the context of any active subroutine call (a shell function or a script)
executed with the . or source builtins.
caller displays the line number and source filename of the current subroutine
call. If a non-negative integer is supplied as expr, caller displays the line
number, subroutine name, and source file corresponding to that position in the
current execution call stack. This extra information may be used, for example,
to print a stack trace.
current frame is frame 0.
return value is 0 unless the shell is not executing a subroutine call or expr does
not correspond to a valid position in the call stack.
is a builtin command