By following the below steps i have resolved the issue

1. Just give the name of the file in 'Shell Command' field

2. After clicking enter, you see 'Files' button. Select the HDFS path to the script in that field and submit the action.

By this way, it will add an element as below, in the workflow xml.

<file>hdfs://${namenode}/tmp/data/file/shellScript.sh</file>