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Recipe 9.31 Sending Messages to the System Logger
9.31.1 Problem
You want to add information about
interesting events to the system log.
9.31.2 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 Recipe 9.32], Perl programs [Recipe 9.33],
or C programs
[Recipe 9.34].
9.31.3 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 of each message. By convention, the
script uses its name "food" as an
identifier that is prepended to each message.It is a good idea to add a process ID to each message, so that a
series of messages can be untangled when several copies of the script
are running simultaneously. For example, consider the log file
entries from a computer named cafeteria :
Feb 21 12:05:41 cafeteria food[1234]: customer arrived: Alison
Feb 21 12:06:15 cafeteria food[5678]: customer arrived: Bob
Feb 21 12:10:22 cafeteria food[1234]: devoured tofu
Feb 21 12:11:09 cafeteria food[5678]: consumed beef
Feb 21 12:15:34 cafeteria food[5678]: ingested pork
Feb 21 12:18:23 cafeteria food[1234]: gobbled up broccoli
Feb 21 12:22:52 cafeteria food[5678]: paid $7.89
Feb 21 12:24:35 cafeteria food[1234]: paid $4.59
In this case, the process IDs allow us to distinguish carnivores and
herbivores, and to determine how much each paid. We use the process
ID of the invoking shell by appending
"[$$]" to the program
name.[12] Other identifiers are possible, like
the customer name in our example, but the process ID is guaranteed to
be unique: consider the possibility of two customers named Bob! The
system logger can record the process ID with each message
automatically.
[12] logger's
own option to log a process ID, -i, is
unfortunately useless. It prints the process ID of
logger itself, which changes on each
invocation.
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system logger are unfortunately common.
Our recipe shows two techniques for capturing stdout and stderr from
such programs, either combined or separately (with different
priorities), using logger. The latter uses
process substitution, which is
available only if the script is run by bash (not
the standard Bourne shell, sh).
9.31.4 See Also
logger(1), bash(1).
