shell脚本中比较并输出最大值

#！/bin/bash

    echo " input first parm:"

    read a

    echo " input second parm:"

    read b

    echo " input third parm:"

    if [ $a -gt $b -a $a -gt $c ]

    then

    echo "max is $a"

    fi

      if [ $b -gt $a -a $b -gt $c ]

    then

    echo "max is $b"

    fi

      if [ $c -gt $b -a $c -gt $c ]

    then

    echo "max is $c"

    fi