Re: Testing Whether $@ Is Empty Or Null

[Greg Wooledge]

On Tue, Oct 11, 2022 at 10:16:21AM -0400, Chet Ramey wrote:
> On 10/11/22 1:00 AM, Jessie Saenz wrote:
> > bash version = 5.1.16(1)-release
> > 
> > I do not understand why the following statement returns 0 despite no
> > arguments given to script:
> > 
> >      [ -n "$@" ] && echo true
> 
> `['  is a builtin, so the arguments undergo word expansion before it's
> invoked.
> 
> "$@" expands to nothing when there are no positional parameters.
> 
> The command ends up being `[ -n ]', which is true because there is one
> non-null argument to `['.

And of course, the command will blow up with an error if there are a
bunch of arguments.

unicorn:~$ set -- one two three
unicorn:~$ [ -n "$@" ]
bash: [: too many arguments

Now, that's the technical answer.  I like to look at the underlying
reason for the question.

If you're trying to detect whether your script/function was called with
no arguments (maybe so you can print a usage message), you should be
using the $# special parameter instead.  This contains the number of
arguments.  Like so:

  if test "$#" = 0; then
    echo "I was called without any arguments."
  fi

Of course, you can use other test variants, like [ "$#" = 0 ], or
[[ $# == 0 ]], or (($# == 0)) if you prefer.

If what you actually want to know is whether your script/function was
called without any NON-EMPTY arguments (i.e. empty arguments don't count),
then you'll need a loop.  It's hard for me to imagine why you might
want such a thing, but it's the best guess I can make about your Subject:
header.

  hasarg=0
  for arg; do
    if test "$arg"; then
      hasarg=1
      break
    fi
  done

  if ((hasarg)); then
    echo "I was called with at least one non-empty argument."
  fi