Which command would you use to modify existing fields in search results?

The command that is used to modify existing fields in search results is eval. This command allows you to create new fields or change the values of existing fields based on expressions you define. For example, you can use eval to calculate new values, adjust existing ones, or transform data types. This flexibility makes it a powerful tool for data manipulation within Splunk searches, enabling more comprehensive data analysis and visualization.

While there are other commands that can alter how fields are presented, eval specifically focuses on modifying field values or creating new fields altogether through various functions and operations. Thus, it stands out as the most appropriate choice for the objective of modifying existing field values in your search results.