How to batch resize all images in a folder (including subfolders)?

Question

I have huge, 12GB, gallery on the server, full of images in various subfolders. Those files are too big and not used in full resolution. I need to resize all the images down to 820px wide (keeping proportions). So my question is - how can I create some kind of crawling script which would resize all images bigger then 820px and save them back overwriting original file?

Hope you can help me :-) Thank you in advance.

Answer 1

ImageMagick tools convert or mogrify will do the job.

You can get them via your package manager or the source/rpms here.

Basic usage (will overwrite the original file):

$ mogrify -resize 820x ./*.jpg

If you need recursion:

find . -name '*.jpg' -execdir mogrify -resize 820x {} +

Answer 2

The script to resize all .png files in a folder to 30%. The original file can either be removed (if delete_original is true), or stays, in that case the resized file would get a name postfix rename_postfix:

#!/bin/bash

rename_postfix="-small"
delete_original=true
substr=".png"
for str in *.png; do
    #reverse strings
    reverse_str=$(echo $str | rev)
    reverse_substr=$(echo $substr | rev)
    
    #find index of reversed substring in reversed string
    prefix=${reverse_str%%$reverse_substr*}
    reverse_index=${#prefix}
    
    #calculate last index
    index=$(( ${#str} - ${#substr} - $reverse_index ))
    
    # Extract the filename without extension
    filename="${str:0:index}"
    if $delete_original; then
        # the name can remain, because the original will be removed
        new_filename="${str}"
    else
        # need a new name, two files will remain
        new_filename="${filename}${rename_postfix}${substr}"
    fi

    echo "extension = ${substr}, original = ${str}, filename = ${filename}, new = ${new_filename}"
    
    # Resize and convert the file to .jpg
    convert -resize 30% "${str}" "${new_filename}"
done

I think you should be able to write a function with the above body that would accept a directory argument, etc (if you want to make it go into subfolders)

The convert function is from the imagemagick library:

sudo apt install imagemagick