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 

 

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
fi

exit 

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
fi

exit 

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
do
    echo $foo
done

exit 

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 

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 

 

 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  

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
done

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

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 

#!/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 

#!/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
            ;;      #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 

 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 

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 

[ -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 

 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 

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 ;;
            n | no  ) return ;;
            *)          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 

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