if [ expression ]
then
Statement(s) to be executed if expression is true
fi

if ... fi 语句
if ... else ... fi 语句
if ... elif ... else ... fi 语句

#!/bin/bash/
a=10
b=20
if [ $a == $b ]
then 
echo "a is equal to b"
elif [ $a -gt $b ]
then
echo "a is greater to b"
else
echo "a is less to b"
fi

a=10;b=20;if [ $a == $b ];then echo "a is equal to b";else echo "a is not equal to b";fi;

#!/bin/bash/
a=10
b=20
if test $a == $b 
then 
echo "a is equal to b"
else
echo "a is not equal to b"
fi

#!/bin/bash/
grade="B"
case $grade in 
"A") echo "Very Good!";;
"B") echo "Good!";;
"C") echo "Come On!";;
*) 
echo "You Must Try!"
echo "Sorry!";;
esac

#include <stdio.h>
int main(){
char grade = 'B';
switch(grade){
case 'A': printf("Very Good!");break;
case 'B': printf("Very Good!");break;
case 'C': printf("Very Good!");break;
default: 
printf("You Must Try!");
printf("Sorry!");
break;
}
return 0;
}

#!/bin/bash
option="${1}"
case ${option} in
"-f") FILE="${2}"
echo "File name is $FILE"
;;
"-d") DIR="${2}"
echo "Dir name is $DIR"
;;
*) 
echo "`basename ${0}`:usage: [-f file] | [-d directory]"
exit 1 # Command to come out of the program with status 1
;;
esac

$./test.sh
test.sh: usage: [ -f filename ] | [ -d directory ]
./test.sh -f index.html
File name is index.html

do
command1
command2
...
commandN
done

#!/bin/bash/
for value in 1 2 3 4 5
do 
echo "The value is $value"
done

The value is 1
The value is 2
The value is 3
The value is 4
The value is 5

for str in 'This is a string'
do
echo $str
done

#!/bin/bash
for FILE in *
do
echo $FILE
done

beijing
tianjin
shanghai
#!/bin/bash
citys=`cat city.txt`
for city in $citys
echo $city
done

#!/bin/bash
c=0;
while [ $c -lt 3 ]
do
echo "Value c is $c"
c=`expr $c + 1`
done

#!/bin/bash
c=0;
until [ $c -eq 3 ]
do
echo "Value c is $c"
c=`expr $c + 1`
done

#!/bin/bash
i=0
while [ $i -lt 5 ]
do
i=`expr $i + 1`
if [ $i == 3 ]
then
break
fi
echo -e $i
done

#!/bin/bash
i=0
while [ $i -lt 5 ]
do
i=`expr $i + 1`
if [ $i == 3 ]
then
continue
fi
echo -e $i
done