help debugging shell script with a variable expansion

[hemlockz]

What I am trying to accomplish is a way to read only the lines that have been added to the file mylog0 since the last time the script looped (5 seconds). I am open to new suggestions too, I have been stuck on this little script for a few hours already. Thanks...

#!/bin/bash
i=0
firstline=0
#program loop
running () 
	{
	date +'%F %H:%M:%S - what i saw in the last 5 seconds:'
	lastline=$(wc -l "$filename"0 | awk '{print $1}')
	sed -n "'$firstline,${lastline}p' ${filename}0"
	firstline=$lastline
	echo -----
	sleep 5
	running	
	}
read filename < /var/tmp/mylogfilename
running

the error happens on the sed command. The same command seems to work from the command line with numbers instead of variables. This is the output of sh -x

+ running
+ date +%F %H:%M:%S - what i saw in the last 5 seconds:
2011-01-20 09:18:59 - what i saw in the last 5 seconds:
+ wc -l mylog0
+ awk {print $1}
+ lastline=16
+ sed -n '4,16p' 
sed: -e expression #1, char 1: unknown command: `''
+ firstline=16
+ echo -----
-----
+ sleep 5

[hemlockz]

eval sed -n "'${firstline},${lastline}p'" ${filename}0

needed the eval in front of the sed i guess, seems to work

[scm]

What you need to do is lose the double quotes on the sed line, and replace the single quotes with double quotes.

Then learn how to use quotes.