[gentoo-amd64] Re: How to use &&

[Duncan]

Richard Freeman <rich0@g.o> posted 4960DC68.1060604@g.o,
excerpted below, on  Sun, 04 Jan 2009 10:57:28 -0500:

> Using && in this way is technically a bit of a hack - the && operator
> was intended to evaluate a boolean expression and not as a replacement
> for "if".  That doesn't stop 99.9999999% of shell scripters from using
> it, however.

Talking about which... for single-clause "if" versions I use &&/|| almost 
exclusively, these days, with {compound statements} wrapped and indented 
for clarity as necessary, to the point that reading shell code with a 
single-clause "if" has me wondering where the "else" is.

Now, every time I come to a multi-clause if/then/else or even a compound 
if (if/then/elseif or if/then/elseif/then/elseif...), I wonder what the 
equivalent logic should be there.  However, it's apparently convoluted 
enough I'm not the only one not using it very often, so I don't believe 
I've come across a proper example to copy.  Every time I try to map the 
logic of what it should look like, my mind gets tangled up.  Thus my 
question is simple enough, are there accepted ways of using &&/|| in 
place of compound if statements, that once I see, I'll probably go "Duh!"?

Some day I really ought to go thru a formal advanced shell scripting 
tutorial.  I learned practical shell scripting based on the intro in 
"Running Linux", the Bash appendix in "Linux in a Nutshell" (which I 
still keep within reach when I'm on a sysadminning binge), and tracing 
the logic of and then splitting into several smaller scripts the big 
Mandrake rcsysinit script... back about Mandrake 8.1 time.  It's amazing 
how effective that was at actually learning the /practical/ side of 
things, to the point I've traced and debugged and proposed for Roy 
several OpenRC patches since I run the ~arch and sometimes hard-masked 
versions (tried the git version for awhile, before deciding that was /
too/ raw for me), but I'm still missing some of the more advanced 
tricks.  I already have some links from Steve Long on [] vs [[]] I need 
to go thru at some point.  If someone could provide a nice link or two 
here covering the above if/then/else &&/|| shortcuts question, I'd 
probably actually go thru it before the []/[[]] stuff, since the &&/|| 
stuff bugs me every time I need an if conditional with both then and else 
clauses.

Or maybe the reason I keep getting tangled up trying to follow the logic 
is that it's not possible, short-circuit evaluation doesn't work that way?

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman