引用变量

 variable2=""    # Empty.

 COMMAND $variable2 $variable2 $variable2        # COMMAND将不带参数执行. 
 COMMAND "$variable2" "$variable2" "$variable2"  # COMMAND将以3个空参数来执行. 
 COMMAND "$variable2 $variable2 $variable2"      # COMMAND将以1个参数来执行(2空格)

在echo语句中, 只有在单词分割(word splitting)或者需要保留的时候, 才需要把参数用双引号括起来.