Linux Security Cookbook

9.31. Sending Messages to the System Logger

Problem

You want to add information about interesting events to the system log.

Solution

Use the logger program. A simple example:

$ logger "using Ethereal to debug a network problem"

Suppose “food” is the name of a program, short for “Foo Daemon.” Log a simple message:

$ logger -t "food[$$]" -p local3.warning "$count connections from $host"

Direct stdout and stderr output to syslog:

$ food 2>&1 | logger -t "food[$$]" -p local3.notice &

Send stdout and stderr to syslog, using different priorities (bash only):

$ food 1> >(logger -t "food[$$]" -p local3.info) \
       2> >(logger -t "food[$$]" -p local3.err)  &

You can also write to the system log from shell scripts [Recipe 9.32], Perl programs [Recipe 9.33], or C programs [Recipe 9.34].

Discussion

The system logger isn’t just for system programs: you can use it with your own programs and scripts, or even interactively. This is a great way to record information for processes that run in the background (e.g., as cron jobs), when stdout and stderr aren’t necessarily connected to anything useful. Don’t bother to create, open, and maintain your own log files: let the system logger do the work.

Interactively, logger can be used almost like echo to record a message with the default user facility and notice priority. Your username will be prepended to each message as an identifier.

Our recipe shows a sample “Foo Daemon” (food) that uses the local3 facility and various priority levels, depending on the importance ...