Shell编程入门(第二版)(下)

流程控制语句

三、select/in[较少用]

格式：

    select [变量] in [关键字]
    do
        command 1
        ... ...
        command n
    done
#select把关键字中的每一项做成类似表单,以交互的方式执行do和done之间的命令

示例-select.sh

#!/bin/bash
# Show select usage

echo "What's your favorate OS?"

select var in "Linux" "Windows" "UNIX" "Other"
do
    break
done

echo "You have selected $var"

四、case/esac

格式：

case 变量 in
字符串1)
    命令列表1
    ;;
    ...
字符串n)
    命令列表n
    ;;
esac

示例-case.sh

#!/bin/bash
# Show Usage for case/esac

echo "*********************************"
echo "Please select a oprator as below:"
echo "C ... copy"
echo "D ... delete"
echo "B ... backup"
echo "*********************************"

read op
case $op in
    C)
        echo "copy...."
        ;;
    D)
        echo "delete...."
        ;;
    B)
        echo "backup..."
        ;;
    *)
        echo "Unknow operator!"
        exit 1
esac

示例-select.case

#!/bin/bash
# A test shell script for select and case

echo "a is 5, b is 3, please select your method"
a=5
b=3;

select var in "a+b" "a-b" "a*b" "a/b"
do
    break
done

case $var in
    "a+b")
        echo "a+b="`expr $a + $b `
        ;;
    "a-b")
        echo "a-b="`expr $a - $b`
        ;;
    "a*b")
        echo "a*b="`expr $a \* $b`
        ;;
    "a/b")
        echo "a/b="`expr $a / $b`
        ;;
    *)
        echo "input error..."
        exit 1
esac

实例-/etc/rc.d/init.d/httpd部分源代码

# See how we were called.
case "$1" in
  start)
    start
    ;;
  stop)
    stop
    ;;
  status)
    ;;
  restart)
    stop
    start
    ;;
  condrestart|try-restart)
    if status -p ${pidfile} $httpd >&/dev/null; then
        stop
        start
    fi
    ;;
  force-reload|reload)
        reload
    ;;
  graceful|help|configtest|fullstatus)
    $apachectl $@
    RETVAL=$?
    ;;
  *)
    echo $"Usage: $prog {start|stop|restart|condrestart|try-restart|force-reload|reload|status|fullstatus|graceful|help|configtest}"
    RETVAL=2
esac

五、while

格式：

while 条件    #无限:while true
do
    命令
done

示例-while.sh

#!/bin/bash
# A usage for while

num=1
while [ $num -le 10 ]
do
    echo $(expr $num \* $num)
    let num++
done

示例-useradd.sh

#!/bin/bash
# A shell script to add user(s)
#echo 123456 | passwd --stdin xiaofang  #用非交互方式设置xiaofang的密码

echo -n "Plese input the user name: "
read username
echo -n "Plese input the sum users: "
read sum

num=1
while [ $num -le $sum ]
do
    /usr/sbin/useradd "$username$num"
    if [ $? -ne 0 ]
    then
        echo "user: $username already exists."
        exit 1
    fi

    let num++
done

echo -n "Please input the passwd for this users: "
read passwd

i=1
while [ $i -le $sum ]
do
    echo $passwd | /usr/bin/passwd --stdin "$username$i"
    let i++
done

示例-userdel.sh

#!/bin/bash
# A shell script for delete user(s)

echo -n "Please input the username: "
read username
echo -n "Please input the user number: "
read num

i=1
while [ $i -le $num ]
do
    /usr/sbin/userdel -r $username$i
    if [ $? -ne 0 ]
    then
        echo "User: $username$i is not exists."
        let i++
        continue
    fi

    let i++
done

六、until

格式:

    until 条件
    do
        命令
    done
#until类似while循环,不同的是until是条件返回值为假时才继续执行。

示例-until.sh

#!/bin/bash
# A script to show until usage.

echo "Please input Y/y to stop..."
read input

until [ "$input" = "Y" ] || [ "$input" = "y" ]
do
    echo "input error, input again!"
    read input
done

七、跳出循环:break和continue 

break:跳出整个循环 

continue:跳过本次循环,进行下次循环

 

示例-break_continue.sh

#!/bin/bash
# A test shell script for break&continue

while true
do
    echo "*****************************"
    echo "Please have a select as blow:"
    echo "1 Copy"
    echo "2 Delete"
    echo "3 Backup"
    echo "4 Quit***********************"
    read op

    case $op in
        "1")
            echo "$op is Copy"
            ;;
        "2")
            echo "$op is Delete"
            ;;
        "3")
            echo "$op is Backup"
            ;;
        "4")
            echo "Exit..."
            break
            ;;
        "*")
            echo "Invalide selectino, please select again..."
            continue
            ;;
    esac
done

 

八、shift指令

参数左移,每执行一次,参数序列顺次左移一个位置,$#的值减1, 用于分别处理每个参数,移出去的参数不再可用

 

示例-shift.sh

#!/bin/bash
# A test shell script for shift

if [ $# -lt 1 ]
then
    echo "No enough parameters"
    exit 1
fi

num=0
while [ $# -gt 0 ]
do
    echo '$1 is '$1
    let num++
    shift
done

echo $num

函数应用

实例-/etc/rc.d/init.d/httpd中的start源代码

 

一、函数的定义: 

函数名 ()
{
    命令序列
}

二、函数的调用:不带() 

函数名 参数1 参数2 ... 参数n

实例-调用

 

 

三、函数中的变量: 

变量均为全局变量,没有局部变量

 

四、函数中的参数:

调用函数时,可以传递参数,在函数中用$1、$2...来引用 

 

示例-function.sh

#!/bin/bash
# A test shell script for function

# function
Help(){
    echo "Usage: sh function \$1 \$2 \$3"
}

Display(){
    echo "three argument: $1 $2 $3"
}

# main
if [ $# -ne 3 ]
then
    Help
else
    echo "Think you for your input"
    Display $1 $2 $3
fi

Shell 脚本调试 

sh -x script  这将执行该脚本并显示所有变量的值。 

sh -n script  不执行脚本只是检查语法的模式,将返回所有语法错误。 

 

最佳实践-命令最好使用绝对路径

 

一个脚本能够执行-

1.对脚本有rx权限,只有r,可以使用sh执行

2.对脚本所在目录至少有rx权限

 

拓展实例-setuid.sh

#!/bin/bash
# After the system installed, please check setuid files first for security
# mkdir /backup
# find / -perm -4000 -o -perm -2000 > /backup/setuid.list

/bin/find / -perm -4000 -o -perm -2000 > /tmp/setuid.list 2> /dev/null

for var in `/bin/cat /tmp/setuid.list`
do
    /bin/grep $var /backup/setuid.list > /dev/null 2> /dev/null
    if [ $? -ne 0 ]
    then
        echo "$var is not in /backup/setuid.list, It's danger!"
    fi
done