mysteries with the placeholder variable _

[Léa Gris]

# GNU bash, version 5.0.17(1)-release (x86_64-pc-linux-gnu)
bash -c 'unset _;_=42;echo $_;unset _;: $((_=666));echo "$_"'

> 666

# ksh version sh (AT&T Research) 93u+ 2012-08-01
ksh -c 'unset _;_=42;echo $_;unset _;: $((_=666));echo "$_"'

> 42
> -3,02546243348e-123

# zsh 5.8 (x86_64-ubuntu-linux-gnu)
zsh -c 'unset _;_=42;echo $_;unset _;: $((_=666));echo "$_"'

> 666

# dash 0.5.10.2-7
zsh -c 'unset _;_=42;echo $_;unset _;: $((_=666));echo "$_"'

> 42
> 666


It raises multiple observations:

- I thought the placeholder variable _ was a sinkhole like /dev/null. It 
seems like it can get assigned values within arithmetic expressions in 
bash, dash and zsh

- The weird working of ksh if out of scope here, look like any number 
not power of 2 produces the strange output that is not even an integer.

- Dash seems to handle _ as a regular variable and accept assignment, 
which is probably conformant to POSIX shell specifications.

--
Léa Gris