Episode 7 ImageMagick

We look at ImageMagick, a powerful suite of command-line tools for doing image processing. With it, we resize, crop, blur, and do format-conversion on a collection of image files.

8 September 2015

[Rhythmic, dark electronic intro music]


Hi, welcome back to Command Line TV. This is episode 7 and today we’re

going to look at a pretty cool suite of tools for doing image manipulation. It’s called ImageMagick.

But first, do we have any follow-up from last time?


I did have some questions. One of them was the – you had something that

showed up in your terminal called reverse-i-search.

I was curious, what does that do and how can we actually use that?


Yeah, so I was doing some fancy stuff there. It has to do with the history

mechanism that bash supports. History is all about getting back to your previous commands.

And we’ve talked about using the up and down arrow to navigate through your previous commands.

But that reverse search is really interesting. What I’m doing there is I hit ‘control-R’ –

so as soon as I hit ‘control-R’, my little prompt changes to say reverse-i-search.

The i there is for ‘incremental’. So what it means now is,

as I start to type it will match up what I’m typing with previous commands from my history.

So maybe if I want to look at a previous cd command that I typed –

I just hit cd and you can see that what I typed – the cd – appears between these weird quotes.

But the command where my cursor is came from the history, and it’s cd Downloads/.

Now if I’ve got another cd command that I’m looking for,

that might be further back in the history, I just hit ‘control-R’ again,

and it will go back one but still matching the cd.

So here’s ‘control-R’ again – there was just a cd command by itself.

‘control-R’ again – Downloads, pics, and so on.

So I can kind of work my way through the history but matching some particular piece of text.

I’ll try it again with – let’s say, a cut command.

So I know in previous episodes we did some of that survey analysis using cut and sort.

So if I look at cut you can see some of those commands that we did in

those previous episodes still come up here. So it’s not even just in the current shell session.

When your shell closes, it saves that history into a file and then reloads

that when I reopen the shell. And I can always go back and find those commands.

If I want to execute a command I found this way, I can just press enter.

If I want to edit it, because maybe I need to change something from the last time I did it,

I can use the arrow keys and come out here and do my normal command line editing,

and then hit enter. Or if you don’t want to run it at all,

you can just hit ‘control-C’ and it’ll come back to your regular prompt.

So that’s the reverse search – it’s pretty useful.


Another thing I saw you do was you cleared the screen without typing clear into the terminal.

What command did you do for that?


That’s just another control command, so ‘control-L’ will get me a clear screen like that.

It’s exactly the same as typing the command clear.

So when things start to get a little messy I might just hit ‘control-L’ and

it brings it back up to a clear screen at the top of my terminal window.


That doesn’t reset it though, does it?


No it’s not the same as reset. So reset, which we learned previously does a lot of other things.

It also clears the screen but it resets a lot of other settings and ‘control-L’ doesn’t do all that.


My last follow-up from the previous episode would be in regards to the way you save the dates.

Is there a specific format you should save things in?


Yeah, so I saved some log files and I used a date in the filename.

So what I was doing there was – like a log file that has as part of its

filename year-month-day in a format like this. The OS doesn’t really care about that –

it doesn’t notice that you named it that way, but what that does is it puts

them in chronological order when you sort things alphabetically.

So the normal ls output, when I have files which have year-month-day and

they use leading zeroes and so forth like this – it has the effect of

putting them also in chronological order, because the years will come first

and then it’ll look at the month and then the day. So that’s why that

format is very popular among programmers and computer users.

It’s just a useful technique when you name something with the date to put

it in this year-month-day format.


So assuming we already have ImageMagick installed, the first command we

could use would be display, correct?


Yeah, ImageMagick comes with a few different commands. One of the simplest is just called display.

So if I type display all by itself, it opens up this little window with

the ImageMagick logo which is this cute wizard. So I can hit escape or –

you probably have a little close window icon or something to close that.

But what this can do is it shows pictures, so I have a couple of pictures

here that I took at the Computer History Museum (I think it’s called) in Silicon Valley.

And if I want to display these I can say display and then a filename,

or I think I can just put *.jpg or something like that.

display *.jpg

It displays other formats, jpg, png, tiff – it supports lots of different image formats.

And then that will open these up – there’s some way to zoom around in them

if it doesn’t quite fit on the screen. There’s some way to move on to the next image,

which – I just hit the space bar to move to the next one.

So this is a simple way to go through and just display some images.

It has a few other features but display is not the most sophisticated

image viewer that’s available. So I actually don’t use it that much.

But it is part of the ImageMagick suite. So we’ll close that.

The next command that’s part of ImageMagick is called identify.

So identify is basically a way of listing the properties of an image file.

So if I try to identify IMAG0* like this – it will take all of those

identify IMAG0*

filenames that match the wildcard and just peer into them,

and tell me things like the resolution and the color model and stuff like that.

So basically it’s doing one-line for each image. It has the filename,

and then a sequence number – which is basically just the ordering of the images it looked at –

the format, the resolution. This is called the ‘geometry’ string.

It’s basically – some image formats allow you to have a certain portion of

the image that is selected, like a crop of the image.

And what this format means is that the crop will be this width and height,

and then the +0 is the X offset, and the next +0 is the Y offset.

So that format allows you to specify a rectangle anywhere within an image.

For all of these, the rectangle is just the full resolution and +0+0 but

there are other image formats where that could be different.

We’re also going to use that when we learn to crop images with ImageMagick.

It’s that same format – if you want to crop an image,

you specify the new width and height, and then the X and Y offset.

We’re using 8-bit color and an RGB color model. Here is the size of the file in human terms –

and whatever all those zeroes are supposed to be.

So identify is a quick way to figure out resolution and other information on images.

It does have a -verbose option. And one thing that’s a little different

about ImageMagick commands – compared to some of the file management commands that we’ve learned –

is that it doesn’t really use the short forms of these options.

So on a lot of GNU commands, you might say -v or --verbose, right?

Or -h or --help – these are equivalent to each other.

ImageMagick doesn’t really support the short forms and so it uses a full

word as its option but only a single dash. It doesn’t use the double dash.

So if I identify -verbose and I put just one of these names –

because that will be plenty of info – so IMAG0212.jpg with the verbose –

identify -verbose IMAG0212.jpg

this is going to dump out a whole ton of information about this image.

And you can see there’s all sort – let’s pipe that through less so that

identify -verbose IMAG0212.jpg | less

we can just get a page at a time. It’s showing me the image format,

that geometry string, resolution means how many dots per inch does this

image expect to be displayed at – or pixels per inch. Color model, stuff like that.

For each of the red, green, and blue channels it gives me like the average colors and such.

There are all kinds of potentially useful information here.

And you can see all of that with the verbose flag.


So a lot of programs and applications nowadays sometimes tend to be very file type specific.

What if you had an image file that you wanted to convert from a jpg to a

different file type or vice versa?


Yeah, one of the real big uses of ImageMagick is just this format conversion.

And it does that very easily. One of the commands for that is called mogrify,

which – as far as I know is a word they just made up. But it means to process some image.

And so what you do is you basically say mogrify and then some flags or

options to specify how you want to process it. So one thing we can do is just change the format.

So I’ve got JPEGs mostly in this folder – what if I want to convert one of them to a PNG format,

right? And then let’s take IMAG0213.jpg. So my command is mogrify -format and then the format –

mogrify -format png IMAG0213.jpg

and, as I said, ImageMagick supports lots of formats – but we’ll use png.

And here’s the original filename. So I do that – it can take a moment if your image file is big,

to do this conversion. Then it comes back – it didn’t say anything,

but I want to see what file it created. So if I do ls,

you see right here it created a png file with the same name as the original jpg.

So that is what mogrify -format can do for us.

Let’s take a look at the identify output of both of those.

So if I do IMAG0213*, I see that both of them have the same resolution

identify IMAG0213*

and the PNG file got a lot bigger, right – so 7 MB instead of 1 MB.

So that’s one thing that happens when I do that conversion.

Try a different format – let’s just convert that same image to a tiff.

mogrify -format tiff IMAG0213.png

That went pretty fast, and I’ve got this tiff file –

so now if I do identify on those again, the TIFF is the same resolution but it became even bigger.

So what we’re seeing here is the difference between compression schemes in these different formats.

JPEG is ‘lossy’ compression, so it can throw out some information from the

original sensors that took the image, and it becomes very small.

PNG is compressed but in a lossless way, and of course since I started with a JPEG,

I can’t get back the information that was lost before I did the JPEG from the camera sensors.

But the PNG will preserve what information is there – it becomes bigger –

and the TIFF becomes bigger yet because it’s, generally speaking, not compressed at all.

Another thing that’s cool about mogrify is that I can just apply it in batch to tons of files.

So I’ve got a whole directory of JPEGs here. And if I want to do that –

I’m doing reverse-i-search with ‘control-R’ – so I’m going to get back my png mogrify.

Let’s say I want to apply that to everything that’s a .jpg, right?

mogrify -format png *.jpg

So what I’m going to end up doing is converting all of these files to png.

And that can take a little while –

Alright, now that that’s finished (through the magic of video editing),

we have a directory that has the original JPEGs as well as the PNGs that were just created.

So I’ve been able to just – in a batch sense, convert a bunch of files

without having to individually open them and save as – that kind of thing.

So I think that’s one of the really great use cases of ImageMagick.


So if we have an image file, for example we see here IMAG0223.jpg

it’s too large to fit onto the screen, what can we do to convert that to a smaller size?

display IMAG0223.jpg

Yeah, so display is showing it to us a piece at a time,

and it doesn’t really fit on the resolution that I’ve got here.

So if I want to shrink it – what you first want to figure it is how much should I shrink it by,

or what’s my target resolution. So I’m going to close that and of course we

identify !$

can run identify to figure out – oh, I just did something a little fancy!

We’ll do more on this later, I just did it automatically [laughs] but uh,

this exclamation and dollar sign !$ – it looks like I’m cursing at the terminal –

but what this means is take the previous command, but only the last word of the previous command.

So whenever I want to operate on the same file, but run a different command –

that’s something that I just automatically do. And I call it “bang dollar”.

So previous command, but only the last word, and you see that it expanded just like that.

At any rate, here is the resolution that I’m starting with.

And let’s say I want to shrink it to like 50% of that.

So that way, if each number is taken by 50% I know I’ll keep the same aspect ratio of the image.

So a good way to do that is the convert command. And you give the input filename first –

so IMAG0223.jpg – and then you can give a series of image processing options to it.

The one that will resize an image is called -geometry.

And the simplest type of -geometry is just to put a percentage, so 50% means shrink it in half.

And then you put an output filename. So we’re going to call this like 223-sm.jpg.

convert IMAG0223.jpg -geometry 50% 223-sm.jpg

And that will open that image, shrink it, and then save the new image.

So I can compare them by putting both of them on the identify command line,

identify IMAG0223.jpg !$

and see – I’m going to use that (!$) again. So the original image is

1950 by 2289 and the new image is 975 by 1145. So that’s halfway.

And I can open it with display and now it fits on my screen pretty well

display 223-sm.jpg

although it’s duplicating for some reason. Yeah, the display command is kind of weird sometimes.

But that’s shrinking.


So just to backtrack, all these commands do support the help option,

and again it’s the -help not -h.


Yeah so if I do convert -help – not double-dash help, or -h, exactly –

but this will give me – let’s do that with less.

convert -help | less

This will give me a very brief synopsis of some of the things that convert can do.

It’s got all sorts of other image processing operations that you can access here.

So why don’t we try a couple of those.

One that I like to do is – sometimes you want to add a border to an image.

And this is a pretty easy thing to do. So let’s take my smaller – well,

let’s combine a shrink with a border. So here’s how we’ll do that.

I’m going to pick one of these and do convert – let’s say IMAG0209.jpg.

And I’m going to do -geometry first to shrink it –

we’re going to shrink it pretty small, so let’s say 33%. And then I can specify -bordercolor.

There are a lot of built-in colors like the standard red, green, yellow, whatever.

So I’ll say red – or you can specify colors more precisely using those

HTML-like hexadecimal specifications – we can look at that another time.

And then you specify the thickness of the border in pixels,

so let’s make it very visible at 8 pixels. So I’m stringing all of this together –

convert IMAG0209.jpg -geometry 33% -bordercolor red -border 8 209border.jpg

putting these operations together will apply them in that order –

and then finally I put the output name. So we’re going to call this 209border.jpg.

And that should work. So I’m going to open that image with –

instead of display I’m going to use this other thing we learned before, called xdg-open.

xdg-open 209border.jpg

And you can see the red border that appears around this image.

So it added that border, it also shrunk it, so it’s not nearly as big as the original was.

Another thing we might like to do is blur or apply other sorts of image filters –

just like you could in Photoshop or something like that.

So there’s a -blur command, and the way blurring works is you give it the geometry –

in other words, the shape that you’re going to apply the blur operator to.

And if it’s a very small rectangle, it just blurs a little bit.

If it’s a big rectangle then it blurs much more. So we’re going to try doing some of that.

Let’s try that on IMAG0205.jpg. We’re going to do -blur – well,

let me again shrink, because it’s easier to deal with if these are a little smaller.

And then we’ll -blur using like a 10x10 pixel filter.

And I’m going to save that as 205blur10.jpg. And let’s open both the

convert IMAG0205.jpg -geometry 33% -blur 10x10 205blur10.jpg

original and the blurred version, then we can see the difference.

So I’m going to actually use my image viewer called eog

this is just a Gnome image viewer, but I could open these separately with

display or xdg-open or something. So here is the – no,

eog IMAG0205.jpg 205blur10.jpg

that’s got to be the blurred image, right. Yeah that’s the blurred image.

There’s the original image. So it – this was 1742 pixels across,

and now it’s 575, so that’s about a third. And it also blurred it pretty substantially.

So you can’t really even tell that this is text on here – it just looks like dirt.

So that’s just one of the many other operations that convert supports.


So I guess the next two things we should discuss are cropping images and

then opening an image in Gimp.


Yeah, so cropping is a little tricky because you’ve got to come up with these geometry strings.

The basic format of the command with convert is –

let’s say I want to take IMAG0207.jpg and I want to crop that –

first of all, I’m going to do identify to figure out the approximate resolution I’m working with.

identify IMAG0207.jpg

And then I can do convert with -crop – let’s say I want to cut a

square image out of the middle of that somewhere. So the first thing you

put is the dimensions that you’re cropping at – 1200x1200 and then I

want to put + something for the X position, so it’s just guesswork really +500+100.

And I could try that – let’s call that 207crop.jpg.

convert IMAG0207.jpg -crop 1200x1200+500+100 207crop.jpg

So I can open both of these files to see what that geometry string produced.

Here is the original version. You can see that it cut out a square from

somewhere around this region. But what if we needed to get it more precise?

What I would recommend in that case is to open a program where you can

actually see the pixel values and move your mouse around.

So that’s not a command line program, that’s going to be a graphical program.

The gimp is basically a free version of Photoshop. It’s available for all Unix systems.

It also might not be installed so you can try using your package management

commands to get that installed. And then we’re going to run that on the original image –

217 I think it was.




207, thank you – .jpg. So it opens that up and this is shrunk a little bit,

gimp IMAG0207.jpg

it says 25% down here at the bottom. But what this can do for me –

well I can just crop it right here, that’s fine. But the advantage of ImageMagick is batch,

right – so if I want to run the same crop on many images I’m going to have

an easier time of that using ImageMagick than Gimp.

But I can get the numbers to make my geometry string from Gimp.

So I’m going to do the rectangle selector and depending on where I want to be,

I look in this lower left corner here. You can’t see the numbers while my

mouse is down there [laughs] but that’s my current position, so that’s the X and Y offset.

So I remember or write down 288 and 360. And then I’m going to get the –

let’s say I want to crop out this piece here. So that’s about –

I keep moving my finger a little bit. But 2636 by 1044.

So I can come back to my – let’s just discard that.

Back to my convert command, and substitute that 2636x ten-something [laughs].




44! Do you remember the original numbers?


No [laughs].


I’m going to make it up, let’s say +300+200. Alright, that’s something close.

convert IMAG0207.jpg -crop 2636x1044+300+200 207crop.jpg

And then I do that convert and we’ll do the comparison again between them.

eog IMAG0207.jpg 207crop.jpg

Now I’ve got a more rectangular, instead of square – and I’ve cut out a bit of this console here.

I didn’t go low enough so my Y-value should have been another 100 pixels lower.

But that’s how we can figure out these geometry strings.

Now I said the strength in ImageMagick is applying that in batch to many different files.

That’s also a little tricky if you always have to specify the from-image

(the original image) as well as the destination image.

So what’s cool about ImageMagick is this mogrify command versus convert.

So I used mogrify with -format to convert an image like this, right?

I convert jpg to png – but I only have to specify the image filename once,

instead of having a source and a destination. But I can apply lots of other

image processing stuff with mogrify. So I can do crop.

If I do -crop from here – I only specify the image file once.

mogrify -crop 1636x1044+200+300 IMAG0207.jpg

What it’s going to do is overwrite that image with the cropped version.

So it’s a destructive operation, mogrify. Unless you’re only doing a format –

if you do -format it gets a new extension, so it doesn’t destroy the original image.

But without the -format it’s going to destroy the original.

So maybe I want to make a copy of the whole folder so I still have my

originals and then I can batch-apply some processing to them in a different folder.

Or, in this case I already made a copy of each image by creating a .png.

So we can apply the -crop to the .png files. So I’m going to do

mogrify -crop let’s say 1636x1044 plus whatever. And I’m going to apply that to *.png.

mogrify -crop 1636x1044+300+200 *.png

And that will crop all of these images exactly the same way. And it can take a little while.

Alright, so now that it finished, I can take a look at these files.

All of these PNGs should have been modified. So if I do my ls -ltr for example,

you see that the .png appear at the end because those are the ones that

were just modified by this. And I’m going to run this –

eog IMAG0*

to view all of my images – so here’s my original JPEG and right next to it is the cropped PNG.

Here’s the original, and then the cropped version. So all of these are

being cropped to the same dimensions and the same position within the image.

So I was able to do very simple batch operations like that.

Yeah, so that’s mogrify vs. convert, and some of the useful things you

can do with these ImageMagick tools.


Thanks for watching today’s episode on image manipulation.

Now ImageMagick is probably not native on your Linux system,

so in the next episode we will learn about package management and how to

use the command line to search for and install programs. See you then.

[Dark electronic beat]

[Captions by Christopher League]