＆＃34; [：操作系统：意外的操作员＆＃34; shell中的错误

Question

#!/bin/sh

echo "List of file which have read ,write and executable permissions in the current directory are: "

for file in *    
do
  if  [ -r $file -a -w $file -a -x $file ]
  then
    echo $file
  fi
done

为什么会出现这样的错误：

os1f.sh: 6: [: OS: unexpected operator