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Shell advanced

2022-07-01 19:37:00 【Upward Wolf】

One 、Shell Operator

1.1、 Classification of operators

Arithmetic operator

Relational operator

Boolean operator

String operators

File test operators

1.2、 Arithmetic operator

expr Is an expression calculation tool , It can be used to evaluate expressions .

#!/bin/bash
a=10
b=20
val=`expr $a + $b` notes : here = No spaces on either side of the sign , And that is + There should be spaces on the left and right , The following are the same rules
echo "a + b : $val"
val=`expr $a - $b` notes : Used here ` Is in Esc The symbol below
echo "a - b : $val"
val=`expr $a \* $b`
echo "a * b : $val"
val=`expr $b / $a`
echo "b / a : $val"
val=`expr $b % $a`
echo "b % a : $val"
if [ $a == $b ] notes : if Follow [ There should be spaces between , [ Follow $a There are also spaces between , Back $b Follow ] Spaces are also required
then
echo "a be equal to b"
fi
if [ $a != $b ]
then
echo "a It's not equal to b"
fi

1.3、 Relational operator

Relational operators only support numbers , String not supported , Unless the value of the string is a number .

#!/bin/bash
a=10
b=20
if [ $a -eq $b ]
then
        echo "$a -eq $b : a be equal to b"
else
        echo "$a -eq $b: a It's not equal to b"
fi
if [ $a -ne $b ]
then
        echo "$a -ne $b: a It's not equal to b"
else
        echo "$a -ne $b : a be equal to b"
fi
if [ $a -gt $b ]
then
        echo "$a -gt $b: a Greater than b"
else
        echo "$a -gt $b: a No more than b"
fi
if [ $a -lt $b ]
then
        echo "$a -lt $b: a Less than b"
else
        echo "$a -lt $b: a Not less than b"
fi
if [ $a -ge $b ]
then
        echo "$a -ge $b: a Greater than or equal to b"
else
        echo "$a -ge $b: a Less than b"
fi
if [ $a -le $b ]
then
        echo "$a -le $b: a Less than or equal to b"
else
        echo "$a -le $b: a Greater than b"
fi

1.4、 Boolean operator

#!/bin/bash
a=10
b=20
if [ $a != $b ]
then
        echo "$a != $b : a It's not equal to b"
else
        echo "$a == $b: a be equal to b"
fi
if [ $a -lt 100 -a $b -gt 15 ]
then
        echo "$a Less than 100 And $b Greater than 15 : return true"
else
        echo "$a Less than 100 And $b Greater than 15 : return false"
fi
if [ $a -lt 100 -o $b -gt 100 ]
then
        echo "$a Less than 100 or $b Greater than 100 : return true"
else
        echo "$a Less than 100 or $b Greater than 100 : return false"
fi
if [ $a -lt 5 -o $b -gt 100 ]
then
        echo "$a Less than 5 or $b Greater than 100 : return true"
else
        echo "$a Less than 5 or $b Greater than 100 : return false"
fi

1.5、 Logical operators

#!/bin/bash
a=10
b=20
if [[ $a -lt 100 && $b -gt 100 ]] notes : If the logical operator is used here &&, || You need two [[ , ]], If
then If you use bitwise operators, you don't need .
        echo " return true"
else
        echo " return false"
fi
if [[ $a -lt 100 || $b -gt 100 ]]
then
        echo " return true"
else
        echo " return false"
fi

1.6、 String operators

#!/bin/bash
a="abc"
b="efg"
if [ $a = $b ] // Be careful : The comparison of strings here is = Number
then
        echo "$a = $b : a be equal to b"
else
        echo "$a = $b: a It's not equal to b"
fi
if [ $a != $b ]
then
        echo "$a != $b : a It's not equal to b"
else
        echo "$a != $b: a be equal to b"
fi
if [ -z $a ]
then
        echo "-z $a : String length is 0"3.1.6. File test operators
else
        echo "-z $a : String length is not 0"
fi
if [ -n "$a" ]
then
        echo "-n $a : String length is not 0"
else
        echo "-n $a : String length is 0"
fi
if [ $a ]
then
        echo "$a : String is not empty "
else
        echo "$a : The string is empty "
fi

1.7、 File test operators

#!/bin/bash
file="/var/node/test.sh"
if [ -r $file ]
then
        echo " Documents are readable "
else
        echo " The file is unreadable "
fi
if [ -w $file ]
then
        echo " Documents can be written "
else
        echo " The document is not writable "
fi
if [ -x $file ]
then
        echo " The document is executable "
else
        echo " The file is not executable "
fi
if [ -f $file ]
then
        echo " The file is a normal file "
else
        echo " Documents are special documents "
fi
if [ -d $file ]
then
        echo " A file is a directory "
else
        echo " File is not a directory "
fi
if [ -s $file ]
then
        echo " The file is not empty "
else
        echo " The file is empty "
fi
if [ -e $file ]
then
        echo " File exists "
else
        echo " file does not exist "
fi

Two 、echo Print data

Shell Of echo Instructions are used for the output of strings

## Show normal string
echo "Hello World"
## Show escape characters
echo "\"Hello World\""
## Show variable
name="zhangsan"
echo "$name Hello World"
## Show wrap
echo -e "OK! \n" notes : If you want to use the line feed function , You need to in echo Add one more -e
echo "Hello World"
## Show no line breaks
echo -e "OK! \c"
echo "Hello World"
## Show results directed to file
echo "Hello World" > myfile
## Output string as is
echo '$name\"'
## Show command execution results
echo `date`

3、 ... and 、test command

Shell Medium test The command is used to check if a condition holds , It can do numerical 、 Character and file testing .

Numbers

character string

File test

# Compare
num1=100
num2=100
if test $[num1] -eq $[num2]
then
echo ' Two numbers are equal ！'
else
echo ' The two numbers are not equal ！'
fi

Four 、Shell Process control ( Can be associated java grammar )

4.1、if

Format :

if condition1
then
command1
elif condition2
then
command2
else
commandN
fi

give an example :

a=10
b=20
if [ $a == $b ]
then
        echo "a be equal to b"
elif [ $a -gt $b ]
then
        echo "a Greater than b"
elif [ $a -lt $b ]
then
        echo "a Less than b"
else
        echo " There are no conditions that meet "
fi

4.2、case

Shell case Statements are multiple choice statements . It can be used case Statement matches a value with a pattern , If the match is successful , Perform matching

Configured command .

Format :

case value in
Pattern 1)
command1
command2
...
commandN
;;
Pattern 2）
command1
command2
...
commandN
;;
esac

give an example :

echo ' Input 1 To 4 Number between :'
echo ' The number you enter is :'
read num
case $num in
        1) echo ' You chose 1'
        ;;
        2) echo ' You chose 2'
        ;;
        3) echo ' You chose 3'
        ;;
        4) echo ' You chose 4'
        ;;
        *) echo ' You didn't enter 1 To 4 Number between '
        ;;
esac

4.3、for

When variable values are in the list ,for A loop executes all the commands at once , Use the variable name to get the current value in the list .
The command can be any valid shell Commands and statements .in The list can contain substitutions 、 String and file name .
in List is optional , If you don't use it ,for Loop through the positional parameters of the command line .

Format :

for var in item1 item2 ... itemN
do
command1
command2
...
commandN
done

give an example :

for loop in 1 2 3 4 5
do
echo "The value is: $loop"
done
for str in 'This is a string','hello moto'
do
echo $str
done

4.4、while loop

while Loop is used to execute a series of commands continuously , It is also used to read data from the output file ; Commands are usually test conditions

while condition
do
command
done

# Bash let command , It is used to execute one or more expressions , There is no need to add $ To represent a variable
#!/bin/bash
int=1
while(( $int<=5 )) Be careful : If you want to use <=, >= Operator to use (( )) To write
do
echo $int
let "int++"
done
# Infinite loop
while true
do
command
done

4.5、break

break Command run jumps out of all loops ( Terminate all subsequent operations )

#!/bin/bash
while :
do
echo -n " Input 1 To 5 Number between :"
read aNum
case $aNum in
1|2|3|4|5) echo " The number you enter is $aNum!"
;;
*) echo " The number you entered is not 1 To 5 Between ! Game over "
break
;;
esac
done

4.7、continue

continue The command will not jump out of all loops , Just jump out of the current loop

#!/bin/bash
while :
do
        echo -n " Input 1 To 5 Number between : "
        read aNum
        case $aNum in
                1|2|3|4|5) echo " The number you enter is $aNum!"
                ;;
                *) echo " The number you entered is not 1 To 5 Between !"
                continue
                echo " Game over "
                ;;
        esac
done

5、 ... and 、Shell function

Linux shell You can customize functions , And then in shell You can call... In the script .
You can take function fun() Definition , It can also be direct fun() Definition , Without any parameters .
Parameter return , Can display plus ：return return , If not , Results will be run with the last command , As return value . return Heel value n(0-255)

#!/bin/bash
## The first function ------------------------------
demoFun(){
        echo " This is my first shell function !"
}
        echo "----- The function starts executing -----"
demoFun
echo "----- The function is finished -----"
## Function return value ------------------------------
funWithReturn(){
        echo " This function will add two input numbers ..."
        echo " Enter the first number : "
        read aNum
        echo " Enter the second number : "
        read anotherNum
        echo " The two figures are $aNum and $anotherNum !"
        return $(($aNum+$anotherNum))
}
funWithReturn
# The return value of the function passes through $? To obtain a .
echo " The sum of the two numbers entered is $? !"
## Function parameter ------------------------------
funWithParam(){
        echo " The first parameter is zero $1 !"
        echo " The second parameter is $2 !"
        echo " The tenth parameter is $10 !"
        echo " The tenth parameter is ${10} !"
}
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