Re: [PLUG] Bash 101 resources

[JP Vossen <jp@jpsdomain.org>]

> Date: Sun, 3 Apr 2011 19:43:30 -0400
> From: Mag Gam<magawake@gmail.com>
> Subject: Re: [PLUG] Bash 101 resources
>
> Very nice slide show.

Thanks.  I hope it's useful.


> One comment about the if statements. Should they be double brackets
> versus single?

It depends...


> #correct way?
> if [[ -d /foo ]]; then
>    echo
> fi

I'd say "new" way.


> instead of
> if [ -d /foo ]; then
>    echo
> fi


See my http://www.bashcookbook.com/bashinfo/ page and pick "The bash 
Reference Guide" link for the version of bash you are using.  I picked 
4.2 just to have the latest, but this stuff hasn't changed in a while. 
See the "NEWS" and/or "CHANGES" links if you care what changed when.

Anyway...

http://www.bashcookbook.com/bashinfo/source/bash-4.2/doc/bashref.html#SEC83

(Hint: search for 'test' when looking for '[', as there are tons of '[' 
characters in the document.)

"Conditional expressions are used by the [[ compound command and the 
test and [ builtin commands. [...]"

So according to Chet, '[[' is a compound command but '[' is a builtin.

What does that mean to you?  I dunno.

My *personal* and admittedly imprecise rule-of-thumb is basically this. 
 I think of '[[' as the "new" one that can do better pattern matching 
and regular expressions.  But I mostly use the older '[' out of habit 
unless I'm doing something that really needs the new one.  And come to 
think of it, '[[' is probably less portable.  Yup.  According to 
http://princessleia.com/plug/2008-JP_bash_vs_dash.pdf, dash doesn't have 
'[['.  If you don't think you care about dash, you're wrong if you use 
Debian or Ubuntu...  See the link as to why.  (Hint: /bin/sh == dash in 
newer Ubuntu & Debian, and bash != Bourne != dash...)


Getting back to the simple example above, except for portability I don't 
think it really matters if you use '[' or '[['.  I'd use '[' out of 
habit and because it's less typing and maybe looks a tiny bit less 
cluttered.

Actually, for something that simple I'd probably write:
	[ -d foo ] && echo 'exists'

Good?
JP
----------------------------|:::======|-------------------------------
JP Vossen, CISSP            |:::======|      http://bashcookbook.com/
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