Work the Shell - Resizing Images, Sort Of

What are shell script programmers to do when they finds themselves constantly scaling image dimensions for their blogs? Write a script to automate the process, of course!
Making It Work as a Script

Here's the basic script at this point:


width="$(file $filename | cut -f5 -d\  )"
height="$(file $filename | cut -f7 -d\  | sed 's/,//')"

width="$(echo "$width * $multiplier" | bc | cut -d. -f1)"
height="$(echo "$height * $multiplier" | bc | cut -d. -f1)"

echo "$filename scaled: width=$width height=$height"

Testing it with the filename specified produces the following:

$ sh
edit.png scaled: width=541 height=539

That's not really exactly what I want, however. First, I want to be able to specify the filename and multiplier on the command line. Second, the output needs a slight tweak to be more useful—the values need to be surrounded by quotation marks.

Here's what I'd like to see:

$ sh 0.75 edit.png
edit.png: width="541" height="539"

That's not too hard to accomplish given the basic script we already have. See if you can do it yourself.

Tip: I actually use a “for name; do; done” loop to step through the file scaling, so I can specify a group of images and calculate them all en masse. Try it, coupled with the shift command, to remove the multiplier value once it's saved into a named variable.

Dave Taylor is a 26-year veteran of UNIX, creator of The Elm Mail System, and most recently author of both the best-selling Wicked Cool Shell Scripts and Teach Yourself Unix in 24 Hours, among his 16 technical books. His main Web site is at, and he also offers up tech support at Follow him on Twitter if you'd like:


Dave Taylor has been hacking shell scripts for over thirty years. Really. He's the author of the popular "Wicked Cool Shell Scripts" and can be found on Twitter as @DaveTaylor and more generally at