Selecting Every nth File from Folder and Duplicating them to New Folder

And here is the AppleScript solution that duplicates the results of the previously submitted Zsh script for the given nth file selection. The AppleScript will run slower than a Zsh solution on larger file collections. I have provided a link in the script back to the 2013 discussion, where this current AppleScript solution was derived, so credit to those who it is due.

-- nth.applescript
(*
	Select nth file of source folder and copy to destination folder
	Derived from reference: https://discussions.apple.com/thread/5553356
	Tested: macOS 10.14.6 (18G6032), macOS 10.15.7 (19H2)
	VikingOSX, 2020-10-22, Apple Support Communities, no warranties expressed/implied
*)

set m to (text returned of (display dialog "Enter your nth file skip value: " default answer "3")) as integer
set srcFold to (choose folder with prompt "Select your source folder:" default location (path to desktop))
set destFold to (choose folder with prompt "Select/Create your destination folder:" default location (path to desktop))

tell application "Finder"
	set nthList to the sort (files of srcFold) by name
	if not (length of nthList) > 0 then return
	
	repeat with n from (m + 1) to (count of nthList) by m
		-- log (item n of nthList) as text
		duplicate (item n of nthList) to folder destFold
	end repeat
end tell
return



As an example, I have a folder with fifty files in sequential naming order from file01.txt ... file50.txt. This script with an nth selection of 3 will result in selected files: file04.txt, file07.txt, file10.txt, ... file49.txt.

The ability to select nth files for processing on macOS requires a programming solution, as modulo file selection is not built into the Finder, and has to be poked at in UNIX shells to achieve it.

Here is a solution that does the following things from a double-clicked, Automator: Application on the Desktop:

1. Prompts for the Nth file integer
2. Prompts for the source directory
3. Prompts for the destination directory, and allows you to create, and select one.
4. Uses strengths of Zsh shell
5. Creates a working array of nth filenames selected from the source directory
6. Sorts the working array by filename, copying files in order to the destination directory
   1. If a file already exists in the destination directory, it will not be copied
7. Workflow ompletion message

Here is the Automator workflow that I have tested on Mojave 10.14.6 (18G6032), but due to the late hour, not on Catalina 10.15.7 (19H2). Right-click on this image and choose View from the secondary menu so you can zoom into it for legibility:

When you add the Automator Utility Library : Run Shell Script action, choose these settings, and remove any defauilt for-loop in the action's body, as it will be replaced with the following Zsh shell script code:

You will copy and paste the entire contents of the following script into the now blank Run Shell Script action:

!/bin/zsh
#
: <<'COMMENT'
Select nth files from the SRCDIR and copy them into the DSTDIR.
COMMENT

typeset -gi mod=0
typeset -ga ary=()

function usage () {
    printf '%s %s\n' "${ZSH_ARGZERO}" sourcefolder destfolder nth_value"
    printf '%s %s\n' "${ZSH_ARGZERO}" "~/Desktop/TestZ ~/Desktop/BAR 3"
    return
}

# three arguments or bust
((${#} == 3)) || ( usage;exit 1 )

# the nth value
mod=$3
# force relative path references to absolute paths
SRCDIR="${1:a:s/\~\//}"
DSTDIR="${2:a:s/\~\//}"

# do these folders exist?
[[ -d "${SRCDIR}" ]] || ( usage;exit 1 )
[[ -d "${DSTDIR}" ]] || ( mkdir "${DSTDIR}" )

# put every nth file into the array
ary=( $(printf '%s\n' "${SRCDIR}"/*.*(.Non) | awk -v m="${mod}" 'NR>1 && NR%m == 1') )

# sort array items in (o) ascending name order
for f in ${(o)ary[@]};
do
    # if file exists in destination folder, don't process it
    [[ -s "${DSTDIR}/${f:a:t}" ]] && continue
    # copy the file from the source folder to the destination folder
    printf '%s -> %s\n' "${f}" "${DSTDIR}/${f:a:t}"
    cp -p "${f}" "${DSTDIR}/${f:a:t}"
done
exit 0

Use code such as:

tell application "Finder"

set the_files to {}

repeat with this_file from 1 to (count files of window 1) by 3

set the_files to the_files & (file this_file of window 1 as string)

end repeat

set dest_folder to (make folder)

repeat with move_file in the_files

duplicate file move_file to dest_folder

end repeat

end tell

(184943)