Capping output redirection log file length

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# 1  
Old 10-12-2009
Capping output redirection log file length

I am trying to write a script which will output notifications to a logfile, but I would like to cap the logfile to, let's say, 200 lines.

Specifically I am using custom firmware, DD-wrt, on my router and I am implementing a script to connect to my work vpn. I have a loop that pings a computer behind the vpn, and then sleeps for a minute before doing it again. If the ping fails, I have it write a brief message to a logfile. There is no real storage on the router, so everything is held in memory. If my log file gets too large, I will crash my router. Is there a way to make my logfile remove the top line when appending to the bottom, if the file is more than 200 lines. Currently I just append with '>>', but there must be a smarter way.

The relevant parts of the script are below.


pingtest () {
 ping -q -c1 $1 >> /dev/null
 echo "$?"


while [ 1 ]; do
  if [ "`pingtest $vpn_keepalive_host1`" == "0" ]; then
    sleep 60
    echo `date` "$vpn_keepalive_host1 unreachable" >> /tmp/etc/vpnc/Status.$vpn_keepalive_host1

    #reconnect to vpn
    vpnc /tmp/etc/vpnc/vpn.conf --dpd-idle 0

# 2  
Old 10-12-2009
You could try:

echo `date` "$vpn_keepalive_host1 unreachable" |cat $logfile - |tail -200> $logfile

or e.g.
log200 () {
  echo "$1"|cat "$2" -|tail -200>"$2"

log200 "`date` $vpn_keepalive_host1 unreachable"  /tmp/etc/vpnc/Status.$vpn_keepalive_host1

# 3  
Old 10-13-2009

I'm not sure if it's a bug in the router OS or if this is true all of the time, but apparently sometimes this method works and sometimes it overwrites the original file with only the last entry. A (less than elegant) method I've found that will work is:

log_clip () {
 echo "$1" | cat "$2" - | tail -n 100 > $pwd/temp
 mv $pwd/temp "$2"

However I feel there must be a more elegant way to avoid temporary files using awk or sed, but I'm just not that familiar with them. Can someone help appease the OCD perfectionist in me?
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