格式：

　　[ 表达式 ]  操作符 [ 表达式2 ] ……

　　命令1  操作符  命令2 ……

常用的操作符 （ 注意：-a和-o放在[]里面用，&&和||放在[]外面用 ）

　　　　-a  或  &&           逻辑与

　　　　-o  或 ||             逻辑或

　　　　　　！               逻辑否

文件测试

格式1:  [  操作符 文件或目录  ]  

格式2：test  操作符 文件或目录 

常用的测试操作符

　　　　-d :测试是否为目录( Directory )

　　　　-e :测试目录或文件是否存在（Exist）

　　　　-f :测试是否为文件（File）

　　　　-r :测试当前用户是否可读（read）

　　　　-w:测试当前用户是否可写（write）

　　　　-x :测试当前用户是否可执行（excute）

#/bin/bash
back_dir=/var/mysql_back

if !test -d $back_dir;then
	mkdir -p $back_dir
fi
echo "开始备份"

格式1：[ 整数1 操作符 整数2 ]

格式2: test 整数1 操作符 整数2

常用的测试操作符

　　　　-eq : 等于 （Equal）

　　　　-ne : 不等于 （Not Equal）

　　　　-gt : 大于（Greater Than）

　　　　-lt : 小于 （Lesser Than）

　　　　-le : 小于或等于（Lesser or Equal）

　　　　-ge : 大于或等于（Greater or Equal）

#/bin/bash
if [ $UID -ne 0];then
	echo "没有权限"
	exit
fi
yum -y install httpd

格式1：[ 字符串1 = 字符串2 ]

　　　  [ 字符串1 != 字符串2 ]

格式2：[ -z 字符串 ]

常用的测试操作符

　　= : 字符串内容相同

　　!= : 字符串内容不同

　　-z : 字符串内容为空

#/bin/bash
if [ $USER = "root"];then
	yum -y install httpd
fi
	echo "没有权限"
	exit

( EXPRESSION )
              EXPRESSION is true

       ! EXPRESSION
              EXPRESSION is false

       EXPRESSION1 -a EXPRESSION2
              both EXPRESSION1 and EXPRESSION2 are true

       EXPRESSION1 -o EXPRESSION2
              either EXPRESSION1 or EXPRESSION2 is true

       -n STRING
              the length of STRING is nonzero

       STRING equivalent to -n STRING

       -z STRING
              the length of STRING is zero

       STRING1 = STRING2
              the strings are equal

       STRING1 != STRING2
              the strings are not equal

       INTEGER1 -eq INTEGER2
              INTEGER1 is equal to INTEGER2

       INTEGER1 -ge INTEGER2
              INTEGER1 is greater than or equal to INTEGER2

       INTEGER1 -gt INTEGER2
              INTEGER1 is greater than INTEGER2

       INTEGER1 -le INTEGER2
              INTEGER1 is less than or equal to INTEGER2

       INTEGER1 -lt INTEGER2
              INTEGER1 is less than INTEGER2

       INTEGER1 -ne INTEGER2
              INTEGER1 is not equal to INTEGER2

       FILE1 -ef FILE2
              FILE1 and FILE2 have the same device and inode numbers

       FILE1 -nt FILE2
              FILE1 is newer (modification date) than FILE2

       FILE1 -ot FILE2
              FILE1 is older than FILE2

       -b FILE
              FILE exists and is block special

       -c FILE
              FILE exists and is character special

       -d FILE
              FILE exists and is a directory

       -e FILE
              FILE exists

       -f FILE
              FILE exists and is a regular file

       -g FILE
              FILE exists and is set-group-ID

       -G FILE
              FILE exists and is owned by the effective group ID

       -h FILE
              FILE exists and is a symbolic link (same as -L)

       -k FILE
              FILE exists and has its sticky bit set

       -L FILE
              FILE exists and is a symbolic link (same as -h)

       -O FILE
              FILE exists and is owned by the effective user ID

       -p FILE
              FILE exists and is a named pipe

       -r FILE
              FILE exists and read permission is granted

       -s FILE
              FILE exists and has a size greater than zero

       -S FILE
              FILE exists and is a socket

       -t FD  file descriptor FD is opened on a terminal

       -u FILE
              FILE exists and its set-user-ID bit is set

       -w FILE
              FILE exists and write permission is granted

       -x FILE
              FILE exists and execute (or search) permission is granted