How to copy every 4th file in a folder

Question

I have a lot of files in a folder, named like 00802_Bla_Aquarium_XXXXX.jpg. Now I need to copy every 4th file to a subfolder, saying in selected/.

00802_Bla_Aquarium_00020.jpg <= this one
00802_Bla_Aquarium_00021.jpg
00802_Bla_Aquarium_00022.jpg
00802_Bla_Aquarium_00023.jpg
00802_Bla_Aquarium_00024.jpg <= this one
00802_Bla_Aquarium_00025.jpg
00802_Bla_Aquarium_00026.jpg
00802_Bla_Aquarium_00027.jpg
00802_Bla_Aquarium_00028.jpg <= this one
00802_Bla_Aquarium_00029.jpg

How do I do this?

Answer 1

With zsh, you could do:

n=0; cp 00802_Bla_Aquarium_?????.jpg(^e:'((n++%4))':) /some/place

POSIXly, same idea, just a bit more verbose:

# put the file list in the positional parameters ($1, $2...).
# the files are sorted in alphanumeric order by the shell globbing
set -- 00802_Bla_Aquarium_?????.jpg

n=0
# loop through the files, increasing a counter at each iteration.
for i do
  # every 4th iteration, append the current file to the end of the list
  [ "$(($n % 4))" -eq 0 ] && set -- "$@" "$i"

  # and pop the current file from the head of the list
  shift
  n=$(($n + 1))
done

# now "$@" contains the files that have been appended.
cp -- "$@" /some/place

Since those filenames don't contain any blank or wildcard characters, you could also do:

cp $(printf '%s\n' 00802_Bla_Aquarium_?????.jpg | awk 'NR%4 == 1') /some/place

Answer 2

Simply with bash, you can do:

n=0
for file in ./*.jpg; do
   test $n -eq 0 && cp "$file" selected/
   n=$((n+1))
   n=$((n%4))
done

The pattern ./*.jpg will be replaced by an alphabetically sorted list of file names as stated by the bash man, so it should fit your purpose.

Answer 3

In bash, a funny possibility, that will work rather well here:

cp 00802_Bla_Aquarium_*{00..99..4}.jpg selected

That's definitely the shortest and most efficient answer: no subshell, no loop, no pipe, no awkward external process; just one fork to cp (that you can't avoid anyway) and one bash brace expansion and glob (that you can get rid of altogether since you know how many files you have).

Answer 4

If you know you will not have newlines in your file names, you could use:

find . -maxdepth 1 -name "*.jpg" | sort | while IFS= read -r file; do
  cp "$file" selected/
  IFS= read -r; IFS= read -r; IFS= read -r
done

Answer 5

If you have ruby installed you can use the following one-liner. Note that it assumes that the directory selected exists.

ruby -rfileutils -e 'files = Dir.glob("*.jpg").each_slice(4) { |file| FileUtils.cp(file.first, "selected/" + file.first) }'

It takes a list of all files with the extension .jpg in the current directory and slices it into lists of four elements and copies the first element from each such list to the directory selected in the current directory.

Answer 6

Looking at stackoverflow.com/q/4553751/789593 which offers a solution to copy every second one as @N.N. mentioned, a very simple solution would be:

• Remove the first file from the starting folder
• Copy every second one from the starting folder to an intermediate folder
• Copy every seconde one from the intermediate folder into the final folder
• Add the first file manually

It may not be very efficient as it requires an extra copy step bit it should be very easy to do.