Re: [tlug] Rsync exclusion question

[Josh Glover <jmglov@example.com>]

2009/6/12 John Fremlin <john@example.com>:

> Josh Glover wrote:
>
>> I think the Best Practise to always use non-interpolating quotes
>> unless you know you need interpolation is the way to go. That way,
>> you don't need to understand the interpolation rules of your
>> programming language all the way down to the metal.
>
> I don't know how you can include not understanding the language you are
> using in a Best Practice ;-)

I hope you know what I meant. Interpolation in the shell can quickly
devolve from "simple rules" to a recursively nastier situation, when
you have scripts calling programs calling scripts.

Of course one *should* understand the basics of interpolation in any
language one uses. But the Best Practice is about avoiding
interpolation altogether unless you *know* you want it in a string,
and then understanding the rules as they apply to that string.

I've seen old Unix hands tripped up by shell interpolation many times.

> (Both over- and under- quoting cause problems.)

This statement is significantly general enough to be true. :)

> I don't understand why the problem was allegedly fixed by changing
> between " and ' quotes; perhaps we haven't heard the full story.

Me neither, but I think it has to do with how Doug is invoking his
script. I'll walk over to his desk later as time allows and see if I
can understand what is happened.

>  From the Bash manpage
[...]
>>        The  special  parameters  *  and  @ have special meaning when in double
>>        quotes (see PARAMETERS below).

PARAMETERS
[...]
   Positional Parameters
       A positional parameter is a parameter denoted by one  or  more
digits,  other
       than  the single digit 0.  Positional parameters are assigned
from the shell's
       arguments when it is invoked, and may be reassigned using the
set builtin com‐
       mand.   Positional  parameters  may  not be assigned to with
assignment state‐
       ments.  The positional parameters are temporarily replaced when
a shell  func‐
       tion is executed (see FUNCTIONS below).

       When  a  positional  parameter  consisting  of  more  than  a
single digit is
       expanded, it must be enclosed in braces (see EXPANSION below).

   Special Parameters
       The shell treats several parameters specially.  These
parameters may  only  be
       referenced; assignment to them is not allowed.
       *      Expands  to  the  positional  parameters,  starting from
one.  When the
              expansion occurs within double quotes, it expands to a
single word with
              the value of each parameter separated by the first
character of the IFS
              special variable.  That is, "$*" is equivalent to
"$1c$2c...", where  c
              is  the  first  character  of the value of the IFS
variable.  If IFS is
              unset, the parameters are separated by spaces.  If  IFS
is  null,  the
              parameters are joined without intervening separators.


But what Doug was doing shouldn't have triggered any magic with *, AFAICT.

I'll leave it at, this: huh.

-- 
Cheers,
Josh