Shell script if, for, and while statements

(1) If statement

[Email protected]:/mnt/shared/shellbox/shellif# cat shellif.sh #!/bin/bash# judgment string if ["$" = "Hello"]then        echo "\$1= $ "fi# judgment number, if () mode can only be used under bash, not under sh if (>) then        echo" \$1: $ > "elif (($ = =) then        echo" \ "= =" Elif (<) then        echo "\$1 <" fi# square brackets judgment statement if [$1-lt]then        echo "\$1 <" elif [$1-g E 20-a $1-le]then        echo "\$1 >= && \$1 <=" elif [$1-gt]then        echo "\$1 >" fi

Execution Result:

[Email protected]:/mnt/shared/shellbox/shellif#./shellif.sh 10
$ < 20
$ < 20
[Email protected]:/mnt/shared/shellbox/shellif#./shellif.sh 20
$ = = 20
$ >= && $ <= 30
[Email protected]:/mnt/shared/shellbox/shellif#./shellif.sh 30
$1:30 > 20
$ >= && $ <= 30
[Email protected]:/mnt/shared/shellbox/shellif#./shellif.sh 40
$1:40 > 20
$ > 30

(2) For statement

[Email protected]:/mnt/shared/shellbox/shellfor# cat shellfor.sh #!/bin/bashfor i in $*do        echo $idonefor Char in {a]. C}do        echo $chardonefor int in {1..3}do         echo $intdone

Execution Result:

[Email protected]:/mnt/shared/shellbox/shellfor#./shellfor.sh
A
B
C
1
2
3

(3) While statement:

[Email protected]:/mnt/shared/shellbox/shellwhile# cat shellwhile.sh #!/bin/bash# Note: (()) This method can only be used in bash, not in SH i= 0while ((I < $))    do echo "i= $i" let    i+=1done# no spaces before or after "=" Num=0while [[$num! =]]do        echo "num= $num, N Um! = \$1 "Let        num+=1donewhile Truedo echo" here in while        true ... "        sleep 2done

Execution Result:

[Email protected]:/mnt/shared/shellbox/shellwhile#./shellwhile.sh 5
I=0
I=1
i=2
I=3
I=4
num=0, num! = $
Num=1, num! = $
num=2, num! = $
num=3, num! = $
num=4, num! = $
Here on while true ...
Here on while true ...

If, for, and while statements of the shell script