I often reboot a development box and then have to sift through syslog.
How can I add a marker beetween the last line before shutdown and the first line of the new boot?
Box is Debian stable with rsyslog 8.4.2-1+deb8u2
If the only point is to find the start of a new boot, you can just look for this regular expression in the logging output
kernel: \[ *0.000000] Linux version
logger program is a command line interface to the logging system, so just run something like
logger -p kernel.notice "---------- Rebooting ---------"
before you reboot. There are lots more options.