I could grep through /etc/passwd but that seems onerous. 'finger' isn't installed and I'd like to avoid that dependency. This is for a program so it would be nice if there was some command that let you just access user info.
You don't specify a programming language, so I'll assume you want to use the shell; here's an answer for Posix shells.
Two steps to this: get the appropriate record, then get the field you want from that record.
First, getting the account record is done by querying the passwd table:
$ user_name=foo
$ user_record="$(getent passwd $user_name)"
$ echo "$user_record"
foo:x:1023:1025:Fred Nurk,,,:/home/foo:/bin/bash
For hysterical raisins, the full name of the user is recorded in a field called the “GECOS” field; to complicate matters, this field often has its own structure with the full name as just one of several optional sub-fields. So anything that wants to get the full name from the account record needs to parse both these levels.
$ user_record="$(getent passwd $user_name)"
$ user_gecos_field="$(echo "$user_record" | cut -d ':' -f 5)"
$ user_full_name="$(echo "$user_gecos_field" | cut -d ',' -f 1)"
$ echo "$user_full_name"
Fred Nurk
Your programming language probably has a library function to do this in fewer steps. In C, you'd use the ‘getpwnam’ function and then parse the GECOS field.