Passing multiple search directories to grep

[hancooper]

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Tuesday, August 3, 2021 1:51 PM, Andreas Kusalananda Kähäri 
<andreas.kahari@abc.se> wrote:

> On Tue, Aug 03, 2021 at 08:55:23AM -0400, Greg Wooledge wrote:
>
> > On Tue, Aug 03, 2021 at 12:10:21PM +0000, hancooper wrote:
> >
> > > On Tuesday, August 3, 2021 11:32 AM, Greg Wooledge greg@wooledge.org 
> > > wrote:
> > >
> > > > So, if this command works on your system:
> > > > grep -rl PATH /tmp /var/tmp
> > > > then the same command generated using an array expansion will also work.
> >
> > > Do you understand how grep distinguishes the search pattern from the 
> > > search directories ?
> >
> > Yes. The search pattern is the first argument string (after options
> > have been processed and removed). All of the argument strings after
> > the pattern are files or directories to be read. If there are no
> > arguments after the pattern, then standard input is read.
> > This is made clear by the man page:
> > SYNOPSIS
> > grep [-E|-F] [-c|-l|-q] [-insvx] -e pattern_list
> > [-e pattern_list]... [-f pattern_file]... [file...]
> >
> >        grep [-E|-F] [-c|-l|-q] [-insvx] [-e pattern_list]...
> >            -f pattern_file [-f pattern_file]... [file...]
> >
> >        grep [-E|-F] [-c|-l|-q] [-insvx] pattern_list [file...]
> >
> >
> > In the absence of -e and -f, you're using the last form, where the
> > pattern_list is a single argument, followed by zero or more files.
> > (The processing of directories with a -r option is a GNU extension.)
> >
> > > I would prefer that my script does not impose any restriction on the 
> > > search patterns allowed
> > > by grep, because currently "$ptrn" is just a user-defined string.
> >
> > In that case, use the -- indicator before the pattern.
> > grep -rl -- "$ptrn" "${dirlist[@]}"
> > This will ensure that grep doesn't treat the pattern as an option, even
> > if it happens to begin with a hyphen.
>
> Alternatively, and IMHO better in a "showing intention" sort of way,
>
> grep -r -l -e "$ptrn" -- "${dirlist[@]}"
>
> I.e., use "-e" to designate the pattern, and then "--" to delimit the
> file operands from the options. But depending on what the pathnames
> that are outputted from this are used for, I might suggest using "find"
> in combination with "grep -q" instead, without either of "-r" or "-l".

How can I determine if there are empty elements in an array?