How to get first n characters from a field in Splunk?

substr() can be used to get first n characters from a field in Splunk.
 
Syntax:
substr(str, start, length)

Syntax with Field Reference:
eval Summary=substr(description,1,57)
 
Example:
eval Summary=substr(description,1,57)

From the above example, Summary will hold first 57 characters of the description field value.