if 语句-条件选择
if 是一个shell的关键字
[root@Centos6app]#type if if is a shell keyword
if 选择执行
if 可以嵌套使用用法
if COMMANDS; then COMMANDS; [ elif COMMANDS; then COMMANDS; ] ... [ else COMMANDS; ] fi 执行 “if COMMANDS” 列表。 如果它的退出状态是零,那么执行“然后命令”列表。 否则,依次执行每个“elif COMMANDS”列表,如果其退出状态为零,则执行相应的“then COMMANDS”列表,然后执行if命令。 否则,执行“else COMMANDS”列表(如果存在)。 整个结构的退出状态是所执行的最后一个命令的退出状态,如果条件不成立,则为零。 if 支持单分支、双分支、多分支用法: if 判断条件1 ; then 条件为真的分支代码 elif 判断条件2; then 条件为真的分支代码 elif 判断条件3; then 条件为真的分支代码 else 以上条件都为假的分支代码 fi 逐条件进行判断,第一次遇为“真”条件时,执行其分支,而后结束整个if语句
示例
根据命令的退出状态来执行命令 if ping -c1 -W2 station1 &> /dev/null ; then echo 'Station1 is UP' elif grep "station1" ~/maintenance.txt &> /dev/null ; then echo 'Station1 is undergoing maintenance‘ else echo 'Station1 is unexpectedly DOWN!' exit 1 fi 判断年龄----->最简单的示例 read -p "Please input your age: " age if [[ ! "$age" =~ ^[0-9]+$ ]] ; then echo "please input right number!" exit 2 elif [ "$age" -le 18 ] ; then echo "you are a child" elif [ "$age" -le 50 ] ; then echo "you are very young" elif [ "$age" -le 80 ] ; then echo "you can go home" elif [ "$age" -le 120 ] ; then echo "you need to feel life" else echo "Go to hell" fi if嵌套最简单示例 read -p "Please input your score: " score if [[ "$score" =~ ^[0-9]+$ ]]; then --------->注意书写时 变量score 上下统一,**别上下写的不统一**,就容易报错 if [ "$score" -le 59 ] ; then echo "your score is so bad" elif [ "$score" -le 70 ] ; then echo "your score is good" elif [ "$score" -le 100 ] ; then echo "unbeliveable" else echo "Go to hell" fi else echo "Please input correct score!" fi
练习题
1、编写脚本/root/bin/createuser.sh,实现如下功能:使用一个用户名做为参数,如果指定参数的用户存在,就显示其存在,否则添加之;显示添加的用户的id号等信息 #!/bin/bash if id $1 &> /dev/null ; then echo "$1 User already exists!" else useradd $1 > /dev/null && echo "$1 User created successfully,$1 info
id $1
" fi 如果不用if语句,id $1 &> /dev/null && echo "$1 user already exists!" || useradd $1 ; echo "User created successfully,$1 infoid $1
"2、编写脚本/root/bin/yesorno.sh,提示用户输入yes或no, 并判断用户输入的是yes还是no,或是其它信息 #!/bin/bash read -p "Do you agree? (yes or no): " ans if [[ "$ans" =~ ^Yy?$ ]] ; then echo "Ok" elif [[ "$ans" =~ ^[Nn][Oo]?$ ]] ; then echo "hehe" else echo "Input error" fi
3、编写脚本/root/bin/filetype.sh,判断用户输入文件路径,显示其文件类型(普通,目录,链接,其它文件类型) #!/bin/bash read -p "Please enter the file name: " file if [ -f "$file" ] ; then echo "The $file is ordinary documents" elif [ -d "$file" ] ; then echo "The $file is table of contents" elif [ -L "$file" ] ; then echo "The $file is link file" else echo "The $file is othe types" fi
4、编写脚本/root/bin/checkint.sh,判断用户输入的参数是否为正整数 #!/bin/bash 1、 read -p "Please input a number: " num if [[ "$num" =~ ^[[:digit:]]+$ ]] ; then if [[ "$num" -gt 0 ]] ; then echo "Entered correctly!" else echo "Input error!" fi else echo "Not a positive integer" fi 2、 read -p "please input :" nub if [[ $arg =~ ^[[:digit:]]+$ ]] && [ $(expr $arg + 0) != 0 ] ; then echo "Bingo!" else echo "Wrong!" fi
case 语句-条件判断
[root@Centos6app]#type case
case is a shell keyword
用法
case WORD in [PATTERN [| PATTERN]...) COMMANDS ;;]... esac case 变量引用 in PAT1) 分支1 ;; PAT2) 分支2 ;; ... *) 默认分支 ;; esac
case支持glob风格的通配符:
*: 任意长度任意字符 ?: 任意单个字符 []:指定范围内的任意单个字符 a|b: a或b 最简单示例 yes or no #!/bin/bash read -p "Do you agree? (yes or no): " ans case $ans in [Yy]|[Yy][Ee][Ss]) echo "Ok" ;; [Nn]|[Nn][Oo]) echo "hehe" ;; *) echo "What do you want to do !" ;; esac