Re: Passing multiple search directories to grep

[Andreas Kusalananda Kähäri]

On Tue, Aug 03, 2021 at 05:31:58PM +0000, hancooper wrote:
> ‐‐‐‐‐‐‐ 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?
> 

You don't put them into the array to start with.

See also U&L user ilkkachu's comment here:
https://unix.stackexchange.com/questions/663140/grep-complaining-grep-no-such-file-or-directory#comment1247548_663140

... especially the bit where he points out that it's difficult to debug
code fragments.

-- 
Andreas (Kusalananda) Kähäri
SciLifeLab, NBIS, ICM
Uppsala University, Sweden

.