Batch resize images when one side is too large (linux) - bash

Question

I know that image resizing on the command line is something ImageMagick and similar could do unfortunately I do only have very basic bash scripting abilities so I wonder if this is even possible:
check all directories and subdirectories for all files that are an image
check width and height of the image
if any of both exceeds X amount of pixels resize it to X while keeping aspect ratio.
replace old file with new file (old file shall be removed/deleted)
Thank you for any input.

Answer 1

Implementation might be not so trivial even for advanced users. As a one-liner:
find \ # 1
~/Downloads \ # 2
-type f \ # 3
-exec file \{\} \; \ # 4
| awk -F: '{if ($2 ~/image/) print $1}' \ # 5
| while IFS= read -r file_path; do \ # 6
mogrify -resize 1024x1024\> "$file_path"; \ # 7
done # 8
Lines 1-4 are an invocation of the find command:
Specify a directory to scan.
Specify you need files only.
Per each found item run file command. Example outputs per file:
/Downloads/391A6 625.png: PNG image data, 1024 x 810, 8-bit/color RGB, interlaced
/Downloads/STRUCTURED NODES IN UML 2.0 ACTIVITES.pdf: PDF document, version 1.4
Note how file names are delimited from their info by : and info about PNG contains image word. This also will be true for other image formats.
Use awk to filter only those files which have image word in their info. This gives us image files only. Here, -F: specifies that the delimiter is :. This gives us the variable $1 to contain the original file name and $2 for the file info. We search image word in file info and print file name if it's present.
This one is a bit tricky. Lines 6-8 read the output of awk line by line and invoke the mogrify command to resize images. Here we do not use piping and xargs, as if file paths contain spaces or other characters which must be escaped,
we will get xargs unterminated quote errors and it's a pain to handle that.
Invoke the mogrify command of ImageMagic. Unlike convert, which is also ImageMagic's command, mogrify changes files in-place without creating new ones. Here, 1024x1024\> tells to resize image to have max size of 1024x1024. The \> part tells to preserve aspect ratio, so that the final image will have the biggest side of 1024px. Other side will be smaller than that, unless the original image is square. Pay attention to the ;, as it's needed inside loops.
Note, it's safe to run mogrify several times over the same file: if a file's size already corresponds to your target dimensions, it will not be resized again. However, it will change file's modification time, though.
Additionally, you may need not only to resize images, but to compress them as well. Please, refer to my gist to see how this can be done: https://gist.github.com/oblalex/79fa3f85f05924017d25004496493adb
If your goal is just to reduce big images in size, e.g. bigger than 300K, you may:
find /path/to/dir -type f -size +300k
and as before combine it with mogrify -strip -interlace Plane -format jpg -quality 85 -define jpeg:extent=300KB "$FILE_PATH"
In such case new jpg files will be created for non-jpg originals and originals will need to be removed. Refer to the gist to see how this can be done.

Answer 2

You can do that with a bash unix shell script looping over your directories. You must identify all the file formats you want such as jpg and png, etc. Then for each directory, loop over each file of the given list of formats. Then use ImageMagick to resize the files.
cd
dirlist="path2/directory1 path2/directory2 ...."
for dir in $dirlist; do
cd "$dir"
imglist=`ls | grep -i ".jpg\|.png"`
for img in $imglist; do
convert $img -resize "200x200>" $img
done
done
See https://www.imagemagick.org/script/command-line-processing.php#geometry