We have already talked about the use of many commands , But there are many commands in the system that we haven't seen , There are many options in each command . We can't put every order 、 Remember every option of the command , Then if you encounter an unknown command or an unknown option , Then we need to get help to solve the problem .
Here is the command to get help ：whatis command 、–help Options 、man command .

4.1 whatis

whatis Commands can help you view the function of a command , Usage is as follows .

whatis  command 

for example , To see ls The role of , The order is as follows .

[[email protected] ~]# whatis ls
ls:  No,  appropriate.
[[email protected] ~]#

If we encounter this problem , Just use root User execution mandb command .

[[email protected] ~]# mandb
 Deleting  /usr/share/man/overrides  Old database entries in ...
	... Large output ...
0 stray cats were added.
19 old database entries were purged.
[[email protected] ~]#

And then execute it again whatis command .

[[email protected] ~]# whatis ls
ls (1)               - list directory contents
ls (1p)              - list directory contents
[[email protected] ~]#

In the second column of the result , We just need to focus on the line with pure numbers in parentheses . You can see from the above results ,ls The function of is to list the contents in the directory .
see date Role of command , The code is as follows .

[[email protected] ~]# whatis date
date (1)             - print or set the system date and time
date (1p)            - write the date and time
[[email protected] ~]# 

Here you can see ,date The command is used to display or set the system date and time .
Execute the following command , You can see cal The command is used to display the calendar .

[[email protected] ~]# whatis cal
cal (1)              - display a calendar
cal (1p)             - print a calendar
[[email protected] ~]# 

whatis The output of is actually from man Intercepted from , About man The back can speak .
adopt whatis We know what orders do , But it doesn't tell us how to use this command and what options are available . The following describes the command –help Options .

4.2 --help Options

–help The usage of options is as follows .

 command  --help

Let's check the order first ls Specific usage of .

[[email protected] ~]# ls --help
 usage ：ls [ Options ]... [ file ]...
List information about the FILEs (the current directory by default).
Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.

 Required parameters apply to both long and short options .
  -a, --all				 Don't hide anything to . Starting projects 
  -A, --almost-all		 In addition to the listed . and  .. Any project outside 
	... Large output ...

[[email protected] ~]#

This shows ls The usage of ls Available options . for example ,-a Equate to –all, Is to list all files including hidden files .

[[email protected] ~]# ls -a
.   aa.txt          .bash_logout  .cache   .dbus     .tcshrc      yy
..  anaconda-ks.cfg  .bash_profile  .config  hosts      .viminfo
11  .bash_history   .bashrc      .cshrc   initial-setup-ks.cfg  .Xauthority
[[email protected] ~]#

There are many files that start with a dot to indicate hidden files , It also includes . And representing the upper level directory ….
ls -A Equate to ls --almost-all, It means to list other than the current directory . And all other files outside the upper directory .

[[email protected] ~]# ls -A
11            .bash_history  .bashrc  .cshrc  initial-setup-ks.cfg  .Xauthority
aa.txt          .bash_logout  .cache   .dbus   .tcshrc     yy
anaconda-ks.cfg  .bash_profile  .config  hosts   .viminfo
[[email protected] ~]#

If you want to prioritize the directory , By checking the help, you can know which options should be used –group-directories-first.

[[email protected] ~]# ls --group-directories-first
11  yy  aa.txt  anaconda-ks.cfg  hosts  initial-setup-ks.cfg
[[email protected] ~]#

here 11 and yy Is a directory , If you want to sort in chronological order , It can be used -t Options .

[[email protected] ~]# ls -t
aa.txt  hosts  11  yy  initial-setup-ks.cfg  anaconda-ks.cfg
[[email protected] ~]# 

The newer the file is, the higher the ranking , Readers can use ls -lt To view the .
If you want to sort by the flashback of time, you can add -r Options .

[[email protected] ~]# ls -t -r
anaconda-ks.cfg  initial-setup-ks.cfg  yy  11  hosts  aa.txt
[[email protected] ~]#

Tips ： You can use ls -ltr To view the . About ls The other options , Readers can check and practice by themselves according to their needs .
So let's see date Usage of .
If direct input date command , The current date and time will be displayed .

[[email protected] ~]# date
2021 year  11 month  20 Japan   Saturday  15:34:57 CST
[[email protected] ~]#

See below date More use of .

[[email protected] ~]# date --help
 usage ：date [ Options ]... [+ Format ]
　 or ：date [-u|--utc|--universal] [MMDDhhmm[[CC]YY][.ss]]
	... Large output ...
 Or use it locally ：info '(coreutils) date invocation'
[[email protected] ~]#

This shows date There are two uses ： The first is “date [ Options ]… [+ Format ]” Used to view date or time . If using format , The format should be preceded by a plus sign “+”, stay date --help All supported formats are listed in the results of , You can check and practice by yourself , Let's demonstrate some .
%Y Indicates the year .

[[email protected] ~]# date +%Y
2021
[[email protected] ~]#
%F It means the day and year of .
[[email protected] ~]# date +%F
2021-11-20
[[email protected] ~]# 
%R Express “ when : branch ”.
[[email protected] ~]# date +%R
15:47
[[email protected] ~]#

%T Express “ when : branch : second ”.

[[email protected] ~]# date +%T
15:47:24
[[email protected] ~]#

We can also combine multiple formats , Usage is date +“ Format 1 Format 2”, Format 1 And format 2 You can specify any separator between .
If the “ year - month - Japan when : branch : second ” The format of , The order is as follows .

[[email protected] ~]# date +"%F %T"
2021-11-20 15:47:53
[[email protected] ~]# 

date There's another usage , It is used to set the time , Usage is as follows .
date [-u|–utc|–universal] [MMDDhhmm[[CC]YY][.ss]]
The format here is “ Month, day, second, year . second ”, Enclosed in brackets in the format , It means you can't write . here “ year ” Don't write , Not writing means this year , It can be written in four digit years, such as 2021, You can also write two digit years, such as 21.“ second ” The position may not be written , If write , It needs to be added in front “.”. But here's the thing , If it is a single digit, fill in the front 0, Such as 1 To write 01.
Suppose the system time is set to “2012-12-21 10:00:08”, The order is as follows .

[[email protected] ~]# date 122110002012.08
2012 year  12 month  21 Japan   Friday  10:00:08 CST
[[email protected] ~]# 
[[email protected] ~]# date
2012 year  12 month  21 Japan   Friday  10:00:09 CST
[[email protected] ~]#

Because by date Changing the time is just changing the system time , It hasn't changed BIOS Time on . adopt hwclock -s Change the system to and BIOS The time on is the same as the current time .

[[email protected] ~]# hwclock -s
[[email protected] ~]#

If you pass date After modifying the system time, I also want to write BIOS in , Then execute the command hwclock -w that will do .
This method of setting time just now doesn't seem to conform to our habits , By checking the help, you can see that there is a -s Options .

 The format is  date -s " year - month - Japan   when : branch : second ".

Next, set the time to “2012-12-21 10:00:08”, You can use the following command .

[[email protected] ~]# date -s "2012-12-21 10:00:08"
2012 year  12 month  21 Japan   Friday  10:00:08 CST
[[email protected] ~]# date
2012 year  12 month  21 Japan   Friday  10:00:09 CST
[[email protected] ~]#

This is much more convenient .
adopt hwclock -s Change the system to current .

[[email protected] ~]# hwclock -s
[[email protected] ~]#

About hwclock Use of commands , Please consult the help by yourself .

4.3 man Use

man yes manual Abbreviation , Is the meaning of the manual , Generally known as manpage. If the same command has multiple meanings ,man Put in different “ chapter ” To show , Pass below whatis see passwd result .

[[email protected] ~]# whatis passwd
openssl-passwd (1ssl) - compute password hashes
passwd (1)           - update user's authentication tokens
passwd (5)           - password file
[[email protected] ~]#

The numbers in parentheses in the second column refer to man It is explained in detail in the chapters of .passwd As a command to change user password , stay man There is a detailed explanation in the first chapter of ,passwd As a password file in man Of the 5 Explained in Chapter .
If you want to look at passwd As the explanation of password file , It can be used man 5 passwd To view the , As shown in the figure below .

Press q sign out .
If you want to look at passwd As an explanation of an order , It can be used man 1 passwd To view the , As shown in the figure below .

Press q sign out , This can be written as man passwd.

Homework

1. Now I want to set the system time to 2022 year 12 month 23 Japan In the morning 10 spot 23 branch 27 second , Which of the following commands is wrong ？
   a. date 12231023202227
   b. date -s “2022-12-23 10:23:27”
   c. date 20221223102327
   d. timedatectrl set-time “2022-12-23 10:23:27”

2. Which of the following commands can list all the files in the current directory , And in reverse chronological order, that is, the first file created , At the end of the line ？
   a. ls -lRt
   b. ls -lrt
   c. ls -alrt
   d. ls -alr