Shell script re-learning

#！ /bin/bash

Two ways to run the shell

Change Permissions chmod u+x

Specifying the interpreter at run time

Shell variables

Variable type

accessing variables

$, ${}

Read-only variables

ReadOnly

Delete a variable

Unset cancels a variable, but cannot cancel a read-only variable

Myurl= "http://lingdandan.blog.51cto.com" unset myurlecho $myUrl

Operation Result:

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Myurl= "readonly myurl unset Myurlecho $MYURL

Operation Result:

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Shell string

Strings can be in single or double quotes, or without quotes

Single quotation marks

Characters in single quotes are output as-is

Single quotation mark cannot have single quotation mark (escape character)

Double quotes

Double quotes inside can have variables

Single quotes can have escape characters

string concatenation

Language=english

echo "Hello ${language}"

String length calculation

echo ${#VAR}

Echo ${var} | Wc-c

echo $TEST _shell_var | awk ' {print length ($)} '

Extract substring

#* characters

##* characters

Characters

Percent characters *

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VAR stands for variable

${var:0:10} represents 10 characters from the beginning of the first character on the left

${var:7} represents all characters after the start of the seventh character

${var:0-3:10} represents 10 characters from the third character of the right number

${VAR:0-14} represents all characters after the start of the 14th character in the right number

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Find string

Shell Array

Array definition

Bash supports only one-dimensional arrays and does not support multidimensional

Subscript starting from 0, can be an integer or an expression

Arr= (value 1 value 2 value 3) parenthesis space separated

You can also define individual components individually

Array Read

${arr[Subscript]}

Use @ To get all the elements of an array

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Get array length

${#arr [@]}

${#arr [*]}

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Shell-Pass parameter

Format of getting parameters in script: $n

$ $

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$* and [email protected] All refer to all parameters, the difference is that $* the argument as a string, [email protected] The argument as a single string

$* "1 2 3" Only one parameter was passed

[Email protected] "1" "2" "3" came out of three parameters

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Shell Basic Operations

Native bash does not support simple arithmetic operations and can be done with other commands such as awk and expr.

Arithmetic operations

+ - * / % = == !=

Var= ' Expr 3 \* 5 '

Relational operators

-eq-nq-gt-lt-ge-le

Relational operators only support numbers, and strings are not supported

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String operations

= =-z-n Str

Expr http://lingdandan.blog.51cto.com/10697032/1791485

Expr match string expression expr string: expression

Expr Index string character

Expr length string

Expr substr string offset length (offset starting from 1)

Boolean operations

！ -o-a

logical operators

&& | |

File Test Operators

[-F file]-d-b-c-p-r-w-x-s (file is greater than 0)-e (directory or file exists)

For

For variable in list

Do

Done

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This article from "Zero Egg" blog, declined reprint!

Shell script re-learning