Tweak the DPI on an image file so that it specifies it to be a certain number of inches.

setdpi.sh

#!/bin/sh
#
# take an image and a desired width in inches, set the DPI
#

input=$1
width=$2

echo $input
echo $width

# get the current DPI, divide by width to get new DPI
density=`identify -format "%w" $input | awk "{print \\$0/$width}"`

echo $density

convert -units PixelsPerInch $input -density $density $input

Sample usage. Create standard rose png file from imagemagick:

$ convert rose: rose.png
$ identify -verbose rose.png | grep Print
Print size: 0.972222x0.638889
$ ../setdpi.sh rose.png 2
rose.png
2
35
$ identify -verbose rose.png | grep Print
Print size: 5.07983x3.33817
$ identify -verbose rose.png | grep Units
Units: PixelsPerCentimeter

5.07cm is 2 inches.

Now with jpg:

$ convert rose: rose.jpg
$ identify -verbose rose.jpg | grep Print
Print size: 0.972222x0.638889
$ ../setdpi.sh rose.jpg 2
rose.jpg
2
35
$ identify -verbose rose.jpg | grep Print
Print size: 2x1.31429
$ identify -verbose rose.jpg | grep Units
Units: PixelsPerInch

Jpegs units are pixels per inch so print size is 2x1.31424 inches.