Mycpen

[root@sanchuang-linux dev]# dd if=/dev/zero of=/tmp/test.dd bs=1M count=5
5+0 records in
5+0 records out
5242880 bytes (5.2 MB, 5.0 MiB) copied, 0.00196718 s, 2.7 GB/s
[root@sanchuang-linux dev]# du -sh /tmp/test.dd 
5.0M	/tmp/test.dd
if where to input, of where to output this file, bs data unit size, count data unit number

Example: Environment Variables
------------------------------------------
[root@sanchuang-linux chenpeng]# which ls
alias ls='ls --color=auto'
	/usr/bin/ls							#(Note: Environment variable)
[root@sanchuang-linux chenpeng]# echo $PATH	#(Note: Environment variable)
/lianxi/sc:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/nginx/sbin:/root/bin:/usr/local/nginx5/sbin:/root/bin
Example 2: Local Variables
-------------------------------------------
a=1
echo $a
echo ${a}

Example:
--------------------------------------------
[root@sanchuang-linux chenpeng]# echo $PATH		#(Note: Environment variable)
/lianxi/sc:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/nginx/sbin:/root/bin:/usr/local/nginx5/sbin:/root/bin

Example: __________________________________
[root@mysql-binary shell_test]# echo $?
1
[root@mysql-binary shell_test]# perror 1
OS error code   1:  Operation not permitted

[root@sanchuang-linux shell_test]# echo $0	#(Note: $0 Current process or script name)
-bash
[root@sanchuang-linux shell_test]# sh canshu.sh "hello" "world"
########This is hello########
hello
########This is world########
world
canshu.sh									#(Note: $0 Current process or script name)
[root@sanchuang-linux shell_test]# cat canshu.sh 
#!/bin/bash

echo "########This is $1########"
echo "$1"
echo "########This is $2########"
echo "$2"

echo "$0"

[root@sanchuang-linux shell_test]# vim canshu.sh
#!/bin/bash

echo "########This is $1########"
echo "$1"
echo "########This is $2########"
echo "$2"
echo "$0"

echo "This is all:$*"			#(Note: $* represents all parameter contents)
echo "Number of parameters: $#"			#(Note: $# represents the number of parameters)
echo "This is @:$@"			#(Note: $@ represents all parameters (extracted one by one))
──────────────────────────────────────────────
[root@sanchuang-linux shell_test]# sh canshu.sh hello world 2020	  #(Note: 3 parameters)
########This is hello########
hello
########This is world########
world
canshu.sh
This is all:hello world 2020
Number of parameters:3
This is @:hello world 2020

[root@sanchuang-linux ~]# vim canshu.py
import sys

print(sys.argv[1],sys.argv[2])		#(Note: 1 receives parameter 1, 2 receives parameter 2)
print(sys.argv[0])					#(Note: 0 is the filename)
---------------------------------------------------------------------- 
[root@sanchuang-linux ~]# python3 canshu.py "hello" "world"
hello world
canshu.py

Example: String Definition__________________
[root@sanchuang-linux ~]# echo abc
abc
[root@sanchuang-linux ~]# a=b
[root@sanchuang-linux ~]# echo $a
b
[root@sanchuang-linux ~]# a="b"
[root@sanchuang-linux ~]# echo $a
b
[root@sanchuang-linux ~]# a='b'
[root@sanchuang-linux ~]# echo $a
b

Example: Number Definition_________________
[root@sanchuang-linux ~]# a=1
[root@sanchuang-linux ~]# a=2

[root@sanchuang-linux ~]# head -n1 /etc/passwd			#(Note: output the first line of passwd)
root:x:0:0:root:/root:/bin/bash
[root@sanchuang-linux ~]# cat /etc/passwd |head -n1		#(Note: not recommended to use this 2 commands)
root:x:0:0:root:/root:/bin/bash

#!/bin/bash

# String Operations
line=`head -n1 /etc/passwd`		#(Note: using backticks ``)(Note: save command output in line)
echo $line
---------------------------------------------------------------------------------
[root@sanchuang-linux chenpeng]# bash test2.sh 
root:x:0:0:root:/root:/bin/bash

echo "The string is: $line"
The string is: root:x:0:0:root:/root:/bin/bash
------------------------------------------
echo 'The string is: $line'
The string is: $line

Substring
Extract the first 4 characters: echo ${line:0:4}
Extract the last 9 characters  echo ${line:0-9}
Extract 4 characters starting from the ninth last character echo ${line:0-9:4}
Extract the last character from left to right: after the character   echo ${line##*:}
Extract the first character from left to right     echo ${line#*:}
Extract the last character from right to left: after the character	echo ${line%%:*}
Extract the first character from right to left	echo ${line%:*}
String length	echo ${#line}

Example: String Operations_______________________________
# String Operations
[root@sanchuang-linux chenpeng]# vim test2.sh
line=`head -n1 /etc/passwd`
echo $line				#(Note: root:x:0:0:root:/root:/bin/bash)
echo "The string is: $line"		#(Note: The string is: root:x:0:0:root:/root:/bin/bash)

echo 'The string is: $line'		#(Note: The string is: $line)

echo "Extract the first 4 characters:"
echo ${line:0:4}			#(Note: root)

echo "Extract the last 9 characters"
echo ${line:0-9}			#(Note: /bin/bash)

echo "Extract 4 characters starting from the ninth last character"
echo ${line:0-9:4}			#(Note: /bin)

echo "Extract the last character from left to right: after the character"
echo ${line##*:}			#(Note: /bin/bash)

echo "Extract the first character from left to right"
echo ${line#*:}			#(Note: x:0:0:root:/root:/bin/bash)

echo "Extract the last character from right to left: after the character"
echo ${line%%:*}			#(Note: root)

echo "Extract the first character from right to left"
echo ${line%:*}			#(Note: root:x:0:0:root:/root)

echo "String length"
echo ${#line}				#(Note: 31)
-----------------------------------------------
[root@sanchuang-linux chenpeng]# bash test2.sh 
root:x:0:0:root:/root:/bin/bash
The string is: root:x:0:0:root:/root:/bin/bash
The string is: $line
Extract the first 4 characters:
root
Extract the last 9 characters
/bin/bash
Extract 4 characters starting from the ninth last character
/bin
Extract the last character from left to right: after the character
/bin/bash
Extract the first character from left to right
x:0:0:root:/root:/bin/bash
Extract the last character from right to left: after the character
root
Extract the first character from right to left
root:x:0:0:root:/root
String length
31

line="http://www.baidu.com/login"

# Extract: login
echo ${line:0-5}				#(Note: take the last 5 characters)
echo ${line##*/}				#(Note: from left to right the last content after /)

# Extract: www.baidu.com/login
echo ${line##*//}

# Extract: http://www.baidu.com
echo ${line%/*}

# Extract: http:
echo ${line%%/*}

Example: ↓↓↓↓↓↓↓↓↓↓↓↓↓↓
[root@sanchuang-linux ~]# a=10
[root@sanchuang-linux ~]# b=20
[root@sanchuang-linux ~]# $(($a + $b))
-bash: 30: command not found
[root@sanchuang-linux ~]# echo $(($a + $b))
30
[root@sanchuang-linux ~]# echo $[ $a +$b ]
30
[root@sanchuang-linux ~]# expr $a + $b
30
[root@sanchuang-linux ~]# expr $a+$b
10+20

Syntax 1: ↓↓↓↓↓↓↓↓
-----------------------
for variable in value1 value2
do
	execute loop statements
done
=======================================
Syntax 2: ↓↓↓↓↓↓↓↓
---------------------------------------
# for ((i=0;i<3;i++))
for ((initialize variable; condition to end loop; operation))
do 
	execute loop statements
done

Syntax 1: ↓↓↓↓↓↓
---------------------------------------
while read line
do 
	execute loop statements
done
=======================================
Syntax 2 ↓↓↓↓↓↓↓↓↓↓
---------------------------------------
while [condition (optional)]:
do
	execute loop statements
done
=======================================
Note: also supports break, continue

Syntax 1: ↓↓↓↓↓↓
-------------------------
if condition
then
	execute statements
fi
=========================
Syntax 2: ↓↓↓↓↓↓
if condition
then
	execute statements
else
	execute statements
fi
==========================
Syntax 3: ↓↓↓↓↓↓↓
----------------------
if [ command ];then
execute statements that meet the condition
elif [ command ];then
execute statements that meet the condition
else
execute statements that meet the condition
fi

Syntax: ↓↓↓↓↓↓________________
case $variable in
condition1)
	execute statement one
	;;
condition2)
	execute statement two
	;;
*）
esac

Example: ↓↓↓↓↓↓↓↓↓_________________________
[root@sanchuang-linux chenpeng]# vim num_test.sh
#!/bin/bash

read -p "Please enter the first number: " num1
read -p "Please enter the second number: " num2

echo "================"
echo "1.add Addition"
echo "2.sub Subtraction"
echo "3.mul Multiplication"
echo "4.exit Exit"
echo "================"

read -p "Please enter your choice: " options
case $options in
1)
    echo "The sum of the two numbers is: $(($num1 + $num2))"
    ;;
2)
    echo "The difference of the two numbers is: $(($num1 - $num2))"
    ;;
3)
    echo "The product of the two numbers is: $(($num1 * $num2))"
    ;;
4)
    echo "Exiting!"
    exit
esac
-------------------------------------------------------------------------------------------
Make it into function form
add(){
    echo "The sum of the two numbers is: $(($num1 + $num2))"
}

case $options in
1)
    add			#(Note: call when needed)
    ;;
2)…………………………

[root@sanchuang-linux chenpeng]# cd /etc/init.d/              
[root@sanchuang-linux init.d]# ls                             
functions  README           

Example: Service restart script using case ↓↓↓↓↓↓__________________
case $mode in
	start)
		start
		;;
	stop)
		stop (using kill command)
		;;
	restart)
		stop
		start
		;;
	reload)
		reload configuration (using kill -HUP)
		;;
esac            

Kill is used to terminate running programs or jobs
Kill can send specified information to the program

# kill -l can view kill signals (kill -L (lowercase))
# kill -0 is used to check if a process exists, when the process does not exist, kill -0 will report an error
# kill -1 pid reload process (commonly used)
# kill -HUP pid and kill -1 pid are the same
# kill -1 pid or kill -HUP pid both indicate reloading this file
# kill -9 force kill
# kill -15 normally stop a process
Kill without a signal defaults to signal 15
Other signals, except signal 9, can be refused by the process!

Example: ↓↓↓↓↓↓↓↓↓↓____________
[root@sanchuang-linux ~]# kill -l
 1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL	 5) SIGTRAP
 6) SIGABRT	 7) SIGBUS	 8) SIGFPE	 9) SIGKILL	10) SIGUSR1
…………………………
63) SIGRTMAX-1	64) SIGRTMAX	

Example: ↓↓↓↓↓↓____________________________________________________________
[root@sanchuang-linux ~]# if id wenyao; then echo "ok"; else echo "error"; fi
id: “wenyao”: no such user
error
--------------------------------------------------------
Equivalent to: ↓↓↓↓↓↓________________________________________________
if id wenyao; 
then 
	echo "ok"; 
else 
	echo "error"; 
fi

Common Judgments:
[ -d FILE ] If FILE exists and is a directory, it returns true.
[ -f FILE ] If FILE exists and is a regular file, it returns true.
[ -e **** ] Check if file/folder exists

String Judgments:
[ -z STRING ] If the length of STRING is zero, it returns true, that is, empty is true
[ -n STRING ] If the length of STRING is non-zero, it returns true, that is, non-empty is true
[ STRING1 ] If the string is not empty, it returns true, similar to -n
[ STRING1 == STRING2 ] If the two strings are the same, it returns true
[ STRING1 != STRING2 ] If the two strings are different, it returns true
[ STRING1 < STRING2 ] If "STRING1" is lexicographically before "STRING2", it returns true.
[ STRING1 > STRING2 ] If "STRING1" is lexicographically after "STRING2", it returns true.

Numerical Judgments
[ INT1 -eq INT2 ] If INT1 and INT2 are equal, it returns true, =
[ INT1 -ne INT2 ] If INT1 and INT2 are not equal, it returns true, <>
[ INT1 -gt INT2 ] If INT1 is greater than INT2, it returns true, >
[ INT1 -ge INT2 ] If INT1 is greater than or equal to INT2, it returns true, >=
[ INT1 -lt INT2 ] If INT1 is less than INT2, it returns true, <
[ INT1 -le INT2 ] If INT1 is less than or equal to INT2, it returns true, <=

Logical Judgments
[ ! EXPR ] Logical NOT, if EXPR is false, it returns true.
[ EXPR1 -a EXPR2 ] Logical AND, if EXPR1 and EXPR2 are both true, it returns true.
[ EXPR1 -o EXPR2 ] Logical OR, if EXPR1 or EXPR2 is true, it returns true.
[ ] || [ ] Use OR to combine two conditions
[ ] && [ ] Use AND to combine two conditions

Example: ↓↓↓↓↓↓↓↓↓↓↓↓↓↓
[root@sanchuang-linux ~]# a=10
[root@sanchuang-linux ~]# b=20
[root@sanchuang-linux ~]# if [ $a -gt $b ];then echo "a>b";else echo "a<b";fi	#(Note: correct)
a<b
[root@sanchuang-linux ~]# if [ $a > $b ];then echo "a>b";else echo "a<b";fi	#(Note: error)
a>b						(Note: using two brackets does not cause an error)
[root@sanchuang-linux ~]# if [[ $a > $b ]];then echo "a>b";else echo "a<b";fi	#(Note: correct)
a<b

[root@sanchuang-linux ~]# if [ $a -gt $b ] && [ $a -ne 20 ];then echo "Output a>b";else echo "Output a<b";fi
Output a<b

Example: ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
Writing 1:
if [ -f a.txt ];
    then echo "File exists"
else
    touch a.txt
fi
-------------------------------------------------
Writing 2: Recommended (similar to Python's if ternary operator)
[ -f a.txt ] && echo "File already exists" || touch a.txt

Example: Previous Exercise ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
[ -f a.txt ] && echo "File already exists" || touch a.txt
[[ -f a.txt ]] && echo "File already exists" || touch a.txt		#(Note: recommended to use two brackets)

Example:
-------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# a=10
[root@sanchuang-linux ~]# b=20
[root@sanchuang-linux ~]# [ $a -gt $b ] && echo "Output a>b"
[root@sanchuang-linux ~]# [ $a -gt $b ] || echo "Output a<b"
Output a<b
[root@sanchuang-linux ~]# [ $a -gt $b ] && echo "Output a>b" || echo "Output a<b"
Output a<b

Example 1: When judging if
-------------------------------------------------------------------------------------------
[root@sanchuang-linux chenpeng]# name="wen yao"
[root@sanchuang-linux chenpeng]# [ $name == "wen yao" ] && echo "ok" || echo "error"
-bash: [: too many arguments		#(Note: automatically does word splitting)
error
[root@sanchuang-linux chenpeng]# [[ $name == "wen yao" ]] && echo "ok" || echo "error"
ok							#(Note: recommended to use two brackets)
[root@sanchuang-linux chenpeng]# [ "$name" == "wen yao" ] && echo "ok" || echo "error"
ok					#(Note: using quotes to connect together indicates a whole)
============================================================================================

Example 2: Numerical Comparison
-------------------------------------------------------------------------------------------
[root@mysql-binary shell_test]# echo $a
10
[root@mysql-binary shell_test]# echo $b
20
[root@mysql-binary shell_test]# [[ $a > $b ]] && echo "ok" || echo "error"
error
[root@mysql-binary shell_test]# [ $a > $b ] && echo "ok" || echo "error"
ok							#(Note: error)
[root@mysql-binary shell_test]# (( $a == $b )) && echo "ok" || echo "error" 
error

Example:
--------------------------------------------------------------------------------------------
[root@mysql-binary shell_test]# a=10
[root@mysql-binary shell_test]# b=20
[root@mysql-binary shell_test]# [[ $a -eq $b ]] && echo "ok" || echo "eroor"
eroor
[root@mysql-binary shell_test]# (( $a -eq $b )) && echo "ok" || echo "eroor"
-bash: ((: 10 -eq 20 : syntax error in expression (error symbol is "20 ")
Eroor
-------------------------------------------------------------------------------
[root@mysql-binary shell_test]# c=102
[root@mysql-binary shell_test]# b=20
[root@mysql-binary shell_test]# [[ $c > $b ]] && echo "ok" || echo "eroor"
eroor
[root@mysql-binary shell_test]# (( $c > $b )) && echo "ok" || echo "eroor"
ok

Example 3: Two Ways of Writing If Condition Judgments
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# a=10
[root@sanchuang-linux ~]# b=20
[root@sanchuang-linux ~]# if [[ $a > $b ]]; then echo "ok"; else echo "error"; fi
error
[root@sanchuang-linux ~]# [[ $a > $b ]] && echo "ok" || echo "error"
error

Example: String Comparison (( )) can also be used
--------------------------------------------------------------------------------------------
[root@sanchuang-linux ~]# a=abc
[root@sanchuang-linux ~]# b=abc1
[root@sanchuang-linux ~]# (( $a > $b )) && echo "ok" || echo "error"
error
[root@sanchuang-linux ~]# a=abc
[root@sanchuang-linux ~]# b=bac1
[root@sanchuang-linux ~]# (( $a > $b )) && echo "ok" || echo "error"
error
[root@sanchuang-linux ~]# a=abc
[root@sanchuang-linux ~]# b=abc
[root@sanchuang-linux ~]# (( $a == $b )) && echo "ok" || echo "error"
ok

Conclusion: It is recommended to use [[ ]] for comparison operations and judgments

Example:
add() {
    echo "The sum of the two numbers is: $(( $num1 + $num2 ))"		#(Note: content of operations inside the function)
}
------------------------------------------------
Call when needed add
case $options in 
1)
	add
	;;
2)……………………
--------------------------------------------------------------------------------------------
add(){
    echo "The sum of the two numbers is: $(($num1 + $num2))"
}

case $options in
1)
    add			#(Note: call when needed)
    ;;
2)…………………………

Example: test
---------------------------------------------------------------------
[root@sanchuang-linux ~]# a=123
[root@sanchuang-linux ~]# b=123
[root@sanchuang-linux ~]# test a==b && echo ok
ok
[root@sanchuang-linux ~]# test a==b && echo ok || echo error
ok