root@lejian:/data# cat demo1 
#!/bin/bash
if date
then
        echo "hello world"
fi
root@lejian:/data# ./demo1 
Fri Dec  2 05:26:27 CST 2016
hello world

root@lejian:/data# cat demo4 
#!/bin/bash
if aaa
then
        echo "hello world"
elif who
then
        echo "hello java"
else
        echo "hello linux"
fi
root@lejian:/data# ./demo4 
./demo4: line 2: aaa: command not found
root     pts/0        2016-12-02 04:39 (122.91.222.126)
hello java

• 数值比较
• 字符串比较
• 文件比较

root@lejian:/data# cat demo3 
#!/bin/bash
val1=10
val2=20
if [ $val1 -gt $val2 ]
then
        echo "The test value $val1 is greater than $val2 "
elif [ $val1 -lt $val2 ]
then
        echo "The test value $val1 is less than $val2"
else
        echo "The test value $val1 is equal $val2  "
fi
root@lejian:/data# ./demo3 
The test value 10 is less than 20

• 大于小于符号必须转义，否则shell会把它们当做重定向符号而把字符串值当做文件名
• 大于小于顺序和sort命令所采用的不同

root@lejian:/data# cat demo5 
#!/bin/bash
val1="hello"
val2="Hello"
if [ $val1 \> $val2 ]
then
        echo "The result is $val1 greater than $val2"
elif [ $val1 \< $val2 ]
then
        echo "The result is $val1 less than $val2"
fi
root@lejian:/data# ./demo5 
The result is hello greater than Hello

root@lejian:/data# cat demo6 
#!/bin/bash
val1="hello"
val2="world"
if [ $val1 \> $val2 ]
then
        echo "The result is $val1 greater than $val2"
elif [ $val1 \< $val2 ]
then
        echo "The result is $val1 less than $val2"
fi
root@lejian:/data# ./demo6 
The result is hello less than world

root@lejian:/data# cat demo8 
#!/bin/bash
val1="testing"
val2=""
if [ -n $val1 ]
then
        echo "The string ‘$val1‘ is not empty"
else
        echo "The string ‘$val1‘ is empty "
fi

if [ -n $val2 ]
then
        echo "The string ‘$val2‘ is not empty"
else
        echo "The string ‘$val2‘ is empty "
fi

if [ -n $val3 ]
then
        echo "The string ‘$val3‘ is not empty"
else
        echo "The string ‘$val3‘ is empty "
fi
root@lejian:/data# ./demo8 
The string ‘testing‘ is not empty
The string ‘‘ is not empty
The string ‘‘ is not empty

代码2-8中：

root@lejian:/data# cat demo9 
#/bin/bash
if [ -d $HOME ]
then
        echo "Your home directory exists"
        cd $HOME
        ls -a
else
        echo "There is a problem with your HOME directory"
fi
root@lejian:/data# ./demo9 
Your home directory exists
.  ..  .bash_history  .bashrc  .cache  .hivehistory  .pip  .profile  .pydistutils.cfg  .rediscli_history  .scala_history  .selected_editor  .spark_history  .ssh  .vim  .viminfo

root@lejian:/data# cat demo1 
#!/bin/bash
file="text"
touch $file
if [ -s $file ]
then
        echo "The $file file exists and has data in it"
else
        echo "The $file exists and is empty"
fi
date > $file
if [ -s $file ]
then
        echo "The $file file exists and has data in it"
else
        echo "The $file exists and is empty"
fi
root@lejian:/data# ./demo1 
The text exists and is empty
The text file exists and has data in it

• [ condition1 ] && [ condition2 ]
• [ condition1 ] || [ condition2 ]

root@lejian:/data# cat demo2 
#!/bin/bash
if [ -d $HOME ] && [ -w $HOME/testing ]
then
        echo "The file exists and you can write it"
else
        echo "I cannot write to the file"
fi
root@lejian:/data# ./demo2 
I cannot write to the file

root@lejian:/data# cat demo3 
#!/bin/bash
val1=10
if(($val1 ** 2 > 90))
then
        ((val2=$val1 ** 2))
        echo "The square of $val1 is $val2"
fi
root@lejian:/data# ./demo3 
The square of 10 is 100

root@lejian:/data# cat demo4 
#!/bin/bash
if [[ $USER=r* ]]
then
        echo "hello $USER"
else
        echo "Sorry. I do not know you"
fi
root@lejian:/data# ./demo4 
hello root

root@lejian:/data# cat demo5 
#!/bin/bash
case $USER in
root|Tom)
        echo "Welcome $USER"
        echo "Please enjoy your visit";;
testing)
        echo "Special testing account";;
jessica)
        echo "Do not forget to log off when you‘re done";;
*)
        echo "Sorry. you are not allowed here";;
esac
root@lejian:/data# ./demo5 
Welcome root
Please enjoy your visit