angularjs - Angular ng-show if string not empty does not work -

i writing web app using angularjs. user submits 3 keywords server. show commas in between keywords if there enough keywords.

for example:

if there no keywords, then:

if there 1 keyword, then: keyword

if there 2 keywords, then: keyword, keyword

if there 3 keywords, then: keyword, keyword, keyword

i have following code:

    <tr>        <td>keywords: </td>        <td>{{post.keyword1}} <span ng-show = "{{post.keyword2}}">,</span> {{post.keyword2}} <span ng-show = "{{post.keyword3}}">,</span> {{post.keyword3}}</td>     </tr> 

why not work?

change to

<td>{{post.keyword1}} <span ng-show = "post.keyword2">,</span> {{post.keyword2}} <span ng-show = "post.keyword3">,</span> {{post.keyword3}}</td>