linux 条件判断式

1.利用if ...then

if [ 判断条件 ];then
    指令
fi

实例一 Y/N:

#!/bin/bash
#Program：
#       This program shows "Hello World!" in your screen.
#History:
#       //      lzyer  First release
read -p "Please input (Y/N)? " yn
if [ "${yn}" == "Y" ] || [ "${yn}" == "y" ];
then
        echo "OK,continue."
        exit
fi
if [ "${yn}" == "N"] || [ "${yn}" == "n" ];
then
        echo "oh,interrupt!"
        exit
fi
echo "I don't know what your choice is " && exit 

实例二：

1. 判断 $1 是否为 hello，如果是的话，就显示 "Hello, how are you ?"；
2. 如果没有加任何参数，就提示使用者必须要使用的参数下达法；
3. 而如果加入的参数不是 hello ，就提醒使用者仅能使用 hello 为参数。

#!/bin/bash
#Program:
#       check $ is equal to "hello"
#History:
#//     lzyer   First Release
if [ "${1}" == "hello" ];then
        echo "Hi, how are you?"
elif [ "${1}" == "" ]; then
        echo "you must input parameter"
else
        echo "hello~~~"
fi

实例三 使用netstat 的指令查看服务的状态

#!/bin/bash
#Program:
#       using netstat and grep to detect www.ssh,ftp and mail service.
#History:
#//     lzyer  First release
echo "Now, I will detect your Linux server' service!"
echo -e "The www,ftp,ssh and mail will be detect!"
testfile=/home/lzyer/test/bin/netstat_file.txt
netstat -tuln > ${testfile}
testing=$(grep ":80" ${testfile})
if [ "${testing}" != "" ];then
        echo "www is running in your system."
fi
testing=$(grep ":22" ${testfile})
if [ "${testing}" != "" ];then
        echo "ssh is running in your system."
fi
testing=$(grep ":21" ${testfile})
if [ "${testing}" != "" ];then
        echo "ftp is running in your system."
fi
testing=$(grep ":25" ${testfile})
if [ "${testing}" != "" ];then
        echo "mail is running in your system."
fi

实例四：计算退伍时间

#!/bin/bash
#Program:
#       You input demobillization date, I calculate how many days before you demobilize.
#History:
#//     lzyer   First release
echo "this program will try to calculate:"
read -p "how many days before you demobillization date (YYYYMMDD>20170805): " date2
date_d=$(echo ${date2} | grep '[0-9]\{8\}')
if [ "${date_d}" == "" ];then
        echo "You input the wrong date format..."
        exit
fi
declare -i date_dem=$(date --date="${date2}" +%s)
declare -i date_now=$(date +%s)
declare -i date_total_s=$((${date_dem}-${date_now}))
declare -i date_d=$((${date_total_s}///))
if [ ${date_total_s} -lt  ];then
        echo "You had been demobilization before : " $((-*${date_d})) "day ago"
else
        declare -i date_h=$((((${date_total_s}-${date_d}**))//))
        echo "You will demobilization after ${date_d} days and ${date_h} hours."
fi

2.利用 case ..... esac 判断

实例一：

#!/bin/bash
#Program:
#       show "hello" from $...by case esac
#History:
#//     lzyer   first release
case ${} in
        "hello")
        echo "hello,how are you?";;
        "")
        echo "please your input sth.";;
        *)
        echo "Usage ${0} hello";;
esac

3.利用 function 功能

#!/bin/bash
#Program:
#       function use
#History:
#//     lzyer  first release
function print(){
        echo  "Your choice is ${1}"
}
echo "This program will print your selection !"
case ${} in
        "one") print ;;
        "two") print ;;
        "three") print ;;
        *) echo "Usage ${0} one|two|three";;
esac