Linux学习笔记（9）之Shell编程入门--判断语句

语法一：

if [ 条件表达式 ]

then

命令序列 1

else

命令序列 2

fi

语法二：

if [ 条件表达式 ]; then

命令序列

fi

语法三：

if test 条件表达式 1

then

命令序列 1

elif [ 条件表达式 2 ]

then

命令序列 2

else

命令序列 3

fi

编写shell脚本时，注意条件表达式与“[“ ”]“之间的空格

实例分析：

#!/bin/bash#script4-1.shvar1="welcome to use Shell script"echo $var1pwdls -i#判断当前目录下是否存在某文件#!bin/bash#script4-2.shecho "Enter a filename"read fileif [-f $file]then        echo "File $file exists."fi#判断当前用户名和输入的用户名是否一致#!/bin/bash#script4-3.shecho -n "Enter your login name:"read nameif test "$name" = "$USER"then        echo "Hello,$name"else        echo "You're not $USER"fi#比较两个数的大小#!/bin/bash#script4-4.shecho "Enter the first integer:"read firstecho "Enter the second  integer:"read secondif test "$first" -gt "$second"    then        echo "$first is greater than $second"    elif test "$first" -lt "$second"    then        echo "$first is less than $second"    else        echo "$first is equal to $second"fi

#判断myfile文件中是否含有“GNU“字符串#!/bin/bash#script4-5.shif grep "GNU" myfile >/dev/nullthen    echo "\"GNU\" occurs in myfile"elseechoecho "\"GNU\"does not  occurs in myfile"fi