shell学习笔记之控制结构（三）

1.if语句

if condition
then
statements
esle
statements
fi

例：

#!/bin/sh

echo "Is it morning? Please answer yes or no"
read timeofday

if [ $timeofday = "yes" ]; then
echo "Good morning"
else
echo "Good afternoon"
fi
exit 0

2.elif语句

#!/bin/sh

echo "Is it morning? Please answer yes or no"
read timeofday

if [ $timeofday = "yes" ]
then
echo "Good morning"

elif [ $timeofday = "no" ]; then
echo "Good afternoon"
else
echo "Sorry!"
exit 1
fi

exit 0

潜在的问题：
如果用户不输入直接按下回车，if [ = "yes" ]这个判断就会出错
所以必须给timeofday变量添加上引号

修改后：

#!/bin/sh

echo "Is it morning? Please answer yes or no"
read timeofday

if [ "$timeofday" = "yes" ]
then
echo "Good morning"

elif [ "$timeofday" = "no" ]; then
echo "Good afternoon"
else
echo "Sorry!"
exit 1
fi

exit 0

echo -n "hello echo"
-n选项去掉换行符

3.for语句

for variable in values
do
statements
done

例1.

#!/bin/sh

for foo in bar fud 43
do
echo $foo
done

exit 0

输出：
bar
fud
43

例2.

#!/bin/sh
for foo in "bar fud 43"#当做一个字符串
do
echo $foo
done
exit 0

输出：
bar fud 43

例3.

#打印当前目录中所有以字母f开头并且.sh结尾的脚本文件
#!/bin/sh
for file in $(ls f*.sh); do
lpr $file
done
exit 0

4.while语句

1.不知道命令序列要执行的次数
2.条件为真时反复执行

while condition do
statements
done

#!/bin/sh

echo "Enter password"
read trythis

while [ "$trythis" != "secret" ]; do
echo "sorry, try again"
read trythis
done
exit 0

5.until语句

循环反复直到条件为真
until condition
do
statements
done

while:循环至少执行一次
unitl：可能根本不需要执行循环

#!/bin/bash

until who | grep "$1" > /dev/null
do
sleep 60
done

echo -e '\a' #响铃发出警报
echo "***$1 has just logged in****"
exit 0

6.case语句

case variable in
pattern [ | pattern ] ...) statements;;
pattern [ | pattern ] ...) statements;;
...
esac

双分号标记前一个语句的结束和后一个模式的开始

#!/bin/bash

echo "Is it morning? Please answer yes or no"
read timeofday

case "$timeofday" in
yes) echo "Good Morning";;
no ) echo "Good Afternoon";;
y  ) echo "Good Morning";;
n  ) echo "Good Afternoon";;
*  ) echo "sorry";;
esac

exit 0

#!/bin/bash

echo "Is it morning? Please answer yes or no"
read timeofday

case "$timeofday" in
yes | y | Yes | YES )     echo "Good Morning";;
n* | N* )                 echo "Good Afternoon";;
*  )                      echo "sorry";;
esac

exit 0

#!/bin/bash

echo "Is it morning? Please answer yes or no"
read timeofday

case "$timeofday" in
yes | y | Yes | YES )
echo "Good Morning"
echo "Up bright and early this morning"
;;
[nN]*)
echo "Good Afternoon"
;;
*  )
echo "sorry"
exit 1
;;      #如果最后一个case模式是默认模式，可以省略最后一个双分号;;
#[yY] | [Yy] [Ee] [Ss])
esac

exit 0

7.命令列表

①AND列表

statement1 && statement2 && statement3 && ...

#!/bin/sh

touch file_one
rm -f file_two

if [ -f file_one ] && echo "hello" && [ -f file_two ] && echo "there
then
echo "in if"
else
echo "in else"
fi
exit 0

②OR列表
statement1 || statement2 || statement3 || ...

#!/bin/sh
rm -f file_one

if [ -f file_one ] || echo "hello" || echo "there"
then
echo "in if"
else
echo "in else"
fi
exit 0

[ -f file_one ] && command for true || command for false
如果测试成功会执行第一条命令，否则执行第二条命令

③语句块

get_confirm && {
grep -v "$cdcatnum" $tracks_file > $temp_file
cat $temp_file > $tracks_file
echo
add_record_tracks
}

8.函数

定义：
必须在调用之前定义
function_name(){
statements
}

#!/bin/sh
foo() {
echo "Function foo is executing"
}
echo "script starting"
foo
echo "script ended"
exit 0

注意：

1.如果函数里面没有return一个值，函数返回的就是执行最后一条命令的返回码。
2.local关键字在函数中声明一个局部变量，局部变量仅在函数的作用范围内有效。
3.函数可以访问全局作用范围内的其他shell变量。
4.如果一个局部变量和一个全局变量名字相同，前者会覆盖后者，但仅限于函数的作用范围内。
5.让函数返回字符串值的常用的方法：
1>让函数将字符串保存在一个变量中，该变量然后可以在函数结束之后被调用
2>echo一个字符串并捕获其结果
foo(){
echo JAY;
}
...
result="$(foo)"

#!/bin/sh
sample_text="global variable"
foo() {
local sample_text="local variable"
echo "Function foo is executing"
echo $sample_text
}

echo "script starting"
echo $sample_text
foo
echo "script ended"
echo $sample_text
exit 0

①参数如何传递
②函数返回值

#!/bin/sh
yes_or_no(){
echo "Is your name $*"
while true
do
echo -n "Enter yes or no:"
read x
case "$x" in
y | yes ) retrun 0;;
n | no  ) return 1;;
*)          echo "Answer yes or no";;
esac
done
}

echo "Original parameters are $*"
if yes_or_no "$1"
then
echo "Hi $1, nice name"
else
echo "Never mind"
fi
exit 0

执行结果
$./my_name Rick Neil
Original parameters are Rick Neil
Is your name Rick?
Enter yes or no:
yes
Hi Rick, nice name