parameter expansion with `:` does not work

[lisa-asket]

>From: Lawrence Velázquez <vq@larryv.me>
>To: lisa-asket@perso.be
>Subject: Re: parameter expansion with `:` does not work
>Date: 08/07/2021 00:39:55 Europe/Paris
>Cc: bug-bash@gnu.org

>On Wed, Jul 7, 2021, at 6:37 PM, lisa-asket@perso.be wrote:
>> So you used `:` at the beginning and it worked?

>Chet and Greg already spelled out your mistake. It has nothing to
>do with the ':' command.



So the : command exists ?



I have some difficulty understanding the difference between ${parameter:-word} 
and ${parameter:=word}, and when it is appropriate to use one as opposed to the 
other.


${parameter:-word}
If parameter is unset or null, the expansion of word is substituted. Otherwise, 
the value of parameter is substituted. 
${parameter:=word}
If parameter is unset or null, the expansion of word is assigned to parameter. 
The value of parameter is then substituted. Positional parameters and special 
parameters may not be assigned to in this way. 

  

Currently I have



dir=${dir:-$PWD} 

fltype=${fltype:="texi,org"}