Shell Script Basics (ii)

I. Logical judgment in shell Scripts 1, Judgment statement IFA) without else

Format:

if 判断语句；then    commandfi

Instance:

[[email protected] shell]# vim if01.sh          //判断数值大小第一种方法用[]，注意前后空格#! /bin/basha=5if [ $a -gt 3 ];then    echo "这个数字大于3"fi#-gt:大于，-lt:小于，-ge:大于或等于，-le:小于或等于，-eq:等于，-ne:不等于[[email protected] shell]# sh if01.sh 这个数字大于3[[email protected] shell]# vim if02.sh                //判断数值大小的第二种方式：(())#! /bin/basha=5if ((a>3));then   echo "这个数字大于3"fi#((a>3)),双括号是shell中的特有格式，只用一个括号或者不用都会报错[[email protected] shell]# sh if02.sh 这个数字大于3

b) with Else

Format:

if 判断语句；then    commandelse   commandfi

Instance:

[[email protected] shell]# vim ifelse.sh #! /bin/bashread -p "请输入分数：" aif [ $a -lt 60 ];then   echo "你没有通过测试！"else   echo "恭喜你，顺利通过测试！"fi#read命令用于和用户交互，它把用户输入的字符串作为变量值 [[email protected] shell]# sh ifelse.sh 请输入分数：60恭喜你，顺利通过测试！[[email protected] shell]# sh ifelse.sh 请输入分数：59你没有通过测试！

c) with Elif

Format:

if 判断语句1;then   commandelif 判断语句2;then   commandelse   commandfi

Instance:

[[email protected] shell]# vim elif.sh #! /bin/bashread -p "请输入分数：" aif (($a<60));then   echo "未通过测试。"elif ((a>=60))&&(($a<85));then   echo "通过测试，成绩良好。"else   echo "通过测试，成绩优秀！"fi# &&表示并且，当然也可以用||表示或者[[email protected] shell]# sh elif.sh 请输入分数：60通过测试，成绩良好。[[email protected] shell]# sh elif.sh 请输入分数：34未通过测试。[[email protected] shell]# sh elif.sh 请输入分数：99通过测试，成绩优秀！

2, and document-related judgments

The If hi in the shell is often used to determine document properties. Common options are:

-E: Determine whether a file or directory exists;
-D: Determine if it is a directory and whether it exists;
-F: Determine if it is a file and whether it exists;
-R: Determine whether there is read permission;
-W; Determine if there is write permission;
-X: Determines whether there are execute permissions.

Format:

if [ -e filename ];then   commandfi

Instance:

[[email protected] shell]# if [ -d /home/ ]; then echo ok; fiok[[email protected] shell]# if [ -f /home/ ]; then echo ok; fi                //home是目录不是文件，所以没有输出ok[[email protected] shell]# touch /root/test.txt[[email protected] shell]# if [ -f /root/test.txt ]; then echo ok; fiok[[email protected] shell]# if [ -r /root/test.txt ]; then echo ok; fiok[[email protected] shell]# if [ -w /root/test.txt ]; then echo ok; fiok[[email protected] shell]# if [ -x /root/test.txt ]; then echo ok; fi[[email protected] shell]# if [ -e /root/test1.txt ]; then echo ok; fi[[email protected] shell]# if [ -d /root/test.txt ]; then echo ok; fi

3. Case Logic Judgment

Format:

case 变量 invalue1)              command                            ;;value2)              command                            ;;value3)              command                            ;;*)              command                            ;;esac

Does not limit the number of values, * denotes other value
Instance:

[[email protected] shell]# vim case.sh#! /bin/bashread -p "请输入一个数字：" na=$[$n%2]case $a in  1)      echo "这数字是奇数"      ;;  0)      echo "这数字是偶数"      ;;  *)      echo "这个不是数字"      ;;esac[[email protected] shell]# sh case.sh请输入一个数字：3这数字是奇数[[email protected] shell]# sh case.sh请输入一个数字：2这数字是偶数

4. Practice (Enter exam results, judge score level)

[[email protected] shell]# vim test01.sh#!/bin/bashread-p "Please input a number:" Nif [-Z "$n"]then echo "Ple    ASE input a number. " Exit # "Exit 1" indicates the return value after executing the partial command # that is, after the command executes, use the value of echo $ Fin1= ' echo $n |sed ' s/[0-9]//g ' #判断用户输入的字符是否为纯数字 # If it is a number, replace it with a null, assign to $ n1if [-N "$n 1"]then echo "please input a number." Exit # determines that $N1 is not empty (that is, $n is not a pure number) again prompts the user to enter the numbers and exits fi# if the user enters a pure number, execute the following command: if [$n- LT] && [$n-ge 0]then tag=1elif [$n-ge] && [$n-lt]then Tag=2elif [$n-ge] &A mp;& [$n-lt]then Tag=3elif [$n-ge] && [$n-le the]then tag=4else Tag=0fi#tag is set for judging conditions    Label, convenient to refer to case later $tag in 1) echo "not OK";;    2) echo "OK";;    3) echo "ook";;    4) echo "Oook";;        *) echo "The number range is 0-100." ;; Esac[[email protected] shell]# sh test01.sh Please input a number:aaplease input a number. [[email protected] shell]# sh test01.sh please input aNumber:78ok[[email protected] shell]# sh test01.sh Please input a number:90oook[[email protected] shell]# sh t est01.sh Please input a number:11not OK

Shell Script Basics (ii)