Linux Shell Programming Basics Notes

1.shell variables
Local variables:
A=1 (no spaces on both sides of the equal sign)
Read-only variables:
ReadOnly variable name = variable value (cannot be changed during use)
Declaring variables:
Linux assigns a value to a variable by default as a character or string, and if a numeric operation is required, you need to declare it beforehand
DECLARE parameter variable name [= Variable value]
Declare parameters:-A: Array-i: INT-r: Read-only
To set an integer variable, in addition to the Declare-i declaration, you can:
a=$ (($a + 1)) or a=$[a+1]
To clear a variable:
unset variable name (read-only variable cannot be cleared)

To access the shell variable:
Set shows all locally defined shell variables
${a} takes the value of variable a
echo Display Variable
Echo's Parameters:
All parameters are printed and converted to the next line
-N does not convert to the next line after printing


To assign a value to a variable from the keyboard read-in:
Read [parameter] variable name
Read variable name variable name variable names ...
If you do not follow the variable name, the input is assigned to $reply
Read parameter:
-a means that the input is stored in the array variable name
-P Displays the prompt string after-p before the value is assigned, such as Read-p "?" a displays:? input


Environment variables:
Export environment variables (available for external invocation):
Export variable Name

Positional parameter variables:
The command-line arguments that are used to save the script when the script is invoked
Echo ${1},${2},...
Position parameter shift:
Shift [move forward], default forward 1
Such as:
Shift 2 Moves the two-bit to the left, usually in the loop structure


Special variables:
$ A: Holds the currently executing script file name, including the path
$#: The number of positional parameters that are passed to the script
$*: Holds all parameters passed to the script
[Email protected]: Same as $*, but each parameter is enclosed in quotation marks
$$: Holds the current process number
$!: The process number that holds the last process running in the background
$?: The exit status of the last executed script or command is stored 0 indicates success, not 0 indicates an error

Reference:
Double quotes: Weak references, to $, ', \ Invalid
Single quotes: Strong references, any special characters are masked
Backslash: Escaped

Variable substitution:
${varname:-word} If varname is undefined or empty use Word instead of varname as the return value
${varname:=word} If varname is undefined or empty, Word is assigned to varname and returned
${varname:?message} output message as error message if varname undefined or empty
${varname:+word} use Word as the return value if VarName is defined and is not empty
${varname:offset} returns the string starting at the beginning of the offset character in $varname to the end, taking the tail string
${varname:offset:length} starts the offset character in $varname, and a string of length is returned, taking a substring

2. Process Control
Sequential structure:
Nothing to say

Condition test:
Test command
Check file status, determine if string matches, numeric test, etc.
Can be used in the branch structure, but also in the loop structure
Test condition
Or
[Test conditions] (a space is required between the test condition and the left and right square brackets)

File test
Test [condition] FILE

-E If file exists
-S if file is not empty
-D If file is a directory
-F If file is a normal document
-B If file is a block device files
-C If file is a character device files
-S if file is a socket
-L If file is a symbolic connection
-P If file is a well-known pipe

-F If file has Read permission
-W If file has Write permission
-X if file has Execute permission
-G If file is owned by a valid group
-O If file is owned by a valid user
-K if file has the sticky bit set
-U if file has the Set-user-id bit set
-G If file has the Set-group-id bit set
-N If file has been modified since it was last accessed

File1-nt FILE2 if FILE1 than FILE2 new
File1-ot FILE2 If FILE1 is older than FILE2
File1-ef FILE2 If FILE1 is a hard connection file for FILE2, they point to the same I node


String test
string1 = string2 strings are equal
String1! = string2 strings are not equal
-N string1 string non-null
-Z string1 string is empty

Numerical test
Expr1-eq EXPR2 expression values are equal
Expr1-ne EXPR2 expression values are not equal
EXPR1-GT EXPR2 expression 1 greater than expression 2
EXPR1-LT EXPR2 expression 1 is less than expression 2
Expr1-ge EXPR2 expression 1 is greater than or equal to an expression 2
Expr1-le EXPR2 expression 1 is less than or equal to an expression 2

logical operators
With-a [$a = "a"-a $b = "B"] if ($a = = "a" && $b = = "B")
Or-o [$a = "a"-O $b = "B"] if ($a = = "a" | | $b = = "B")
Non -! [! a= "A"] if ($a! = "a")


Branching structure: (if branch, Case branch)
If branch structure:
If condition test 1;then
Conditional Test 1 is true when the statement
[elif condition Test 2;then]
Conditional Test 2 is true when the statement
...
[Else]
Sequence of statements when a condition test is false
Fi
#如果条件测试语句与then在一行, then use; separate from then
#条件测试语句与if, there must be at least one space separated between elif,else,then
#elif, else optional, else can only appear once
#if可以嵌套

Case Branching structure:
Case value in
Mode 1)
Statement Group 1
...
;;
Mode 2)
Statement Group 2
...
;;
......
*)
Statement Group N
Esac


Loop structure
For loop:
For variable in value 1 value 2 ... Value n
Do
Statement 1
Statement 2
...
Done

A format similar to the C language
For ((i=1;i<=6;i++))
Do
...
Done


While loop:
While condition
Do
Statement 1
Statement 2
...
Done


Select loop:
Select name in [in list]
Do
Statement list
Done

3.shell function
Function definition:
function funcname
{
 shell commands
}
or
FuncName ()
{
 shell commands
}

Parameter:
Similar to the parameters of the script, using the $1,$2 ...
such as

#!/bin/bash
My_func ()
{
 a=$1
 echo $a
}

Read input
My_func () $input The return of the

function can be used:
return