第四章 控制流结构
简介
控制结构
if then else语句
case语句
for循环
until循环
while循环
break循环
ocntinue控制
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
if语句格式
if 如果条件一为真，那么执行命令1
then
elif 如果条件一不成立，条件二为真，那么执行命令2
then
else 如果条件一二都不成立，那么执行命令3
fi 完成（if语句必须以fi终止）
例句：
#!/bin/bash
#if test
#this is a comment line,all comment lines start with a #
if [ "10" -1t "12" ]
then
#yes 10 is less than 12
echo "Yes,10 is less than 12"
else
echo "No"
fi
例句2：
#!/bin/bash
#if test2
echo -n "Enter your name:"
read NAME
#did the user just hit return
if [ "$NAME" == "" ];then
echo "You did not enter any information"
else
echo "Your Name is ${NAME}"
fi
if
elif
elif
else
fi
例：（注意[]两边都有空格
chmod 755 ifelif
./ifelif
vi中fset nu可以显示行号）
#!/bin/bash
#ifelif
echo -n "Enter your name:"
read NAME
if [ -z $NAME ] || [ "$NAME" = "" ];then
echo "You did not enter a name."
elif [ "$NAME" = "root" ];then
echo "Hello root"
elif [ "$NAME" = "chinaitlab" ];then
echo "Hello chinaitlab"
else
echo You are not root or chinaitlab,but hi,"$NAME"
fi
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
case语句
case 值 in ----------------取值后面必须跟in
模式1) ---------------每个模式必须以 ) 结束
命令1
;; ---------------终止提示符
模式2)
命令2
;;
esac
例子：
#!/bin/bash
#case select
echo -n "Enter a number from 1 to 3:"
read ANS
case $ANS in
1)
echo "You select 1"
;;
2)
echo "You select 2"
;;
3)
echo "You select 3"
;;
*)
echo "`basename $0`: This is not beteen 1 and 3" >&2
exit;
;;
esac
例子解释：
例子中*)表示除了1) 2) 3)外的所有其他情况
例子中可以在 2) 命令结束后打入
y|Y)
echo "You select $ANS"
;;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
for 循环 ----------- 将 列表 依次带入 变量名中，执行命令
for 变量名 in 列表
do
命令1
命令2
done
例：
#!/bin/bash
#forlist1
for loop in 1 2 3 4 5
do
echo $loop
done
例子延伸：将上例中1 2 3 4 5 更换成 `cat forlist1` （``内的内容可以表示linux命令）, 则
./forlist1之后屏出forlist1文件内容，注意for循环一次读入空格之间的字符。
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
until 循环
until 条件
do
命令
命令
done
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
while 循环
while 命令
do命令1
do命令2
done
例子：
#!/bin/sh
#whileread
echo "press ctrl +d to quit"
while echo -n "type ur fav movie";read FILM
do
echo "Yeah,${FILM} is ur fav movie"
done
例子2:
#!/bin/sh
#whilereadline
while read LINE
do
echo $LINE
done < names.txt
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
break & continue
ex:
~~~~~~~~~~~~
#!/bin/bash
#breakout
while :
do
echo -n "Enter any number [1...5]:"
read ANS
case $ANS in
1|2|3|4|5)
echo "You enter a number between 1 and 5."
;;
*)
echo "Wrong number,Bye."
break
;;
esac
done
~~~~~~~~~~~~~~
ex2:
~~~~~~~~~~~
#!/bin/bash
#breakout
while :
do
echo -n "Enter any number [1...5]:"
read ANS
case $ANS in
1|2|3|4|5)
echo "You enter a number between 1 and 5."
;;
*)
echo -n "Wrong number,continue <y/n>?:"
read IS_CONTINUE
case $IS_CONTINUE in
y|yes|Y|Yes)
continue
;;
*)
break
;;
esac
esac
done
~~~~~~~~
