# yum install ImageMagick

# cd /img/dir

for file in */*.jpg; do
 to_middle_file="${file%.*}_m.${file##*.}"; //文件名加后綴 如 hello_m.jpg
 to_small_file="${file%.*}_s.${file##*.}";
 convert ${img} -resize 300x180! ${to_middle_file};
 ls ${to_middle_file};
 convert ${img} -resize 100x60! ${to_small_file};
 ls ${to_small_file};
done

# convert resize argument
# 100x200       # width = 100, height = 200
# 300x          # width = 300, height = proportional
# x300          # width = proportional, height = 300
# ! 表示強制轉換大小,不加“!”會按照原圖片比例壓縮圖片