Create the script:

# vi /usr/bin/tweet

#!/bin/sh

	tweet="${@}"
	user="YourUsername"
	pass="YourPassword"

	if [ $(echo "${tweet}" | wc -c) -gt 140 ]; then
		echo "FATAL: The tweet is longer than 140 characters!"
		exit 1
	fi

	curl -k -u ${user}:${pass} -d status="${tweet}" http://twitter.com/statuses/update.xml > /dev/null 2>&1

	if [ "$?" == "0" ]; then
		echo "Successful tweet!"
	fi

	echo "Usage: $0 "{tweet "My tweet update"}""

# chmod +x /usr/bin/tweet

Usage:

# tweet "Update from bash script"