Addition in bash

[Ray Parrish]

David N. Lombard wrote:
> Ray Parrish wrote:
>   
>> David N. Lombard wrote:
>>     
>
>   
>>> Interval=${1:-1}
>>>   
>>>       
>> Could you please explain the above line, and the one below for Duration, 
>> as I do not see what that code is supposed to be doing.
>>     
>
> This uses bash's built-in capability to test for a null value or no 
> value and make an alternate assignment.
>
> So, Interval is given the value of $1 if and only if $1 is non-null; if 
> $1 is not set or is "blank" is sets Interval to 1.
>
> Thus Duration=${2:-300} sets Duration to the non-null value of $2 
> otherwise Duration is 300
>   
Thanks for the leg up on the testing for null, and assigning alternate 
values. Pretty handy to have it all in line. I've updated the script to 
use your method.
>   
>>>    [[ $Seconds -ge 60 ]] && (( Seconds -= 60 )) && (( Minutes += 1 ))
>>>   
>>>       
>> That last line did not work correctly, it decremented Seconds, but 
>> neglected to change the value of Minutes... got any idea why??? This 
>> error was with Interval set to 1 second, with Interval set to 7 seconds 
>> Minutes increments correctly.
>>     
>
> Oops, my bad.  Needed to reverse Minutes and Seconds.
> When Seconds *was* 60, the new value of 0 caused it failed the &&.
>
>    [[ $Seconds -ge 60 ]] && (( Minutes += 1 )) && (( Seconds -= 60 ))
>   
Thank you, my only guess had been that the -ge operator was somehow 
typoed. 8-)
> && is a short-circuit AND operator, as in C.  The exit code of the 
> command/operation before the  && is tested.  If true (exit==0), then the 
> command/operation following the && is executed; if not (exit!=0), the 
> command/operation following the && is ignored.
>
> || is a short-circuit OR operator.  If the command/operation before the 
> || is not true (exit!=0), the command/operation following the || is 
> executed; if true (exit==0), the command/operation following the || is 
> not executed.
>
> These are idiomatic examples used in preference of single-operation if 
> statements
>
>    [[ conditional_statement ]] && do_something_if_true
>
> and
>
>    [[ conditional_statement ]] || do_something_if_false
>
> This also works
>
>    [[ conditional_statement ]] && do_if_true || do_if_false
>
> But be careful you understand how that last one works!
>   
Thanks for all the help streamlining my code. I've added a web page to 
Ray's Links with the new timer/stop watch script available as open 
source software.

Later, Ray Parrish

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