Re: Expansions based on variable existence (e.g. ${var+exists}) yield inconsistent results.

[Chet Ramey]

On 4/20/20 11:55 AM, andrej--- via Bug reports for the GNU Bourne Again
SHell wrote:

> Bash Version: 5.0
> Patch Level: 16
> Release Status: release
> 
> Description:
>   Variable existence testing doesn't match variables that are (only) declared.
>   Unless the declared variables are redeclared as arrays. Other variable
>   existence tests yield surprising results as well. (This is all about 
> existence
>   tests, i.e., ones without the extra ':' that tests for emptiness.)
> 
>   In particular, Example 4 below exposes a situation in which a variable 
> doesn't
>   seem to be set, yet cannot be set. Example 5 below shows that empty arrays
>   are treated differently in terms of variable existence. Also, redeclaration 
> of
>   a variable as an array seems to have an "assignment effect" on it (in the
>   sense of making it detectable by existence tests). Redeclaration as an 
> integer
>   doesn't have this effect.
> 
> Repeat-By:
>   Example 1:
>   declare var                   # The same problem occurs with -i, -a and -A.
>   echo "${!var@}"               # empty [unexpected]
>   echo "${var+exists}"          # empty [unexpected]
>   echo "${var-nonexistent}"     # nonexistent [unexpected]

var is unset; you have not assigned a value.

> 
>   Example 2:
>   declare var
>   declare -A var

This is a bug; var should remain unset since it has no value. It happens as
part of the process converting the variable to an associative array.

>   echo "${!var@}"               # var [expected] << Difference from 
> Experiment 1
>   echo "${var+exists}"          # empty [unexpected]
>   echo "${var[@]+exists}"       # empty [somewhat unexpected]
>   echo "${var-nonexistent}"     # nonexistent [unexpected]
>   echo "${var[@]-nonexistent}"  # nonexistent [somewhat unexpected]


> 
>   Example 3:
>   declare var=
>   echo "${!var@}"               # var [expected]
>   echo "${var+exists}"          # exists [expected]
>   echo "${var-nonexistent}"     # empty [expected]

var is set with an empty value.

> 
>   Example 4:
>   declare -r var
>   echo "${!var@}"               # empty [unexpected]
>   echo "${var+exists}"          # empty [unexpected]
>   echo "${var-nonexistent}"     # nonexistent [unexpected]
>   var=                          # Error! [inconsistent]

var is unset until you attempt to assign it a value. It exists only as an
unset variable with attributes.

> 
>   Example 5:
>   var=()
>   echo "${!var@}"               # var [expected]
>   echo "${var+exists}"          # empty [unexpected]
>   echo "${var[@]+exists}"       # empty [somewhat unexpected]

An empty array is set (or, more precisely, it is not unset). It's like
assigning a null string to a variable.

>   echo "${var-nonexistent}"     # nonexistent [unexpected]

This is the same as var[0].

>   echo "${var[@]-nonexistent}"  # nonexistent [somewhat unexpected]

There aren't any values there. I suppose it could expand to the empty
string, but it doesn't.


-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/