bash 编程 if

 

面向过程

          顺序执行

          条件判断     if  case

          循环执行     for while  until

条件判断     

[ 表达式 ]   表达式左右有空格

test  表达式

命令

变量

          本地变量   local 声明  

          局部变量

          环境变量-全局变量

          位置变量   $1  $2

          特殊变量   $?   命令返回结果     $@  全部参数      $# 参数个数

if

          if 条件 ；then

                         语句1

                         语句2

          fi

                              例题1  写一个脚本，实现如下功能：     如果用户存在，就说明其存在

                                        #!/bin/bash

                                        #

                                        UserName=user1

                                        if grep "^$UserName\>" /etc/passwd &> /dev/null; then

                                                    echo "$UserName exists."

                                        fi

                                        #!/bin/bash

                                        UserName=user1

                                        if id $UserName &> /dev/null; then

                                                     echo "$UserName exists."

                                        fi

          双分支语句

          if  条件；then

                         语句1

                         语句2

          else     

                         语句1

                         语句2

          fi

          数字测试           数值1  比较符号  数值2

                                             小于     -lt

                                             小于等于     -le

                                             大于     -gt

                                             大于等于     -ge

                                             等于     -eq

                                             不等于          -ne

例子：写一个脚本，生成两个随机数，比较其大小；显示大数；     bash有个内置变量：$RANDOM

                                                                      #!/bin/bash
                                                                      #
                                                                      A=$RANDOM
                                                                      B=$RANDOM

                                                                      if [ $A -ge $B ]; then
                                                                                       echo "Max number is $A."
                                                                      else
                                                                                       echo "Max number is $B."
                                                                      fi

练习：给定一个用户，如果其ID号大于499，就说明其是普通用户，否则，就说明其是管理员或系统用户；

                                                                        #!/bin/bash

#

UserName=daemon

Uid=`id -u $UserName`

if [ $Uid -gt 499 ]; then

  echo "A common user: $UserName."

else

  echo "admin user or system user: $UserName."

fi   

               字符测试

                             等于  ==     

                             不等于 ！=

                              -z  string     string 为空 ture

                              -n  string    string 不为空  ture

                                                   #!/bin/bash

                                                   #

                                                   Shell=`grep "^$1:" /etc/passwd | cut -d: -f7`

                                                   if [ -z $Shell ]; then

                                                             echo "No such user or User's shell is null."

                                                             exit 10

                                                   fi

                                                   if [ "$Shell" == "/bin/bash" ]; then

                                                                 echo "Bash User."

                                                                    Ret=0

                                                         else

                                                                  echo "Not Bash User."

                                                                     Ret=9

                                                          fi

                                                                      exit $Ret      

               组合条件测试

                                   -a  与  [ 条件1 -a  条件2 ]     

                                   -o 或

                                   -i  非

                                   一般用在一个[]内

                                   && 与 [ 条件1 ] && [ 条件2 ]

                                   ||   或

              多重判断

                                   if 条件 ；then

                                                  语句

                                   elif 条件；then

                                                  语句

                                   else 

                                                   语句

                                   fi

               文件测试

                                   -f  是否普通文件

                                   -d 是否目录文件

                                   -e 文件是否存在

                                   -r 对当前用户是否有可读权限

                                   -w对当前用户是否有可写权限

                                   -x 对当前用户是否有可执行权限

                                   -s 文件是否存在且非空

               cmd1 && cmd2   命令1执行正确，开始执行命令2

               cmd1 ||   cmd2   命令1执行不正确，开始执行命令2

      cmd1 ||   cmd2  &&  cmd3  命令1执行正确 直接执行命令3 ，

                                               命令1执行不正确，执行命令2 ，如命令2 执行正确，再执行命令3

            case语句

                            case   变量  in

                                        value1 )

                                                    语句

                                                    ；；

                                        value2 )

                                                    语句

                                                    ；；

                                          * ）

                                                    语句

                                                        ；；

                               esac

                例子：写一个脚本
                            1、显示如下菜单给用户：
                                    m|M) show memory usages;
                                    d|D) show disk usages;
                                    q|Q) quit
                            2、  如果用户选择了第一项，则显示内存使用信息；
                                   如果选择了第二项，则显示磁盘挂载及使用相关信息；
                                   如果是第三项，退出，并显示选择退出；
                                   其它任何内容，均说明错误选项；

                #!/bin/bash
                cat << EOF 
                m|M) show memory usages;

                d|D) show disk usages;
                q|Q) quit
                EOF

                read -p "Your choice: " Choice

                case $Choice in
                                    m|M)
                                             free -m ;;
                                    d|D)
                                             df -lh ;;
                                    q|Q)
                                              echo "Quit..."
                                              exit 0
                                               ;;
                                        *)
                                               echo "Invalid input."
                                               exit 5
                                                ;;
                   esac