[ale] Bash confusion

[Geoffrey Myers]

The Don Lachlan wrote:
> On Wed, Jul 20, 2011 at 03:29:42PM -0400, Al Snow wrote:
>> Try if [ "X01" = "X${date}" ] then if $date is empty you still have "X".
> 
> That is unnecessary and confusing.
> 
>   if [ "01" = "$date" ]
> 
> is simpler and more clear.

Not that I agree with the "X01" approach, but it is a standard practice 
throughout a lot of the scripting in Linux.

> 
> -L
> 
>>> Date: Wed, 20 Jul 2011 15:22:40 -0400
>>> From: ale-at-ale.org at unpopularminds.org
>>> To: ale at ale.org
>>> Subject: Re: [ale] Bash confusion
>>>
>>> On Wed, Jul 20, 2011 at 01:04:10PM -0400, Brian MacLeod wrote:
>>>> You are doing a string comparison here, so you should take the correct
>>>> precautions:
>>>>> if [ $date = "01" ];
>>>> should be like this:
>>>> if [ "01" = "$date" ];
>>>> Why?
>>>> If for some ungodly reason your variable ends up as null, the whole
>>>> statement (and function, etc) breaks if you have that null in front of
>>>> the equal sign.  Flip that around and it "properly" evaluates even if
>>>> null. Otherwise you'll see things like "unary operator" and such.
>>>> The other thing is making sure you treat your variable as a string,
>>>> thus, quote it so that you are sure it isn't being further evaluated
>>>> and/or evaluated differently.
>>> Order is irrelevent; quoting the string protects against null values.
>>>
>>>   if [ "$date" = "01" ]
>>>
>>> is safe, while
>>>
>>>   if [ "01" = $date ]
>>>
>>> fails if $date is a null string.
>>>
>>>
>>> Regarding the original problem, as written, yes, it should work. But...
>>>
>>> Are there any commands between the date command and the test? There's no
>>> explicit path used to date - could it be executing a different version of
>>> date which is evaluating improperly? IOW, what are you telling us? ;)
>>>
>>> -L
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>>  		 	   		  
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-- 
Until later, Geoffrey

"I predict future happiness for America if they can prevent
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