The convert program will convert a file from one image format to another:

    $ convert INPUT_FILE OUTPUT_FILE

Here's an example of this:

    $ convert file1.jpg file1.png

We can resize an image by specifying the scale percentage or the width and height of the output image. To resize an image by specifying WIDTH or HEIGHT, use this:

    $ convert imageOrig.png -resize WIDTHxHEIGHT imageResized.png

Here's an example of this:

    $ convert photo.png -resize 1024x768 wallpaper.png

If either WIDTH or HEIGHT is missing, then whatever is missing will be automatically calculated to preserve the image aspect ratio:

    $ convert image.png -resize WIDTHx image.png

Here's an example of this:

    $ convert image.png -resize 1024x image.png

To resize the image by specifying the percentage scale factor, use this:

    $ convert image.png -resize "50%" image.png

This script will perform a set of operations on all the images in a directory:

#!/bin/bash 
#Filename: image_help.sh 
#Description: A script for image management 

if [ $# -ne 4 -a $# -ne 6 -a $# -ne 8 ]; 
then 
  echo Incorrect number of arguments 
  exit 2 
fi 

while [ $# -ne 0 ]; 
do 

  case $1 in 
  -source) shift; source_dir=$1 ; shift ;; 
  -scale) shift; scale=$1 ; shift ;; 
  -percent) shift; percent=$1 ; shift ;; 
  -dest) shift ; dest_dir=$1 ; shift ;; 
  -ext) shift ; ext=$1 ; shift ;; 
  *) echo Wrong parameters; exit 2 ;; 
  esac; 

done 



for img in `echo $source_dir/*` ; 
do 
  source_file=$img 
  if [[ -n $ext ]]; 
  then 
    dest_file=${img%.*}.$ext 
  else 
    dest_file=$img 
  fi 

  if [[ -n $dest_dir ]]; 
  then 
    dest_file=${dest_file##*/} 
    dest_file="$dest_dir/$dest_file" 
  fi 

  if [[ -n $scale ]]; 
  then 
    PARAM="-resize $scale" 
  elif [[ -n $percent ]];   then 
    PARAM="-resize $percent%"  
  fi 

  echo Processing file : $source_file 
  convert $source_file $PARAM $dest_file 

done 

The following example scales the images in the sample_dir directory to 20%:

$ ./image_help.sh -source sample_dir -percent 20%
Processing file :sample/IMG_4455.JPG
Processing file :sample/IMG_4456.JPG
Processing file :sample/IMG_4457.JPG
Processing file :sample/IMG_4458.JPG

To scale images to a width of 1024, use this:

$ ./image_help.sh -source sample_dir -scale 1024x

To scale and convert files into a specified destination directory, use this:

# newdir is the new destination directory
$ ./image_help.sh -source sample -scale 50% -ext png -dest newdir