UNIX for Dummies Questions & Answers

I've googled the hell out of this, and in my quest to advance my knowledge and expertise in modifying phones to make them more awesome, I ended up here.
I've found answers about patterns and whatnot that seem really complex for what I am trying to do, and basically it is this:
if the line says

Code:

ro.ril.gprsclass=10

I want to change it to

Code:

ro.ril.gprsclass=12

- if I understand right then that would be easy -

Code:

sed 's/^ro.ril.gprsclass=*/ro.ril.gprsclass=12/g' /system/build.prop

(the file I am modifying)
But what would be the easiest way to add that line IF there was no line that contained "ro.ril.gprsclass="?
I know this may be a dumb question, but I am looking for the SIMPLEST way of doing this(there will be many lines modified like this, and I want to keep it simple to avoid possible errors)?
I am only trying to modify/add the lines that start with RO(read-only), I can create another file that will supercede any files that don't start with that..
In case it helps, what I have so far is:

Code:

#!/sbin/sh
busybox echo "" >> /system/build.prop
busybox echo "# xLoud" >> /system/build.prop
busybox echo "ro.semc.sound_effects_enabled=true" >> /system/build.prop
busybox echo "ro.semc.xloud.supported=true" >> /system/build.prop
busybox echo "persist.service.xloud.enable=1" >> /system/build.prop
busybox echo "com.sonyericsson.xloud_enabled=true" >> /system/build.prop
busybox echo "xloud_enabled=true" >> /system/build.prop
busybox echo "ro.semc.xloud.default_setting=true" >> /system/build.prop
sed 's/^ro.product.version=*/ro.product.version=Revolutionized Nonsense 2.1Final/g' /system/build.prop
busybox echo "" >> /system/build.prop

The last line will replace a line I KNOW is there, but I need this answer to make sure that I don't create multiple entries of the same code(and the first entry takes precedence anyways)
If anything here is unclear let me know, but for me I am clearly going from developing for a phone to coding- and all the instructions I have found thus far are complex or just plain confusing..

---------- Post updated at 09:01 AM ---------- Previous update was at 12:22 AM ----------

Someone in the phone forums said I could use grep + sed to get it done, they didn't give me a solid example, but I'm guessing it would look something like this?

Code:

if grep ro.ril.gprsclass= /system/build.prop ; then 
sed -d 's/^ro.ril.gprsclass=*/ro.ril.gprsclass=12/g' /system/build.prop 
else 
busybox echo "ro.ril.gprsclass=12" >> /system/build.prop

I'm unsure about the formatting, but if the above worked and there WAS a line that said "ro.ril.gprsclass=8" it would change it to 12, or if there was no line it would add it to the end of the original file.. am I correct?
I'd appreciate any feedback, it seems much easier to find complex ways of doing these things that may work, much harder to find the simplest way that WILL work..
Thanks in advance!

SBD

Hi, this:

Code:

sed 's/^ro.ril.gprsclass=*/ro.ril.gprsclass=12/g' /system/build.prop

should be this:

Code:

sed 's/^ro\.ril\.gprsclass=.*/ro.ril.gprsclass=12/g' /system/build.prop

probably your busybox version of sed has a -i option so the file can be edited in-place..

I don't think you need to use "busybox echo" since echo is a shell built-in

for grep I would you "grep -q"

I'm not sure if there is a rule against linking to a different forum, but if anyone happens upon this thread looking for answers, I got some really in depth ones here that should solve my problem:
It won't let me post a link, but as I said this is to be helpful - to this thread showthread.php?t=1647656 at xda

I for one get frustrated when I find so more posts on the internet that don't have an answer than ones that do.