname
13      --pretty            Only set pretty hostname
14 
15 Commands:
16   status                 Show current hostname settings
17   set-hostname NAME      Set system hostname
18   set-icon-name NAME     Set icon name for host
19   set-chassis NAME       Set chassis type for host
20   set-deployment NAME    Set deployment environment for host
21   set-location NAME      Set location for host
22 [root@Centos8 ~]# hostnamectl set-hostname Centos9
23 [root@Centos8 ~]# exit
24 Last login: Fri Apr 16 14:25:45 2021 from 10.0.0.1
25 [root@Centos9 ~]#