Common shell rules

1. system environment variables after the user logs on to the system:
$ Home user's own directory
$ Path: the directory to be searched during Command Execution
$ TZ Time Zone
$ Mailcheck the number of seconds to check whether a new letter exists
$ PS1 prompt number in the Command Column
$ PS2 indicates the prompt number when shell requests to re-input when the command has not been completed
$ Manpath man command search path

Ii. Special variables:

$0 execution name of the program
$ N the nth parameter value of this program, n = 1 .. 9
$ * All parameters of this program
$ # Number of parameters of this program
$ PID of the program
$! PID used to execute the previous command
$? The Return Value of the previous command.

3. Variable in shell:
* Any string
? Any character
[ABC] One of the three: A, B, and C
[A-N] any character from A to N

4. Special characters

/B Return
/C: When printing a row, there is no line break. This is often used.
/F form feed
/R press ENTER
/T tabulation
/V vertical tabulation
// Backslash itself

V. Determining file attributes

Format:-operator filename
-If the file-e exists, 1 is returned; otherwise, 0 is returned.
-If the r file is readable, 1 is returned; otherwise, 0 is returned.
-W file write return 1, otherwise return 0
-1 is returned for executable-X Files; otherwise, 0 is returned.
-O: if the file belongs to the user, 1 is returned; otherwise, 0 is returned.
-If the length of the Z file is 0, 1 is returned; otherwise, 0 is returned.
-If the f file is a normal file, 1 is returned; otherwise, 0 is returned.
-If the D file is a directory file, 1 is returned; otherwise, 0 is returned.

6. Test the string
String 1 = string 2 true when two strings are equal
String 1! = String 2: true when two strings are not equal
-N string: true when the string length is greater than 0
-Z string: true when the string length is 0
String is true when the string is not empty

7. Test the relationship between two integers
Number 1-EQ Number 2 equal to true
The number 1-ne number 2 is not true.
Digit 1-GT digit 2 digit 1 greater than digit 2 true
Number 1-ge number 2 Number 1 greater than or equal to number 2 true
Number 1-lt number 2 Number 1 less than number 2 true
Number 1-Le number 2 Number 1 less than or equal to number 2 true

8. Logic Testing
-A and
-O or
! Non

Introduction to shell special characters
====================================
Special characters in Shell include:

1. $ dollar sign
2./backslash
3. 'quotation marks
4. Double quotation marks
5. & lt;, & gt ;;,*,?, [,]

The following is a one-to-one description.
1. $ symbol
1. Echo $? Displays the exit status of the previous command.
2. Echo "$? "Same effect
3. Echo '$? 'Is $?
4. Echo/$? $?
5. Echo "/$? "Show $?

You may have seen that the $ symbol has a special meaning in double quotation marks. Double quotation marks do not work for the $ symbol.
The single quotation marks can block the special meanings of special characters so that they can be displayed as characters themselves.
The bar can also block the special meanings of special characters, so that special characters do not have special meanings.

Ii./backslash
The backslash is used to block the special meaning of a special character so that it is the original character.
A = 1234
Echo/$ A is displayed as $ A. If no value is added, 1234 is displayed.
Echo/'display'
Echo/"is displayed as double quotation marks
Echo // displayed/

Iii. 'quotation marks
The function of anti-quotation marks is to replace commands and execute strings in the anti-quotation marks as commands. We often use shell programming to assign the execution result of system commands to a variable.

A = 'date'
Echo $ A shows the time string instead of the date.
For example, the content of file a is as follows:
Abcdefg
1234456
Abcdefg

B = 'cat A | grep 234 '# retrieve the row where file a contains string 234
Echo $ B is displayed as 1234456
Echo "$ B" will show why?
Echo "/$ B" will show why? Try it by yourself

Iv. Double quotation marks
There are some special characters in the system. To avoid referencing these special characters, these special characters are often caused by double quotation marks or single quotation marks, so that they do not have special meanings.
However, some special characters still have special meanings in quotation marks, which are caused by double quotation marks and do not work. The first four special characters listed in this article are in double quotation marks or special characters. In order to make it have no special meaning, one is to use single quotation marks, and the other is to use/backslash to make it useless.

For example, we want to output these special characters as they are.

Echo """
Echo "$"
Echo "/"
Echo "'"
The above is not the expected result, because double quotation marks do not work for them, you can only output the prototype of these special characters
Echo '"'
Echo '$'
Echo '/'
Echo '''
Or
Echo "/""
Echo "/$"
Echo "//"
Echo "/'"
Will be displayed as "$ /'
5. Other special characters
We noticed that except the first four special characters, I put all the other special characters in one piece. This is because the first four special characters still have special meanings in double quotation marks, so I will explain them separately, if you want to output the prototype of these special characters, you can use double quotation marks or single quotation marks to make them lose their special meaning.
& Lt;, & gt ;;,*,?, [,] It has a special meaning for shell, but you can use double quotation marks to input these prototypes.

Have you noticed that all special characters have no special meaning in single quotes? If you want to output the original form of special characters but cannot remember that these special characters cannot output the original form in double quotes, we recommend that you use single quotes.

Introduction to conditional test statements today

I. If condition statements
Format:
If condition expression
Then # execute the following statement when the condition is true
Command list
Else # execute the following statement when false
Command list
Fi

If statements can also be nested

If condition expression 1
Then
If condition expression 2
Then
Command list
Else
If condition expression 3
Then
Command list
Else
Command list
Fi
Fi
Else
Command list
Fi

You can perform multi-layer nesting. An if statement must end with a fi to indicate the condition of the layer. Otherwise, syntax errors may occur.
The following are examples:
Here we first talk about the command test used in a condition statement to test whether the condition after test is true.

If test-F "$1"
Then
LPR $1
Else
If test-d "$1"
Then
CD $1
LPR $1
Else
Echo "$1 is not a file or directory"
Fi
Fi

The above example can also be changed to the following:

If test-F "$1"
Then
LPR $1
Elif test-d "$1" # Elif is the same as else if
Then
(CD $1; LPR $1)
Else
Echo "$1 is not a file or directory"
Fi

Do you understand the above examples?
If we save this example as prfile
Chmod + x prfile
Execute the program
./Prfile aaa

In this example, check whether your input parameter is a file. If it is a file, print it. If it is a directory, convert it to a directory and then print it. If it is not a file or a directory, a prompt is given.