Re: [Ltsp-discuss] Sed substitution in /usr/share/ltsp/init-ltsp.d script yields partially duplicate results

[Andreas S. <sch...@fa...>]

Hi John,

I don't know much about LTSP but I know _sed_ well enough to be able 
to provide hopefully helpful answers.

On Thu, 20 Dec 2012, at 10:43, John Hupp wrote:

> > > I'm using a client boot script at /usr/share/ltsp/init-ltsp.d/50-apcupsd
> > > with sed to modify a configuration file.  The script:
> > > 
> > > sed -i \
> > >     -e 's/UPSCABLE usb/UPSCABLE ether/' \
> > >     -e 's/UPSTYPE usb/UPSTYPE net/' \
> > >     -e 's/DEVICE/DEVICE Lubuntu1:3551/' \
> > >     -e 's/TIMEOUT 105/TIMEOUT 60/' \
> > >     -e 's/NETSERVER on/NETSERVER off/' \
> > >     -e 's/NISIP 0.0.0.0/NISIP 127.0.0.1/' \
> > > /etc/apcupsd/apcupsd.conf

You said this worked. And it should. Note, though that sed "rules" 
starting with 's/' are substitutions. If what is to be substituted 
does not exist in the first place, you'll never get the results you 
want. E.g., if the TIMEOUT in the original file is not 105, the result 
will never be 60. This line could be written as

    -e 's/TIMEOUT [1-9][0-9]*/TIMEOUT 60/' \

then TIMEOUT 0 is untouched, but any other integer value is replaced 
with 60.

> > > It all works fine except for this substitution:
> > >     -e 's/DEVICE/DEVICE Lubuntu1:3551/'
> > > 
> > > It takes this line:
> > >     DEVICE
> > > and replaces it with this:
> > >     DEVICE Lubuntu1:3551 Lubuntu1:3551
> > > 
> > > However, if I run the same script on the server, it does not 
> > > produce the duplicate "Lubuntu1:3551".  Does anyone know why?

There are only 2 possible explanations:

A) The script runs twice.
B) The original line was not "DEVICE" but "DEVICE Lubuntu1:3551"
   (maybe because another script changed it).

's/DEVICE/DEVICE Lubuntu1:3551/' replaces the 1st "DEVICE" with 
"DEVICE Lubuntu1:3551". If you apply this twice you get "DEVICE 
Lubuntu1:3551 Lubuntu1:3551"

> > > Seeking a work-around, I added an /etc/apcupsd/apcupsd_slave.conf file
> > > which was already modified with the parameters I want clients to boot
> > > with. Then I edited /usr/share/ltsp/init-ltsp.d/50-apcupsd to remove the
> > > sed command and added:
> > > 
> > > cp /etc/apcupsd/apcupsd_slave.conf  /etc/apcupsd/apcupsd.conf
> > > 
> > > But this does not seem to execute.

Double check that files are indeed where they should be 
(server-client-wise). This should execute. Maybe add a logger 
statement to debug.

> > > local session xterminal and run:
> > > $ su root cp /etc/apcupsd/apcupsd_slave.conf /etc/apcupsd/apcupsd.conf
> > > it returns:
> > > /bin/cp: cannot execute binary file

I guess this should be

  su root -c cp ...

> > Someone points out that with the sed command, I would get the 
> > duplicate result I report if the script ran twice.

Ah, so someone said it before :)

> > Does that trigger any thoughts?

I don't have the same setup. But I'd check what is in 
/opt/ltsp/i386/etc/apcupsd/apcupsd.conf

> Someone else wondered if the colon in the replacement expression 
> needed to be escaped.

Definitely no.

> But I still have the thinnest possible understanding of the finer 
> points of sed and the substitution syntax, especially about escaping 
> special characters.

Your script is fine. There's a lot that needs to be escaped in general 
terms but no in your specific case.

> I don't even know if the backslash would go before or after the
> colon!

As I said, do not touch the colon. Backslashes are required at the end 
of the lines, because the whole command is made of multiple lines. The 
Backslashes are actually for Bash (or whatever shell) not for sed.

> But I had wondered about the possible effect of that colon myself.
> To test that idea, I replaced the colon with a space:
>     sed -e 's/DEVICE/DEVICE Lubuntu1 3551/'
> Now on the LTSP client, the resulting line is:
>     DEVICE Lubuntu1:3551 Lubuntu1 3551

That's interesting. If this was the result then the script with 
the "missing"/replaced colon ran first. Another substitution took 
place afterwards. This excludes the scenario "A" mentioned above!

> I don't know what the syntax means, but someone also suggested the 
> following:
> 
>     sed -e 's/DEVICE$/DEVICE Lubuntu1:3551/'
> 
> And that had no effect whatsoever.  The result was still "DEVICE 
> Lubuntu1:3551 Lubuntu1:3551".

The suggestion is good. It replaces "DEVICE" only if it is the last 
word on the line. It did not help you because, apparently, your script 
finds "DEVICE" and only "DEVICE" and replaces it correctly and as 
intended. There must be another script that messes it all up.

> Does LTSP use something like a writable overlay filesystem?

AFAIK, yes, but someone with a better understand should fill you in on 
this.

HTH,
-- 
-- Andreas