2017.10.7 Linux Basic commands

1. Date Time command:

   Date

   Function: Displays or sets the time date.

   Show time or Date:

   Date [option] ... [+format] Displays the time and date in the specified format.

   Option: Options

   Format: Formatting symbols.

   Example:
   

   [Email protected]:~$ Date +%d
   10/07/17 Month/day/year
   [Email protected]:~$ Date +%f
   2017-10-07 year-month-day
   [Email protected]:~$ Date +%t
   09:01:37 hours: minutes: seconds
   Set the time or date:

   Date [MMDDHHMM[[CC]YY][.SS]]

   MM: DD: The number of HH: hours mm: minutes YY: Two-bit year CCYY: four-bit year. SS: Seconds

   Example:

   [Email protected]:~$ sudo date 123009182017.40//sudo is executed as Administrator
   [sudo] XXL password://need to enter a password
   Saturday, December 30, 2017 09:18:40 CST
   
   Two types of clocks for Linux:

System clock: The timing of the CPU's operating frequency by the Linux kernel.

Hardware clock

hwclock: Display hardware clock

-S,--hctosys

-W--SYSTOHC

cal: Calendar

Cal: Displays the calendar for the current month.

Cal Year: Displays the calendar for the current year.

2. Directory-related commands

Current directory or working directory

Home directory: Home

Root:/root

Normal User:/home/username:/HOME/XXL

~: User's home directory

For example:

[Email protected]:~$ whoami
xxl
[Email protected]:~$ CD ~
[Email protected]:~$ pwd
/home/xxl

CD: Switch directories

CD or CD ~: Back to the current user's home directory

CD ~username: Switch to the home directory where the user is being developed

CD-: Switch back and forth between the previous directory and the current directory

. Represents the current directory

.. Represents the parent directory of the current directory

ls : list displays the file listing under the specified path.

list [option] ... [DIR] ...

-A,--All: Displays all files, including hidden files;

-L: Long format

Example: Drwxr-xr-x 3 root root 4096 October 4 20:16 opt
Drwxr-xr-x:

Leftmost first bit: file type-, D, L, B, C, p, s

9 bits in the back: access rights

Number: The number of times a file has been hard-linked.

Left Root: Owner of File

Right root: Group of files

4096: Size of File

October 4 20:16 The time the file was last modified

Opt: file name or directory name.

-H, Unit conversion

-D, which displays the relevant properties of the directory itself, usually used with-l;

-R, Reverse display

-R, Recursive

Sata/path/to/somefile: Gets the metadata for the specified file

3. File View command cat TAC

Cat [option] ... [File] ...

-E: Display line terminator $

-N: Numbering each line displayed

4. Document Content View command: file

File/path/to/somefile

5. Echoing command: echo

-N: Prevent automatic line break addition

-e: Allow escape character (\ n: newline \ t: tab)

echo "$VAR _name": variable is replaced, double quotation mark table weak reference.

Echo ' $VAR _name ': The variable is not replaced by a strong reference.

which: Displays the file path for the command

which [option] COMMAND

--skip-alias: Disable alias display

Whatis : Create a database of all help manuals and corresponding keywords on the current system.

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2017.10.7 Linux Basic commands