sysrc -- a sysctl(8)-like utility for managing /etc/rc.conf et. al.

[Devin Teske]

On Oct 9, 2010, at 10:25 PM, Julian Elischer wrote:

> Ah grasshoppers...
> 
> /me wonders if anyone will get the full significance of that..
> 
> 
> On 10/9/10 3:39 PM, Devin Teske wrote:
>> On Oct 9, 2010, at 1:25 PM, Garrett Cooper wrote:
>> 
>> 
>>> Why not just do...
>>> 
>>> if [ "x$rc_conf_files" = x -o "x$varname" = x ]
>>> then
>>>    return ${FAILURE-1}
>>> fi
>> I think you'll find (quite pleasantly) that if you intonate the lines...
>> 
>> 	"rc_conf_files [is non-null] OR return failure"
>> 	"varname [is non-null] OR return failure"
>> 
>> Sounds a lot better/cleaner than the intonation of the suggested replacement:
>> 
>> 	"if x plus rc_conf_files expands to something that is not equal to x OR x plus the expansion of varname is not x then return failure"
>> 
> 
> 
> For what it matters, I'v enever found the [ "x$foo" = "x" ] construct to be useful.
> the quoting seems to work for everything I've ever worked on.

I agree... I think that the "x" syntax came around for when people were using non-quoted syntax... for example...

[ x$foo = x ]

is still very useful in that it prevents the error when $foo expands to "-e".

However, enclosing the argument (as the 'x$foo' portion is really just the first argument to the '[' built-in) in quotes:

[ "$foo" = x ]

makes it so that the expansion is taken as:

[ "-n" = x ]

rather than:

[ -n = x ]

The former not causing an error, while the latter does.

Quite functionally, at a C-level, if you have your array, ...

argv[0] = "[\0";
argv[1] = "\"-n\"\0"; /* quoted syntax */
argv[2] = "=\0";
argv[3] = "x\0";

and

argv[0] = "[\0";
argv[1] = "-n\0"; /* non-quoted syntax */
argv[2] = "=\0";
argv[3] = "x\0";

The C-code that switch'es on the argv element can tell the difference between a leading quote and a leading dash and does indeed do the right thing for all expansions of the variable within a double-quote block ("...").

Though, the non-quoted syntax should be avoided at all costs, imho unless you know for a fact that the expansion of the variable will never include a character from $IFS (defaults to space, tab, newline).

If it does, then it'll evaluate to a new positional argument for the C-code.

Take the following example:

$ foo="abc = abc"
$ [ $foo ] && echo true
true
$ foo="abc = 123"
$ [ $foo ] && echo true
# no output (not true)

Whereas the quoted syntax:

$ [ "$foo" ]

will always evaluate to true because there is an implied "-n" operator for the case where the first-and-last positional argument is a double-quoted string, and...

$ [ -n "$foo" ]

always evaluates to true in the above cases for foo ("abc = abc" then later "abc = 123").




> ...
> 
> Now One thing that should be bourne in mind (heh) is that
> as there is a 'usual' form of format for perl there is one for sh as well so it's not "polite"
> to make one's sh code look like perl.  :-)

Perusing sh(1) today... found examples of the lazy operators:

[ expr ] || expr
[ expr ] && expr

Search under "Short-Circuit List Operators"

I'd say that the description therein is a green-light to treat sh like perl ^_^


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--
Cheers,
Devin Teske

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