What is the difference between the Eval and the FIELDALIAS commands?

[shayhibah]

I am wondering what is the difference between eval & fieldalias commands?

For example:
EVAL-app = if(isnull(service),app,service)
FIELDALIAS-log = service AS app

In the first case, if service is null, app stay as is.
In the second case, if service is null, app is null.

Can someone explain it please?
Thanks

[kamlesh_vaghela]

@shayhibah

Search-time operation sequence will help you to understand this. 🙂

http://docs.splunk.com/Documentation/Splunk/7.2.0/Knowledge/Searchtimeoperationssequence

Thanks

[shayhibah]

I read this page and it gave me some more information but I still cannot understand why the output of FILEDALIAS is null in some cases.

[lwest_splunk]

@shayhibah please see the answer I placed on your other answers ask found here:

https://answers.splunk.com/answers/657473/fieldalias-override-another-fieldalias.html?childToView=76...

Hope this helps give some clarity to you for the FIELDALIAS function and its limitations!

[kamlesh_vaghela]

@shayhibah

1)

EVAL-app = if(isnull(service),app,service)

In this case, we are deciding app field on the basis of the service. If service is not available then app will be the same value available in the event. which can be null (if app not found in the event) or not null (if app found in the event). So if service field is available then service will be assigned to the app field. if service field is not available then app will be assigned to app (if the event has app field ). if app field is not available then app will be null.

2

FIELDALIAS-log = service AS app

In this case, we are deciding app field on the basis of the service field regardless of field available or not. So if service field is available then service will be assigned to the app. if service field is not available then app will be null.