[gentoo-server] Re: Scripting fun

[Steve Long]

JD Gray wrote:

> I'm running the below script on my gentoo servers to email me whenever
> there are GLSA's affecting me. It works like a charm, but I have one
> beef with it. Newlines are not preserved, so I get a lovely Wall Of
> Text (tm) when ever it sends me the GLSA. I'm guessing this is because
> of the way bash handles variables. Anyone have any insight on how to
> correct this?
>
> #!/bin/bash
>
> /usr/bin/glsa-check -t all &> /dev/null
> CHECK_RESULT="`/usr/bin/glsa-check -t all 2>&1`"
>
> if [ "$CHECK_RESULT" != "" ]; then
>   echo $CHECK_RESULT | /bin/mail -s "Frog glsa-check" kahdgarxi@×××××.com;
> fi
>
Simply quote the variable you are echo'ing:  echo "$CHECK_RESULT"
What's happening here is called "word-splitting"; bash is sending each part of 
the string as a separate parameter to echo. See: 
http://www.bash-hackers.org/wiki/doku.php/syntax/words for more.

The best explanation I've seen for why this happens (the one that helps 
beginners ime) is:
"At its base, a shell is simply a macro processor that executes commands. The 
term macro processor means functionality where text and symbols are expanded 
to create larger expressions."
..from the bash reference manual: 
http://tiswww.tis.case.edu/~chet/bash/bashref.html which I don't recommend as 
a beginning read :) but is definitely worth bookmarking.

http://www.grymoire.com/Unix/Quote.html shows how to deal with quoting in 
various situations; make sure you understand the difference between strong 
(single) quotes and weak (double) quotes. For instance, "Frog glsa-check" 
doesn't need double-quotes and single-quotes are a bit faster to parse and 
evaluate (why lose /any/ speed when you don't need to?) Strong quotes can 
also be really useful for stuff echo'ed to the terminal, eg a help message 
where you need to include some script the user might look at or use. 
update[1] does this for the various help options.

I've also heard that mutt is a very useful utility if you need to send mail 
from shellscripts; I haven't worked with it but aiui it makes it easy to add 
attachments and so on.

BTW you can simply use: if [[ $foo ]] to do the check; see ''help test'' in 
your terminal; -n is the default, and no word-splitting happens inside [[ so 
quotes are not needed, which is one of the things that makes it much more 
convenient.

http://wooledge.org/mywiki/BashGuide is the best tutorial I've seen on the 
web; it's written and maintained by people from #bash on irc.freenode.net 
which is an invaluable resource, if a little rude sometimes ;)
The bash hackers wiki mentioned, is also a good place to learn:
http://www.bash-hackers.org/wiki/

The FAQ: http://wooledge.org/mywiki/BashFaq is legendary, and this:
http://wooledge.org/mywiki/BashPitfalls is worth reviewing, especially once 
you've got the basics.

Minor style point: varnames in caps are usually reserved for environment vars, 
or constants; within ebuilds they're also used for standard config things, or 
variables that cross functions or phases.

Oh, one other thing: backticks are deprecated, and $(..) is much preferred, 
eg: foo=$(cmd 'some param' "$blah" 2>"$errFile") since it's easier to nest 
and maintain. This is true for POSIX sh as well as BASH; see:
http://www.opengroup.org/onlinepubs/009695399/utilities/contents.html
..for the definitive reference on portability. Sticking to the parameters 
given there for commands (aka utilities), will do wonders for the portability 
of your BASH scripts.

HTH,
steveL (I _love_ BASH, can you tell? ;)

[1] http://forums.gentoo.org/viewtopic-t-546828.html | You might like 
update -A which does the glsa-check for you. I'd love feedback from -server 
users; update -x is an automated mode which logs to the syslog, designed for 
use under cron. (It's called server in the code.)