mkdir ~/bin

Now let's go to it:

cd ~/bin and create a file text.sh

#!/bin/bash

MESSAGE=$1
# Replace with their number (be sure to include the +1)
PHONE_NUMBER=+11234567

if [ "$#" -ne 1 ]; then
     echo "Usage: $0 MESSAGE"
     exit 1
fi

osascript -e "tell application \"Messages\" to send \"$MESSAGE\" to buddy \"    $PHONE_NUMBER\""

Now we need to make the file executable:
chmod +x text.sh

Now we can use it!

./text.sh "I LOVE YOU"

Alias

To alias it so that you could say something like rach "I LOVE YOU"

Add to your .zshrc or .bashrc the following:

# local bin
export PATH="$HOME/bin:$PATH"
alias rach='text.sh'

Now we can say rach "I LOVE YOU" and it sends!