from
man bash
...
unset [-fv] [name ...]
For each name, remove the corresponding variable or
function. If no options are supplied, or the -v
option is given, each name refers to a shell vari
able. Read-only variables may not be unset. If -f
is specifed, each name refers to a shell function,
and the function definition is removed. Each unset
variable or function is removed from the environ
ment passed to subsequent commands. If any of RAN
DOM, SECONDS, LINENO, HISTCMD, FUNCNAME, GROUPS, or
DIRSTACK are unset, they lose their special proper
ties, even if they are subsequently reset. The
exit status is true unless a name does not exist or
is readonly.