Does `declare -p ` check if a variable (including an array variable) is set?

Question

Does declare -p check if a variable (including an array variable) is set?

Why does it behave differently for an array variable and a non-array variable?

For an array variable:

$ unset ar
$ declare -a ar=()
$ declare -p ar
declare -a ar='()'
$ echo ${ar[@]-This is a new value}
This is a new value

and

$ unset ar
$ declare -a ar
$ declare -p ar
bash: declare: ar: not found
$ echo ${ar[@]-This is a new value}
This is a new value

For a non-array variable:

$ unset var
$ declare -i var
$ declare -p var
bash: declare: var: not found
$ echo ${var-This is a new value}
This is a new value

Thanks.

---

I am running Bash 4.3.48.

From https://unix.stackexchange.com/a/381491/674, I heard that in 4.4, after running unset ar and declare -a ar, the output of running declare -p var is declare -a ar.

I would like to know whether declare -p in bash 4.4. handles array differently from other types. In 4.4, after running unset var and declare -i var , what is the output of running declare -p var. Does it output error such as bash: declare: var: not found or declare -i var?

Answer 1

All bash versions do handle arrays correctly if the array is given a list ().
Or an array element is given a value (even null).
This code (using typeset to get it to run in ksh):

unset ar; typeset -a ar=()
printf '%-20s%s ' "$(typeset -p ar)" '--'

unset ar; typeset -a ar; ar[3]=""
printf '%-30s%s ' "$(typeset -p ar)" '@@'

unset ar; typeset -a ar; ar[3]="val"
printf '%-31s ' "$(typeset -p ar)"

Will print this results:

zsh/sh          : typeset -a ar=(  )  -- typeset -a ar=( '' '' '' '' ) @@ typeset -a ar=( '' '' '' val )
b203sh          : declare -a ar='()'  -- declare -a ar='([3]="")'      @@ declare -a ar='([3]="val")'
b204sh          : declare -a ar='()'  -- declare -a ar='([3]="")'      @@ declare -a ar='([3]="val")'
b205sh          : declare -a ar='()'  -- declare -a ar='([3]="")'      @@ declare -a ar='([3]="val")'
b30sh           : declare -a ar='()'  -- declare -a ar='([3]="")'      @@ declare -a ar='([3]="val")'
b32sh           : declare -a ar='()'  -- declare -a ar='([3]="")'      @@ declare -a ar='([3]="val")'
b41sh           : declare -a ar='()'  -- declare -a ar='([3]="")'      @@ declare -a ar='([3]="val")'
b42sh           : declare -a ar='()'  -- declare -a ar='([3]="")'      @@ declare -a ar='([3]="val")'
b43sh           : declare -a ar='()'  -- declare -a ar='([3]="")'      @@ declare -a ar='([3]="val")'
b44sh           : declare -a ar=()    -- declare -a ar=([3]="")        @@ declare -a ar=([3]="val")
ksh93           : typeset -a ar       -- typeset -a ar=([3]='')        @@ typeset -a ar=([3]=val)
attsh           : typeset -a ar       -- typeset -a ar=([3]='')        @@ typeset -a ar=([3]=val)
zsh/ksh         : typeset -a ar=(  )  -- typeset -a ar=( '' '' '' '' ) @@ typeset -a ar=( '' '' '' val )
zsh             : typeset -a ar=(  )  -- typeset -a ar=( '' '' '' )    @@ typeset -a ar=( '' '' val )

As you can see, all shells (with some diferences) run the code and provide equivalent results.

---

Also, all shells work with this code:

unset ar; typeset -i ar              ; printf '%-17s%s ' "$(typeset -p ar)" '=='
unset ar; typeset -a ar              ; printf '%-19s%s ' "$(typeset -p ar)" '++'

And print:

zsh/sh          : typeset -i ar=0  == typeset -a ar=(  ) ++
b203sh          : declare -i ar="" == declare -a ar='()' ++
b204sh          : declare -i ar="" == declare -a ar='()' ++
b205sh          : declare -i ar="" == declare -a ar='()' ++
b30sh           : declare -i ar="" == declare -a ar='()' ++
b32sh           : declare -i ar="" == declare -a ar='()' ++
b44sh           : declare -i ar    == declare -a ar      ++
ksh93           : typeset -i ar    == typeset -a ar      ++
attsh           : typeset -i ar    == typeset -a ar      ++
zsh/ksh         : typeset -i ar=0  == typeset -a ar=(  ) ++
zsh             : typeset -i ar=0  == typeset -a ar=(  ) ++

The rough patch is in bash series 4. Both 4.1 and 4.2 work for the array:

b41sh           : declare -a ar='()' ++
b42sh           : declare -a ar='()' ++

But not for a simple variable. And bash 4.3 fails in both tests:

b43sh           : script: line 3: typeset: ar: not found ==
                  script: line 4: typeset: ar: not found ++

So, for this tests, please move out of Bash 4.3 --.