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