The fillnull command replaces null values in one or more fields with a specified value. The values option allows you to specify a comma-separated list of values to fill the null values in the corresponding fields. The fields option allows you to specify a comma-separated list of fields to apply the fillnull command to. The eval statement in the question uses the if and isnull functions to check if field1 and field2 have null values and replace them with 0 and “NO-VALUE” respectively. The equivalent expression using fillnull is to use the values option to specify 0 and “NO-VALUE” and the fields option to specify field1 and field22

1: Splunk Core Certified Power User Track, page 9. 2: Splunk Documentation, fillnull command.