# reject invalid vars
# (set argv0 to "SHELL" to avoid "/path/to/shell: blah" in error messages)
"$THIS_SH" -c 'echo ${1q}' SHELL
"$THIS_SH" -c 'echo ${&}' SHELL
#"$THIS_SH" -c 'echo ${$}' SHELL -- this is valid as it's the same as $$