Add messages to syslog (/var/log/syslog).

# To log a message to syslog:
logger message

# To take input from stdin and log to syslog:
echo log_entry | logger

# To send the output to a remote syslog server running at a given port. Default port is 514:
echo log_entry | logger -h hostname -P port

# To use a specific tag for every line logged. Default is the name of logged in user:
echo log_entry | logger -t tag

# To log messages with a given priority. Default is user.notice. See man logger for all priority options:
echo log_entry | logger -p user.warning