Shell Learning Note 1 "Linux shell programming from beginner to proficient in 2nd edition"

1. number of logged-in users: who | Wc-l
2. See which users are logged on to the system : who
3. Create a new file /bin/ct11, enter a command in the file,chmod Set the Execute permission for the file

# cat >/bin/ct11

W.H.O. | Wc-l

Press Ctrl+d

# Cd/bin

# chmod +x ct11

#./ct11

Execution Result:

2

1. # echo escape sequence:

\c : Indicates that echo does not print the last line break symbol

\a : Warning character, usually the BEL character of ASCII

\b : Backspace

\f : Page Change

\ n : line break

\ r: Enter

\ t: Horizontal tab

\v: Vertical tab

\ \ : backslash character

\ODDD: Converts a character into a 3 -bit octal value

1. printf statement printf "I said '%s,%s1,%s2,%s3 ' \ n" This is my pen
2. Clear Screen :Clear
3. Echo input A text branch append to file cc11: echo-e "WERW\NSFSD" >>cc11
4. Remove the ASCII carriage return from the Aa.txt file and write the contents to file.txt

# tr-d ' \ R ' < Aa.txt > file.txt

1. Remove the carriage return from the Aa.txt file and then use the pipeline to sort its file contents and enter it into the file file.txt

# tr-d ' \ r ' <aa.txt | Sort >file.txt

1. turn off the ability to automatically print input characters Stty-echo
2. reply to the function of automatically printing input characters stty Echo
3. trace function set-x is to perform trace function open,set +x is turn off tracking function

Eg:cat > aa.sh

Set-x

Echo SS Echo

Set +x

Echo SDF Echo

Ctrl +d

Chmod +x aa.sh

./aa.sh Execution

1. Automated Test Course Catalog

2. Shell Pass Parameters

Create a new tst.sh script,

Nano-w tst.sh

The contents of the edit script are as follows, and then the first second parameter, and so on, is the file name parameter

#!/bin/sh

Name=$1

echo "My name is ${name}!"

Save Ctrl +o When edit is finished, Ctrl + C when exiting

Chmod +x tst.sh has execute permission for script

./tst.sh Zhang

Execution Result:

My name is Zhang

if it is vi mode edit mode press i, save and exit press ESC: wq!

1. unset command can delete variable, cannot be used again after deletion,unset command cannot delete read-only variable
2. Example:echo-e "Jdkfja \ n" output result:Jdkfja
3. here -E indicates the substitution of the escape character. If you do not use the-e option, it is output as-is: jdkfja\n
4. command substitution refers to the The shell can execute the command first, save the output temporarily, and output it in the appropriate place. Syntax for command substitution:

' Command '

Note that it is an anti-quote, not a single quotation mark, this key is located Below the ESC key.
In the following example, the command execution results are saved in a variable:

• If...then...fi is a conditional statement that will be explained later.

List of arithmetic operators
Operator	Description	Example
+	Addition	The ' expr a+a+b ' result is 30.
-	Subtraction	The ' expr a?a?b ' result is 10.
*	Multiplication	The ' expr a\*a\*b ' result is 200.
/	Division	The ' expr b/b/a ' result is 2.
%	Take surplus	The result of ' expr bba ' is 0.
=	Assign value	A= $b assigns the value of variable B to a.
==	Equal. Used to compare two numbers, the same returns true.	[A==a==b] returns FALSE.
!=	Not equal. Used to compare two numbers, and returns true if they are different.	[A!=a!=b] returns TRUE.

1. 
   NOTE: Conditional expressions are placed between square brackets and have spaces, such as [A==a==b] is wrong and must be written in [A==a==b].

Shell Annotations

1.1 Shell Comment # The entire code comment can be enclosed in curly braces, similar to a function

1.2 Shell String

1.2.1 Single quote str= ' This is a string ' single quote cannot appear in single quotation marks

1.2.2 double quotes str= "This is a string" double quotation marks can have variables that can appear escape characters

1.2.3 Stitching string yourname= "JKHJ" greeting= "Hello," $yourname "!"

1.2.4 Get string length string= "DFA" Echo ${#string}

1.2.5 extract substring string= "My name is ending" echo ${string:1:4} output:y na

1.2.6 Find string string= "My name is ane" echo ' Expr index "$string" is "

1.3 Shell Array

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

1.3.1 defining Arrays

Array_name={1 2 3 4}

or array_name={

1

2

3

4

}

Each component of an array is defined individually:

Array_name[0]=1

array_name[0]=2

1.3.2 reading an array

Value=${array_name[0]}

use @ or * to get all the elements in the array

Value=${array_name[*]}

Value=${array_name[@]}

1.3.3 Gets the length of the array

num=${#Array_name [@]} gets the number of array elements

length=${#Array_name [0]} gets the length of the first element of the array

1.3 Echo Command

1.3.1 echo "\" It is a test\ "" output: "It is a test"

1.4 Line break

1.4.1 echo "ok!\n" echo "It is a cat"

Output Result:

Ok!

It is a cat

1.5 Show No Line break

1.5.1 echo "ok! \c "echo " It is a dog "

output Result: ok! It is a dog

1.6 Display results redirected to file echo "It is a test" > myfile

1.7 output String as- is (not escaped), using single-quote echo ' $name \ '

1.8 Display current date echo ' Date '

1.9 Arithmetic Operations

1. Echo ' Expr 2 + 2 '
2. multiplication,echo ' expr 4 \* 2 '

1.10 Relational Operations

If [3-lt 8]

Then

Echo ' true '

Else

Echo ' false '

Fi

1.11 Shutting down the firewall

Systemctl Stop Firewalld.service

1.12 View shell types cat/etc/shells

1.13 new File touch filename

Shell Learning Note 1 "Linux shell programming from beginner to proficient in 2nd edition"