Resize multiple images proportionally with FFmpeg and a shell function

December 26, 2021 ≈ 2 minutes 6 seconds

You probably already know that FFmpeg can resize images proportionally, and also that it can't overwrite them. So you either save resized files under new names or into a new folder, and it becomes tedious... When you repeat the same pattern multiple times over the week, it's time to search for a way to automate it.

So I wrote that little shell function for myself and I share it with the hope that it helps you too (you can include it into your .aliases file). I included comments for better understanding.

# resize_images -w400 -h800 -tpng
# resize_images -h800 -tjpg
function resize_images()
    width='-1';  height='-1'; type=''; # -1 if we scale proportionally in ffmpeg
    folder="tmp_$(date +%s)"; # tmp_ + timestamp
    files_count=0; resized_files_count=0; # count of files in folder and tmp_ folder

    # read width, height and type
    while getopts "w:h:t:" opt; do # w - width, h - height, t - type, : - expect value
        case ${opt} in
            w ) width=${OPTARG} ;;
            h ) height=${OPTARG} ;;
            t ) type=${OPTARG} ;;

    if [[ $type == '' ]]; then # type can't be empty
        echo "No image file type provided. Example: resize_images -w100 -h50 -tpng"; return;

    if [[ $width == '-1' && $height == '-1' ]]; then # either width or height should be provided
        echo "No width or height provided. Example: resize_images -w100 -tpng "; return;

    files_count=$(ls *.$type | wc -l | tr -d ' ') # count all files of type in current folder

    if [[ $files_count == 0 ]]; then # should be at least one file of type
        echo "No $type files in a folder"; return;

    mkdir $folder # create temporary folder: tmp_ + timestamp
    # resize files and save in temporary folder
    for i in *.$type ; do ffmpeg -i "$i" -vf scale="$width:$height" "$folder/$i" ; done    

    resized_files_count=$(ls $folder/*.$type | wc -l | tr -d ' ') # count files of type in temporary folder

    # if count of files in current folder different from count of files in temporary, display error
    if [[ $files_count != $resized_files_count ]]; then 
        echo "Error while resizing: files count mismatch"; return;

    # Promt confirmaiton
    echo "Do you want to rewrite original files with files from $folder (y/n)?"
    read -q choice # read answer in choice variable
    if [[ $choice != 'y' ]] return

    rm *.$type # Remove all files of type from current folder
    mv ./$folder/*.$type . # Move all files from temporary to the current folder
    rm -rf $folder # Remove temporary folder

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