Eval & 404 error code search Splunk Commands example:

index ="main" | status ="404" | top limit = 50 url

Naming concatenation: 

index ="main" | eval new_field = 'Firstname' + 'LastName' | table "Firstname" + "Lastname", new_field

. for integer concatenation.

index ="main" | eval new_field = 'Firstname' + 'LastName'.100 | table "Firstname" + "Lastname", new_field

override:

index = "main"| eval "firstname" = 'Firstname'.100 | table "firstname"

index ="main" | eval F1 = "Karthik", F2 = "Sid" | table "F1","F2"