[NTLUG:Discuss] sed question

[fredjame]

Ralph Green, Jr wrote:

>Howdy,
>  sed is one of those neat tools I always thought I should know, but
>never managed to use.  I have a good application for it now and there is
>one thing I have not figured out.  If you have a suggestion, please let
>me know.  If you think this is the wrong tool, I'll listen to that.  I
>want to do this in an efficient way.
>
>  The problem is this.  I have a web page written in PHP.  It allows the
>user to specify two NTP servers to use to keep the machine
>synchronized.  When a user changes the servers on the web page, I need
>to update the /etc/ntp.conf file and then restart the NTP service.  My
>only probleem is updating the config file.  The two lines that need to
>be changed of /etc/ntp.conf are shown here between the dashed lines:
>- - - - - - - - - - - - - - - - - - -
>server  ntppub.tamu.edu
>server  tick.usno.navy.mil
>- - - - - - - - - - - - - - - - - - -
>  There are a couple of things to keep in mind.
>1. There is another line in the file that starts with server.  It says
>server  127.127.1.0 or some such.  It needs to be left unchanged.
>2. While I will know what the two server lines said the last time I
>updated the file because I have database entries for them, the user
>could have made manual changes.  The script need to work in any case.
>
> I know one way to solve this would be to issue a command like
>sed '/server[ \t][^0-9]'/d <ntp.conf >ntp.new
> This would delete any active server lines and then I could append my
>new ones to the bottom of the file.  I'll go with this plan if I have
>too,It os not pretty, but it is functional.
>
>  It looks like there might be a way to tell sed to go to the first
>matching line and then insert text(my new server lines) and then delete
>the matching lines.  This has the advantage that the server lines appear
>in their normal place in the file, below the appropriate comment block. 
>But, I have not gotten this to work.
>
>  Do you understand what I am trying to accomplish?  Can you offer a
>suggestion?
>Thanks,
>Ralph
>  
>
Just a couple of quickies off the top of my head ...
(1) If the two lines in question are:
(1-a) always in the same place, you could use the line numbers as the 
addresses instead of the pattern
(1-b) not always in the same place, is there anything else in the line 
that is constant in addition to "server"?
That is, can you make a unique identification of each line?  If you can, 
you could do a search/replace.
(2) Would you want to keep of history of the lines replaced/changed?

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