⚓ ⚓ ⚓ | set sail! Set sail! Let's learn to Shell language

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Why should I learn Shell Programming

A Linux o&M engineers need to write shell programs to manage server clusters
B for JavaEE and Python programmers, your supervisor will ask you to write some shell scripts for programs or server maintenance, such as writing a script to periodically back up a database
C For big data programmers, you need to write shell programs to manage the cluster

02 variable

#!/bin/Bash # case1Let's define the variable A, A=100$echo A=$A echo"A=$A"Case #2Undo variable A unset A echo"A=$A"

a A=`date`A b A=$(date) is equivalent to the backquote # P.S. No Spaces on either side of the equal sign1The variable name can consist of letters, digits, and underscores, but cannot start with a number. 5A=500It is wrong to2No Spaces on either side of the equals sign (note: this is different from the Java syntax for declaring variables)3The variable name is usually uppercaseCopy the code

03 Position parameter variables

#!/bin/bash
echo "0 = $0 1 = $1 2 = $2"
echo "All parameters =$*"
echo "$@"
echo "The number of arguments is $#"
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04 operator.

#!/bin/Bash # case1Calculation (:2+3) X4Use the first method: RES1=$(()2+3) *4))
echo "res1=$RES1"RES2=$[()2+3) *4]
echo "res2=$RES2"# Third way RES3=`expr 2 + 3 `
echo "RES3=$RES3"
echo "res4=`expr $RES3 \* 4`"Case #2: Requests the sum of the two arguments [integers] on the command line20 50
SUM=$[$1+ $2] where $1Represents the value of the first argument20 $2Represents the value of the second argument50
echo $SUM
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05 Process control

#!/bin/Whether bash # "man" is equal to"man"# Judge statements using =if [ "man" = "man" ]
then
echo "equal"
if [ 23‐ ge22 ]
then
echo "More than"
else
echo "Less than"
fi
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‐f: /root/shcode/aaa.txt: /root/shcode/aaa.txt: /root/shcode/aaa.txt: /root/shcode/aaa.txtif‐ echo. [‐f /root/shcode/aaa.txt] then echo"There"
else
echo "Nonexistent"
fi
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Flow control statement

Case statements and if… elif… The else statement is a multi-branch condition, but unlike the if multi-branch condition, the case statement can only determine one conditional relationship, whereas the if statement can determine multiple conditional relationships.

For loop of flow control

# Basic syntax1:forvariablein 值1 值2 值3..do
echo "On Monday"
;;
"2")
echo "Tuesday"
;;
"3")
echo "On Wednesday"
;;
"4")
echo "Thursday"
;;
"5")
echo "Friday"
;;
"6")
echo "Saturday"
;;
"Seven")
echo "Sunday"
;;
*)
echo "You entered something wrong.";; Esac program/code done # basic syntax2:

for(initial value; Circulation control conditions; Variable change)doProcedures are doneCopy the code

#!/bin/Bash # case1Print the arguments entered on the command line. #$* treats the arguments entered as a wholefor i in "$*"
do
echo "num is $i"Done # uses $@ to retrieve the input arguments. Note that this is treated separately, so each argument outputs several echo sentences"= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = ="
for j in "$@"
do
echo "num is $j"
done
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07 Flow control of while loop

whileLoop basic syntaxwhile[Conditional expression]doProcedure done note:whileThere is space between and [, as well as between conditions and [Copy the code

#!/bin/Bash # case1: Enter a number n from the command line for statistics1+2+... What is the value of plus n? SUM is equal to0
i=0
while [ $i ‐le $1 ]
doSUM = $[$SUM + $I] or written SUM = $(($SUM + $I)) # I since I = $increasing [$I +1] or I =$(($I +1 ))
done
echo $SUM =$SUM
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08 Read Reads console input

#!/bin/Bash # case1Read a num1 value entered by the console read ‐p"Please enter a number NUM1=" NUM1
echo "You enter NUM1=$NUM1"Case #2Read console input for a num2 value in10Input read ‐t in seconds10‐ p"Please enter a number NUM2=" NUM2
echo "You typed NUM2=$NUM2"
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09 function

#!/bin/Bash # calculates the sum of the input two arguments, and getSum # defines the function getSumfunction getSum() {
SUM=$[$n1+$n2]
echo "And is = $SUM"} # Input two values read ‐p"Please enter a number n1="N1 read ‐ p"Please enter a number n2="N2 # call getSum $n1 $n2Copy the code

09 random number

echo $[$RANDOM%3#] produce0-3Echo $[$RANDOM%4#] produce1-50Between $[$RANDOM%50+1[$RANDOM%50The resulting number is0-49# produce10-99Between $[$RANDOM%89+10]
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So let’s practice with a couple of problems.