Take a look at the /etc/passwd file. This file shows you how user information is stored. Your user information may or may not be stored here (There are several different databases that Unix uses for storing users), but the format is the same.
Basically, Unix uses the User ID (UID) to store what user is what. The next entry was the old password entry, then the UID, the primary Group ID, the GECOS field, the $HOME directory, and the user's shell. (There are three extra entries displayed in the id -P command in MacOS. I don't know what they are, but they make the GECOS field the eighth field instead of the fifth field).
Using the id -P command on your system gave you this entry. Some systems use getent or even getpwent as a command. What you need to do is parse this entry. Each field is separated by colons, so you need either the fifth or eighth the entry (depending upon the command you had to use).
The awk and cut commands do this quite nicely. cut is probably more efficient, but awk is more common, so I tend to use that.
In awk, the standard field separator is white space, but you can use the -F parameter to change this. In Awk, each field in a line is given a number and preceded by a dollar sign. The $0 field is the entire line.
Using awk, you get:
id -P | awk -F: '{print $8}'
This says to take the id -P command, and use the : as a field separator, and to print out the eighth field. THe curly braces surround all AWK programs, and the single quotes are needed to keep the shell from interpreting the $8.
In BASH, you can use $( ) to run a command and return it's output, so you can set environment variables:
$USER_NAME=$(id -P | awk -F: `{print $8}`) echo $USER_NAME
