convert

# instal convert tool
sudo apt-get install imagemagick

# change image format
convert howtogeek.png howtogeek.jpg
convert howtogeek.png -quality 95 howtogeek.jpg


# resize image
convert example.png -resize 200x100 example.png


# resize image with width
convert example.png -resize 200 example.png

# resize image with height
convert example.png -resize x100 example.png

# Rotating an Image
convert howtogeek.jpg -rotate 90 howtogeek-rotated.jpg


# batch process
for file in *.png; do convert $file -rotate 90 rotated-$file; done