Keep in mind that Linux is just a kernel. A Linux distribution (what you download and run) is made up of a kernel, and a bunch of other programs packaged together.
If you want Linux's source (ie. the kernel source), head to kernel.org.
If you look at the manual page for 'write' (
man write), you'll see this:
CODE
WRITE(1) BSD General Commands Manual WRITE(1)
NAME
write - send a message to another user
SYNOPSIS
write user [ttyname]
DESCRIPTION
The write utility allows you to communicate with other users, by copying
lines from your terminal to theirs.
When you run the write command, the user you are writing to gets a mes‐
sage of the form:
Message from yourname@yourhost on yourtty at hh:mm ...
Any further lines you enter will be copied to the specified user’s termi‐
nal. If the other user wants to reply, they must run write as well.
When you are done, type an end-of-file or interrupt character. The other
user will see the message ‘EOF’ indicating that the conversation is over.
You can prevent people (other than the super-user) from writing to you
with the mesg(1) command.
If the user you want to write to is logged in on more than one terminal,
you can specify which terminal to write to by specifying the terminal
name as the second operand to the write command. Alternatively, you can
let write select one of the terminals - it will pick the one with the
shortest idle time. This is so that if the user is logged in at work and
also dialed up from home, the message will go to the right place.
The traditional protocol for writing to someone is that the string ‘-o’,
either at the end of a line or on a line by itself, means that it’s the
other person’s turn to talk. The string ‘oo’ means that the person
believes the conversation to be over.
SEE ALSO
mesg(1), talk(1), wall(1), who(1)
HISTORY
A write command appeared in Version 1 AT&T UNIX.
BSD June 6, 1993 BSD
Googling for "BSD source code" brings up the FreeBSD site (good enough), and you can find the source file for 'write'
here.
Any other programs you're looking for will probably be one directory up.