What is bash declare built-in reporting here?

Question

Our RHEL8 servers don't have nc available, but at some time in the past someone using this shared account defined an nc function:

$ declare -f nc | head -1
nc ()
$

I am trying to determine where this function is defined:

$ (shopt -s extdebug; declare -F nc)
nc 0 environment
$

I was expecting a line number and a filename there, and as you can see I got 0 environment instead. Can somebody tell me what this means, please?

Answer 1

That tells us the nc function was defined by bash on startup when it found a BASH_func_nc%% variable in its environment:

$ env BASH_FUNC_nc%%='() { echo nc; }' bash -c 'shopt -s extdebug; declare -F nc'
nc 0 environment

That typically happens when bash was invoked by another bash instance (somewhere up its ancestry) that defined a nc function and exported it with export -f nc.

bash-5.2$ nc() { echo nc; }
bash-5.2$ export -f nc
bash-5.2$ shopt -s extdebug
bash-5.2$ declare -F nc
nc 1 main
bash-5.2$ printenv BASH_FUNC_nc%%
() {  echo nc
}
bash-5.2$ bash -c 'shopt -s extdebug; declare -F nc'
nc 0 environment