{"id":14434,"date":"2026-07-20T02:00:00","date_gmt":"2026-07-20T06:00:00","guid":{"rendered":"https:\/\/www.both.org\/?p=14434"},"modified":"2026-07-13T17:09:08","modified_gmt":"2026-07-13T21:09:08","slug":"resize-images-at-the-command-line","status":"publish","type":"post","link":"http:\/\/www.both.org\/?p=14434","title":{"rendered":"Resize images at the command line"},"content":{"rendered":"
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I like to include screenshots in my articles. I find that a visual element helps the reader to understand what I’m describing. When showing how to do something in an app, I try to include full-size screenshots where I can. For example, in my article about how to format a book interior in LibreOffice<\/a>, I used lots of full-size screenshots.<\/p>\n\n\n\n

But sometimes, I need to adjust a screenshot. One common example for my writing is when I make a screenshot of a DOS application, such as one running on FreeDOS. My other article about using the FED editor on FreeDOS<\/a> included several screenshots of the editor, running in a QEMU virtual machine. This is where I run into a problem: QEMU always represents pixels as square. This works well for graphics like the screenshots of Windows 3.11 running on FreeDOS<\/a>, but doesn’t work well for plain text windows\u2014like the “console mode” screenshots in the same article.<\/p>\n\n\n\n

Classic PCs in the 1980s and 1990s used monitors with a 4:3 aspect ratio. Pixels were not square, so text mode did not use square pixels. Instead, the monitor would have displayed “console mode” text in the 4:3 aspect ratio. To make these screenshots look right<\/em> for the 1980s and 1990s, I prefer to resize “console mode” screenshots to use the correct aspect ratio.<\/p>\n\n\n\n

Resizing with ImageMagick<\/h2>\n\n\n\n

If I have a lot of screenshots to work with, I prefer to use a command line tool like ImageMagick<\/a> to resize my images. ImageMagick should be available for every Linux distribution; if it is not installed on your system by default, you can install it using your system’s package manager, like this for Fedora-based systems:<\/p>\n\n\n\n

$ sudo dnf install imagemagick<\/code><\/pre>\n\n\n\n
Using the command line is just faster when I might have a dozen or more screenshots. But for this example, I’ll use just two images: one “console mode” screenshot and one that is from graphics mode.<\/p>\n\n\n\n
\"editing
a “console mode” screenshot (screenshot1.png)<\/figcaption><\/figure>\n\n\n\n
\"several
a graphics mode screenshot (screenshot2.png)<\/figcaption><\/figure>\n\n\n\n
The identify<\/code> subcommand<\/h3>\n\n\n\n
Older versions of ImageMagick provided separate commands for each action, such as convert<\/code> or identify<\/code>, but newer versions provide just one command name (magick<\/code>) and the actions are usually subcommands<\/em>. Let’s start with the identify<\/code> subcommand to see the size of each screenshot.<\/p>\n\n\n\n
$ ls -1 *.png\nscreenshot1.png\nscreenshot2.png\n\n$ magick identify screenshot*.png\nscreenshot1.png PNG 720x400 720x400+0+0 8-bit sRGB 9497B 0.000u 0:00.000\nscreenshot2.png PNG 640x480 640x480+0+0 8-bit sRGB 8295B 0.000u 0:00.000<\/code><\/pre>\n\n\n\n
The “console mode” screenshot uses 720×400 size, and the graphics mode screenshot is 640×480 (a standard VGA resolution).<\/p>\n\n\n\n
The -resize<\/code> action<\/h3>\n\n\n\n
You might wonder about the 720×400 size. This is because VGA fonts used 9×16 glyphs<\/a>, so each character was 9 pixels wide and 16 pixels tall. DOS “console mode” screens were typically 80×25, so 80 columns and 25 lines. If you do the math, 9×80 = 720 and 16×25 = 400.<\/p>\n\n\n\n
But original VGA monitors only displayed video in 4:3 aspect ratio. That means I need to resize the “console mode” screenshot to display it in its original 4:3 aspect ratio. If I don’t want to lose the y<\/em> resolution (400) I only need to resize the x<\/em> dimension. Doing a little more math, that means the x<\/em> size needs to be 400×4\/3 = 533 pixels.<\/p>\n\n\n\n
I can use the -resize<\/code> action to change the dimensions of an image. However, by default this will only resize the image to fit within<\/em> a certain size. If I try to resize the screenshot1.png<\/em> image to fit in a 533×400 size, the image will actually be too small:<\/p>\n\n\n\n
$ magick screenshot1.png -resize '533x400' new.png\n\n$ magick identify new.png \nnew.png PNG 533x296 533x296+0+0 8-bit sRGB 59859B 0.000u 0:00.000<\/code><\/pre>\n\n\n\n
To force the new image to be exactly<\/em> a certain size, I need to append an exclamation point to the end of the resized dimension:<\/p>\n\n\n\n
$ magick screenshot1.png -resize '533x400!' new.png\n\n$ magick identify new.png\nnew.png PNG 533x400 533x400+0+0 8-bit sRGB 38046B 0.000u 0:00.000<\/code><\/pre>\n\n\n\n
Converting a set of images<\/h3>\n\n\n\n
If I only have a few images to convert to the new size, I might run that command separately for each image. In that case, using identify<\/code> to check the size and the -resize<\/code> action to change the size for a few “console mode” screenshots is not too much trouble. But when I make screenshots for an article, I usually capture more than a dozen screenshots, and sort through them later. Converting these one at a time is too much work for me; I’d rather automate that.<\/p>\n\n\n\n
To automate a set of images, I start with a feature of the identify<\/code> subcommand to display certain image characteristics using fields<\/em>. For example, %w<\/code> and %h<\/code> will show the width and height of an image. Use these fields in a -format<\/code> action to print just the dimensions:<\/p>\n\n\n\n
$ magick identify -format '%wX%h' new.png\n533X400<\/code><\/pre>\n\n\n\n
Or, if I wanted to automate that for a set of screenshots, I could use Bash shell expansion<\/em> with $( )<\/code> to save the output of the identify<\/code> subcommand in a variable, so I can use it later:<\/p>\n\n\n\n
$ for f in screenshot*.png; do dim=$(magick identify -format '%wX%h' $f); echo $dim $f; done\n720X400 screenshot1.png\n640X480 screenshot2.png<\/code><\/pre>\n\n\n\n
And that is how I automate resizing a collection of screenshots, but preserving the resolution for graphics mode screenshots while resizing any “console mode” screenshots to use 4:3 aspect ratio. To do this, I use Bash conditional evaluation<\/em> to compare a variable’s value (like dim<\/code>) and take action if the variable is (or is not) the 720X400<\/code> value. Again, I only have two screenshots in this example, but the process is the same for two images as it would be for a dozen or more:<\/p>\n\n\n\n
$ for f in screenshot*.png; do dim=$(magick identify -format '%wX%h' $f); [ $dim = '720X400' ] && magick $f -resize '533x400!' new$f; done<\/code><\/pre>\n\n\n\n
There’s a lot going on in this (very long) command line, but the only new part is the conditional evaluation:<\/p>\n\n\n\n
[ $dim = '720X400' ] && magick $f -resize '533x400!' new$f<\/code><\/pre>\n\n\n\n
This performs a test: is the dim<\/code> variable equal to the 720X400<\/code> value? If it is, then Bash executes the magick<\/code> command with the -resize<\/code> action to force the screenshot to the 4:3 aspect ratio. If the image is some other size, nothing happens.<\/p>\n\n\n\n
After running this for<\/code> command on my two screenshots, I actually end up with three images: the two original screenshots, plus one “console mode” screenshot that has been resized to 533×400 resolution:<\/p>\n\n\n\n
$ magick identify *screenshot*png\nnewscreenshot1.png PNG 533x400 533x400+0+0 8-bit sRGB 38046B 0.000u 0:00.000\nscreenshot1.png PNG 720x400 720x400+0+0 8-bit sRGB 9497B 0.000u 0:00.000\nscreenshot2.png PNG 640x480 640x480+0+0 8-bit sRGB 8295B 0.000u 0:00.000<\/code><\/pre>\n\n\n\n
\"editing
the original “console mode” screenshot (screenshot1.png)<\/figcaption><\/figure>\n\n\n\n
\"editing
the resized “console mode” screenshot (newscreenshot1.png)<\/figcaption><\/figure>\n\n\n\n
Command line for the win<\/h2>\n\n\n\n
I could have used an app like GIMP to resize the “console mode” screenshot; this doesn’t take very long, probably a few seconds. And that’s not a big difference for just one or two screenshots.<\/p>\n\n\n\n
But if you’re working with a dozen or more screenshots, “a few seconds” for each image quickly adds up to a lot of time. It’s much faster to use the command line for these kinds of “batch” operations.<\/p>\n","protected":false},"excerpt":{"rendered":"
Here’s how to selectively resize images from the command line<\/p>\n","protected":false},"author":33,"featured_media":4464,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_lmt_disableupdate":"","_lmt_disable":"","footnotes":"","_members_access_role":[],"_members_access_error":""},"categories":[100,5],"tags":[104,91],"class_list":["post-14434","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-command-line","category-linux","tag-command-line","tag-linux"],"modified_by":"Jim Hall","_links":{"self":[{"href":"http:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/14434","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/users\/33"}],"replies":[{"embeddable":true,"href":"http:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=14434"}],"version-history":[{"count":6,"href":"http:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/14434\/revisions"}],"predecessor-version":[{"id":14440,"href":"http:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/14434\/revisions\/14440"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/media\/4464"}],"wp:attachment":[{"href":"http:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=14434"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=14434"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=14434"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}