If Bash arrays can't contain a null string, how does this array do it?

Question

I've recently discovered that you can't pass a null string to a Bash array. This is tricky, as I'm trying to insert an empty string value into an array.

I've noticed that the Bash completion variable COMP_WORDS, however, can contain an empty value.

I've written a dummy completion script to illustrate this, with the debugging options on:

 1 #!/bin/bash 2 _comp() { 3 set -xv 4 for i in ${!COMP_WORDS[@]}; do 5 echo "<${COMP_WORDS[$i]}>" 6 done 7 set +xv 8 local cur=${COMP_WORDS[$COMP_CWORD]} 9 10 local arr=(apple banana mango pineapple) 11 COMPREPLY=() 12 COMPREPLY=($(compgen -W "${arr[*]}" -- $cur)) 13 14 } 15 16 complete -F _comp dummy 

If I source this file, and then enter the following:

dummy apple banana 

And then if I move the cursor back to the middle space between apple and banana (indicated below by an underscore) ...

dummy apple _ banana 

And then if I hit tab, I get the following output from the resulting debug, showing four array elements, 'dummy' 'apple' '' 'banana'

for i in '${!COMP_WORDS[@]}' + echo '<dummy>' <dummy> + for i in '${!COMP_WORDS[@]}' + echo '<apple>' <apple> + for i in '${!COMP_WORDS[@]}' + echo '<>' <> + for i in '${!COMP_WORDS[@]}' + echo '<banana>' <banana> 

So the COMP_WORDS array is clearly capable of storing a null string of some kind. Although interestingly, this does not happen if I use a regular for loop, accessing the array elements rather than the indices. I.e., if I change the for loop in the debug section to this ...

for i in ${COMP_WORDS[@]}; do echo "<$i>" done 

Then I get the following debug output showing only three array elements: 'dummy' 'apple' 'banana'

for i in '${COMP_WORDS[@]}' + echo '<dummy>' <dummy> + for i in '${COMP_WORDS[@]}' + echo '<apple>' <apple> + for i in '${COMP_WORDS[@]}' + echo '<banana>' <banana> 

And I've tried echoing the length of the array with ${#COMP_WORDS[@]} - for this input, it shows a length of four. So it clearly does contain four elements, one being a null string.

This seems like really strange behaviour to me. My question is twofold:

1. What is the explanation for the COMP_WORDS array being able to store null strings when normal arrays apparently cannot?

2. Why am I unable to see the null string when I use a regular for loop, but I can see it when I access the elements of the array by their indices?

Answer 1

You seem to be confusing the empty string "" with the NULL character \0. They are not the same. You can use empty strings in arrays and variables:

$ a=(aa "" "b") $ echo "${#a[@]}" 3 $ printf -- '- %s\n' "${a[@]}" - aa - - b $ for i in "${a[@]}"; do > printf '%s\n' "$i" > done aa b 

Some of the conditional expressions in Bash even allow testing whether a string is empty:

-z string

True if the length of string is zero. 

-n string string

True if the length of string is non-zero. 

See also When is double-quoting necessary?