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GNU Screen logtstamp string

By Jon Jensen
July 24, 2013

A short note on GNU Screen configuration:

You can add configuration to ~/.screenrc or another configuration file named by -c filename upon invocation, and among the many options are some to enable logging what happens in the screen windows. This is useful when using screen as a reattachable daemonizer.

Consider this configuration:

logfile path/to/screen-output.%Y%m%d.log
logfile flush 1
logtstamp on
logtstamp after 5
log on

That works nicely. With logfile we specify the name of the logfile, using some % escapes as per “STRING ESCAPES” in the manpage to put the date in the logfile name.

With logfile flush 1 we request that every 1 second the output be flushed to the log, making it easier to follow with tail -f.

logtstamp on turns on timestamping which writes a timestamp to the log after a default 2 minutes of inactivity. We shorten that to 5 seconds with logtstamp after 5.

Finally, log on turns on the logging.

Now, what if we want to customize the timestamp? The default looks like this:

-- 0:process-name -- time-stamp -- Jul/24/13  9:09:56 --

Which the manpage says can be customized with logtstampt string ..., where the default is ‘-- %n:%t -- time-stamp -- %M/%d/%y %c:%s --\n’.

The manpage earlier says that arguments may be separated by single or double quotes. Doing so with the default shown doesn’t work, because a literal \n shows up in the logfile.

The solution I worked out by trial and error is that you must double-quote the string and use an octal escape value \012. Single-quoting that will output a literal backslash 0 1 2, and \n simply is not a recognized escape. Thus our final configuration directive is:

logtstamp string "-- time-stamp -- %Y-%m-%d %0c:%s --\012"

which results in output like:

-- time-stamp -- 2013-07-24 09:59:35 --

If you use more than one screen window with this configuration, all windows’ output will go into the same logfile. Use the %n escape string to include a window number in the logfile name if you’d like them kept separate.

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