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Top Forums Shell Programming and Scripting Grabbing variables and comparing Post 302225810 by kerpm on Sunday 17th of August 2008 04:27:00 AM
Old 08-17-2008
Grabbing variables and comparing

I have two computers with dynamic IP addresses and am using dyndns so that they are identifiable as the same computer even if their IPs change (we'll call them and I also have a remote server which I would like to store my computers' IP addresses on. There is a file on my remote server which has the IPs that the two hostnames point to. So the file would look like this

I need a script that will run this command which gets the IPs of my computers:

/usr/bin/dig +short | /usr/bin/tail -n 1
/usr/bin/dig +short | /usr/bin/tail -n 1

Then compare the result (which is an IP address) to the file that I have described above. If the IP is different from the old one then remove that line and replace it with the new IP. I thought I could do this myself but grabbing the old IP and storing it in a variable and then comparing it to the new IP was beyond me. I don't even know if that is how you should do it. sed/awk is preferable but any language will do Smilie

Thanks in advance.

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SUBST(1)						      General Commands Manual							  SUBST(1)

subst - substitute definitions into file(s) SYNOPSIS
subst [ -e editor ] -f substitutions victim ... DESCRIPTION
Subst makes substitutions into files, in a way that is suitable for customizing software to local conditions. Each victim file is altered according to the contents of the substitutions file. The substitutions file contains one line per substitution. A line consists of two fields separated by one or more tabs. The first field is the name of the substitution, the second is the value. Neither should contain the character `#', and use of text-editor metacharacters like `&' and `' is also unwise; the name in particular is best restricted to be alphanumeric. A line starting with `#' is a comment and is ignored. In the victims, each line on which a substitution is to be made (a target line) must be preceded by a prototype line. The prototype line should be delimited in such a way that it will be taken as a comment by whatever program processes the file later. The prototype line must contain a ``prototype'' of the target line bracketed by `=()<' and `>()='; everything else on the prototype line is ignored. Subst extracts the prototype, changes all instances of substitution names bracketed by `@<' and `>@' to their values, and then replaces the tar- get line with the result. OPTIONS
-e Substitutions are done using the sed(1) editor, which must be found in either the /bin or /usr/bin directories. To specify a dif- ferent executable, use the ``-e'' flag. EXAMPLE
If the substitutions file is FIRST 111 SECOND 222 and the victim file is x = 2; /* =()<y = @<FIRST>@ + @<SECOND>@;>()= */ y = 88 + 99; z = 5; then ``subst -f substitutions victim'' changes victim to: x = 2; /* =()<y = @<FIRST>@ + @<SECOND>@;>()= */ y = 111 + 222; z = 5; FILES
victimdir/ new version being built victimdir/substtmp.old old version during renaming SEE ALSO
Complains and halts if it is unable to create its temporary files or if they already exist. HISTORY
Written at U of Toronto by Henry Spencer. Rich $alz added the ``-e'' flag July, 1991. BUGS
When creating a file to be substed, it's easy to forget to insert a dummy target line after a prototype line; if you forget, subst ends up deleting whichever line did in fact follow the prototype line. 25 Feb 1990 SUBST(1)

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