Shell script entry notes

1. condition test

Two formats: test condition or [condition] (Note that spaces are added on both sides)
Condition can be a logical operation of multiple sub-conditions. The logical operator numbers include:

-A logic and

-O logic or

! Logic No

&, |

2. File Test

File testing uses a conditional test to test the status of a file or directory, such as whether the file exists or is readable. The specific test parameters are as follows:

-D directory

-F normal file (regular file)

-L Symbolic Link

-R readable (readable files and directories)

-Block B dedicated files

-E file exists

-G: If the set-group-ID bit of the file is set, the result is true.

-S file length is greater than 0, not empty

-Z file length = 0

-W writable (Writable Files And Directories)

-The U file has SUID settings

-X executable (File executable and directory browsed)

-C character dedicated File

-L Symbolic Link

For example, to test whether the file file.txt exists, you can test it as follows:

$ Test-e file.txt or [-e file.txt]

3. String Testing

The string test can test whether the string is null and whether the two strings are in phase. The specific test parameters are as follows:

= The two strings are equal

! = Two strings

-Z blank string [zero]

-N non-empty string [nozero]

4. Numerical Test

The value test is used to test the size and phase of two parameters. The format is "Number" numberic_operator "Number" or ["Number" numberic_operator "Number"]. The value of numberic_operator is as follows:

-Equal (equal)

-NE (not equal)

-Gt a> B (greater)

-Lt a <B (less)

-Le A <= B (less, equal)

-Ge a> = B (greater, equal)

(Question: It seems that a variable cannot be compared with a numerical constant. You need to assign the value to the variable and then compare the two variables for further research !)

5. If Structure

If condition 1
Then
Command 1
Elif condition 2
Then
Command 2
Else
Command 3
Fi

6. Case Structure

Case variable in
Pattern [| pattern]...) # Use "|" to separate matching Modes
Command ....
;;
Mode 2)
Command .....
;;
*) # Match all
.......
;;
Esac

7. For Structure

For variable name in list
Do
Command 1
Command 2
Done

Example:

Used for numerical sequence:
For Loop in 1 2 3 4 5
For I in {1 .. 10}
For (I = 1; I <= 10; I ++ ))

Used for string sequence:
For Loop in "orange red blue gray"

Command Execution result:
For Loop in 'LS'

Infinite Loop:
For files in *

8. Until Structure

Until Condition
Do
Command .....
Done

9. While Structure

While Condition
Do
Command 1
Command 2
Done

10. Number of bash groups (array)

Generally, variables like a = "a B C Def" only replace $ A with a single string,
But to a = (a B C DEF), it is to define $ A as the number of groups...
The following method can be used to replace the number of bash groups:
$ {A [@]} or $ {A [*]} can obtain a B c def (number of all groups)
$ {A [0]} returns a (number of the first group), and $ {A [1]} returns the number of the second group...
You can get 4 for $ {# A [@]} or $ {# A [*]} (number of all groups)
$ {# A [0]} can get 1 (that is, the length of the first number of groups (), $ {# A [3]} can get 3 (length of the fourth number (DEF)
A [3] = xyz is to redefine the fourth number of groups as XYZ...

11. Three important symbols

Hard quote: ''(single quotes). All the Meta in hard quotes are disabled.
Soft quote: "" (double quotation marks). In Soft quoe, most of the Meta will be disabled, but some will be retained (for example, $ ).
Escape: \ (backslash). A single Meta is disabled only after escape (Escape Character.

For example, the echo '$0 outside function is' $0, the first $0 won't be unlocked, and the character "$0" is output directly, and the second $0 will be unlocked, output the path of the current shell script;

If you run echo "$0 outside function is" $0, both $0 will be unlocked, so the expected result cannot be reached.

(Note: When writing a shell script in Vim, the color prompt of VIM can be used to check whether the symbol is unlocked, that is, it is processed by regular strings or meta)

[Other key rules]

• (): Place the commands in it in the nestedsubshell for execution, or use it for calculation or command replacement. (Variable modification is temporary)
• {}: Place the commands in the non-namedfunction for execution, or use them in the defined range of variable replacement. (Variable modification is valid for this shell)
  
• $ {} Or '': used to replace the command, that is, execute the command line in the brackets, and then return the command line execution result.
• $ (): Used for integer calculation. For example:
  $ A = 5; B = 7; C = 2
  $ Echo $ (a + B * C ))
• (): You can redefine variable values or perform conditional testing. For example:
  A = 5; (A ++) You can redefine $ A as 6
  A = 5; B = 7; (a <B) returns 0 (true.
  
• There are only two return values for commands, functions, or shell scripts: 0 is true (true), and non-0 is false (false)
  
• $ #: Number of parameters added to Shell
• $: Shell PID (processid)
• $! : PID of the background process last run by Shell
• $? : End code of the last running command (return value)
• $ *: List of all parameters. For example, when "$ *" is included in... $ N "to output all parameters.
• $ @: List of all parameters. For example, when "$ @" is included in "... All parameters are output in the form of "$ N.