shell 判断语句

语法一：
if []
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
判断当前目录下是否存在某文件：
#!/bin/bash
echo “Enter a file name:”
read file
if [-f $file]
then
echo “File $file exists”
fi
判断当前用户名是否和输入的用户名一致：
#!/bin/bash
echo -n “Enter your login name:”
read name
if [“$name” = “$USER”];
then
echo “Hello,$name.How are you today?”
else
echo “You are not $USER,so who are you?”
fi
比较两个数的大小：
#!/bin/bash
echo “Enter the first integer:”
read first
echo “Enter the second integer:”
read second
if [“$first” -gt “$second”]
then
echo “$first is greater than $second”
elif [“$first” -lt “$second”]
then
echo “$first is less than $second”
else
echo “$first is equal to $second”
fi
在文本文件中查找字符串：
#!/bin/bash
if grep “GNU” myfile > /dev/null
then
echo “\”GNU\” occurs in myfile”
else
echo “\”GNU\” does not occurs in myfile”
fi