The first lesson of shell script learning

The shell is a programming language and a service that accesses the operating system kernel.

The types of shell that are common to Linux are:

Bourne Shell (/usr/bin/sh or/bin/sh)

Bourne Again Shell (/bin/bash)

C Shell (/USR/BIN/CSH)

K Shell (/usr/bin/ksh)

Shell for Root (/sbin/sh)

Two ways to execute a shell script

./shell.sh executes according to the shell specified in the first line of the shell script

/bin/sh Test.sh executes according to the shell specified in the command line

/bin/php test.php

Shell variables

Define variable: your_name= "runoob.com" for file in ' ls/etc '

Use variable: Echo $your _name echo ${your_name} echo "I am good at ${skill}script"

Delete variable: unset variable_name

Variable type

1) Local Variables

2) Environment variables

3) Shell variables

Shell string

Single quote: Str= ' This is a string '

Double quotes:

Your_name= ' QINJX '
Str= "Hello, I know your is \" $your _name\ "! \ n "

Stitching strings:

Your_name= "QINJX"
greeting= "Hello," $your _name "!"
greeting_1= "Hello, ${your_name}!"
echo $greeting $greeting _1

Get string Length:

string= "ABCD"
echo ${#string} #输出 4

To extract substrings:

String= "Runoob is a great site"
echo ${string:1:4} # Output Unoo

To find a substring:

String= "Runoob is a great company"
echo ' expr index ' $string ' is ' # output 8

Shell Array

Bash supports one-dimensional arrays, does not support multidimensional arrays, and does not limit the size of arrays

Define array:: Array name = (value 1 value 2 ...) Value N) array_name= (value0 value1 value2 value3)

or ARRAY_NAME[0]=VALUE0

Array_name[1]=value1
Array_name[n]=valuen

Read array: ${array name [subscript]} Valuen=${array_name[n]}

Use the @ symbol to get all the elements of the array: Echo ${array_name[@]}

Gets the length of the array:

# Gets the number of array elements
length=${#array_name [@]}
# or
length=${#array_name [*]}
# Gets the length of an array of individual elements
lengthn=${#array_name [n]}

Shell Pass Parameters

When we execute a shell script, we pass parameters to the script, and the format of the parameters obtained in the script is: $n. n represents a number, 1 is the first argument to execute the script, 2 is the second argument to execute the script, and so on.

Example:

＃！ /bin/bash

echo "Shell pass parameter instance! ";

echo "Executed the file name: $";

echo "The first parameter is: $ zero";

echo "The second parameter is: $";

echo "The third parameter is: $ $";

Execution results

./test.sh 1 2 3

Shell Pass argument instance!

File name executed:./test.sh

The first parameter is: 1

The second parameter is: 2

The third parameter is: 3

In addition, several special characters are used to handle parameters

Parameter description	Description
$#	The number of arguments passed to the script
$*	Displays all parameters passed to the script in a single character. If "$*" is enclosed in ["], output all parameters in the form of" $ $ ".
$$	The current process ID number for the script to run
$!	ID number of the last process running in the background
[Email protected]	Same as $*, but quoted when used, and returns each parameter in quotation marks. such as "[email protected]" in the case of ["], with" $ "" $ "" $ "..." $n "to output all parameters
$-	Displays the current options used by the shell, same as the SET command function
$?	Displays the exit status of the last command. 0 means there is no error, and any other value indicates an error.

Example:

#!/bin/bash

echo "Shell pass parameter instance! ";

echo "The first parameter is: $ zero";

echo "Number of parameters: $#";

echo "The number of arguments passed as a string display: $*";

Execute the script with the following results:

./test.sh 1 2 3

Shell Pass argument instance!

The first parameter is: 1

Number of parameters: 3

The number of arguments passed is displayed as a string: 1 2 3

The difference between $* and [email protected]

Same point: All parameters are referenced

Different points: only in quotation marks. Assuming that the script was written with the parameters 1, 2, 3, then "*" is equivalent to "1 2 3" (passing a parameter), and "@" is equivalent to "1" "2" "3" (three parameters passed).

#!/bin/bash

echo "-\$* Demo"

For i in "$*";d o

Echo $i

Done

echo "--\[email protected] Demo"

For i in "[email protected]";d o

Echo $i

Done

Execute the script with the result as shown below

./test.sh 1 2 3

--$* Demo--

1 2 3

[Email protected] Demo--

1

2

3

Shell Array

The shell array is represented by parentheses, and the elements are separated by a "space" symbol.

Shell operators

Expr is an expression evaluation tool that uses it to perform evaluation operations on expressions.

Val= ' Expr 2 + 2 ';

echo "The sum of two numbers: $val";

Two note: There is a space between the expression and the operator, for example, it is not correct and must be written in 2 + 2, which is not the same as most of the programming languages we are familiar with.

The complete expression is to be contained, note that this character is not a common single quote, below the ESC key.

Arithmetic operators

Operator	Description	Example
+	Addition	' Expr $a + $b ' result is 30
-	Subtraction	' Expr $a-$b ' result is-10
*	Multiplication	' Expr $a \* $b ' result is 200
/	Division	' Expr $b/$a ' result is 2
%	Take surplus	' Expr $b% $a ' result is 0
=	Assign value	A= $b will assign the value of variable B to a.
==	Equal. Used to compare two numbers, the same returns true.	[$a = = $b] returns false
!=	Not equal. Used to compare two numbers, and returns true if they are different.	[$a! = $b] returns to True

Relational operators

Relational operators only support numbers, and strings are not supported unless the value of the string is a number

Operator	Description	Example
-eq	Detects whether two numbers are equal and returns true for equality	[$a-eq $b] returns false
-ne	Detects whether two numbers are equal and returns true if they are not equal	[$a-NQ $b] returns True
-gt	Detects if the number on the left is greater than the right and, if it is, returns true	[$a-gt $b] returns false
-lt	Detects if the left number is less than the right, and returns true if it is	[$a-lt $b] returns True
-ge	Detects if the left number is greater than or equal to the right, and returns true if it is	[$a-QE $b] returns false
-le	Detects if the left number is less than or equal to the right, and returns true if it is	[$a-le $b] returns True

Boolean operator

Operator	Description	Example
!	Non-arithmetic, expression true returns false, otherwise true	[!false] Returns True
-O	Or operation, an expression of true is true	[$a-lt 20-o $b-GT 100] Returns True
-A	With arithmetic, two expressions are true to return true	[$a-lt 20-a $b-GT 100] returns false

logical operators

Operator	Description	Example
&&	Logic and	[[$a-lt && $b-GT 100]] returns false
||	Logical OR OR	[[$a-lt | | $b-GT 100]] returns True

string Operators

A= "abc" b= "EFG"

Operator	Description	Example
=	Detects whether a string is equal and returns true for equality	[$a = $b] returns false
!=	Detects whether two strings are equal, not equal returns True	[$a! = $b] Returns True
-Z	Detects whether the string length is 0 and returns true for 0	[-Z $a] returns false
-N	Detects whether the string length is 0 and does not return true for 0	[-N $a] returns true
Str	Detects if the string is empty and does not return true for null	[$a] returns True

File Test Operators

The variable file represents the document "/var/www/runoob/test.sh", which is 100 bytes in size and has RW permissions. The following code detects the properties of the file.

Operator	Description	Example
-B File	Detects if the file is a block device file, and returns True if it is	[-B $file] returns false
-C file	Detects if the file is a character device file, and returns True if it is	[-C $file] returns false
-D File	Detects if the file is a directory, and returns True if it is	[-D $file] returns false
-F File	Detects if the file is a normal file (neither a directory nor a device file), and if so, returns True	[-F $file] returns True
-G file	Detects if the file has a sgid bit set, and returns True if it is.	[-G $file] returns false
-K File	Detects if the file has a sticky bit set (sticky bit), and returns True if it is	[-K $file] returns false
-P File	Detects if the file is a well-known pipe, and if so, returns True	[-P $file] returns false
-U file	Detects if the file has a suid bit set, and returns True if it is	[-U $file] returns false
-R File	Detects if the file is readable and returns true if it is	[-R $file] returns true
-W File	Detects if the file is writable and returns true if it is	[-W $file] returns True
-X File	Detects if the file is executable and returns true if it is	[-X $file] returns True
-S file	Detects whether the file is empty (file size is 0), NOT NULL returns True	[-S $file] returns True
-E File	Detects whether the file (including the directory) exists and, if so, returns True	[-e $file] returns True

Shell echo Command

1. Display normal string

echo "It is a test"

The double quotes here are completely omitted, and the following commands are identical to the previous instance:

Echo It is a test

2. Show escape characters

echo "\" It is a test\ ""

The result will be

"It is a test"

3. Display variables

#!/bin/bash

Read name

echo "$name It is a test"

Name receives a variable of standard revenue, the result will be:

SH test.sh

OK #标准输入

OK It is a test #输出

4. Show line break

Echo-e "ok! \ n "#-e Open escape

echo "It is a test"

Output results

Ok!

It is a test

5. Show No Line break

#!/bin/bash

Echo-e "ok! \c "#-e Open escape \c No Line break

echo "It is a test"

Output results

Ok! It is a test

6. Display results directed to file

echo "It is a test" > myfile

7. Output a string as is, without escaping or taking a variable (in single quotes)

echo ' $name \ '

Output results

$name \ "

8. Show command Execution results

Echo ' Date '

The result will show the current date

Mon Dec 18:33:04 UTC 2016

The first lesson of shell script learning