shell command to get pixel size of an image

It's been added to act:

$ npm install @lancejpollard/act -g
$ act read image.png --size

    2000x2000

For those like me who want the size in megapixels :

perl -le "printf "=> fileName = %s size = %.2f Mpixn", "$fileName", $(identify -format '%w*%h/10**6' $fileName)"

This was a helpful snippet (I didn't write) that does returns dimensions for every png and jpg in the folder:

file ./* | perl -ne '@m = /^.*.jpg|^.*.png|[0-9][0-9]*[ ]?x[ ]?[0-9][0-9]*/g; print "@mn"'

Rather than parsing the output of identify by eye, or by text utilities, you can use its -format option to output the width and height in whatever format suits you best. For example:

$ identify -format '%w %h' img.png
100 200
$ identify -format '%wx%h' img.png
100x200

A list of image properties that you can output can be found on this page, but for the question here, it seems all you need are %w and %h, which give the image's width and height, respectively, in pixels.

---

The flexibility afforded by -format came in handy for me in finding the largest images in terms of pixels, by outputting %[fx:w*h] for a number of images and sorting the output.

You might want to specify the -ping option if you're processing many images, using more complicated escapes, and want to make sure the program doesn't waste time loading the entire images. With simple escapes, -ping should be the default. More information on the choice between -ping and +ping can be found here.

You can also try GraphicsMagick, which is a well maintained fork of ImageMagick used at e.g. Flickr and Etsy:

$ gm identify a.jpg
a.jpg JPEG 480x309+0+0 DirectClass 8-bit 25.2K 0.000u 0:01

It is faster than ImageMagick's identify (in my tests about twice).

Use identify to see the sizes :

$ identify color.jpg 
> color.jpg JPEG 1980x650 1980x650+0+0 8-bit DirectClass 231KB 0.000u 0:00.000

Extract value via cut | sed, from field 3:

identify ./color.jpg | cut -f 3 -d " " | sed s/x.*// #width
identify ./color.jpg | cut -f 3 -d " " | sed s/.*x// #height

Asign to variable:

W=`identify ./color.jpg | cut -f 3 -d " " | sed s/x.*//` #width
H=`identify ./color.jpg | cut -f 3 -d " " | sed s/.*x//` #height
echo $W
> 1980
echo $H
> 650

Both display and file are quite slow, and have the potential to bring even quite capable systems to their knees dealing with many multiple files. A small test:

     $ du -h *.png --total | tail -n 1
     9.2M    total

     $ ls -l *.png | wc -l
     107

     $ /usr/bin/time file *.png
-->  0.37user 0.26system 0:06.93elapsed 9%CPU (0avgtext+0avgdata 37232maxresident)k
     22624inputs+0outputs (9major+2883minor)pagefaults 0swaps

     $ /usr/bin/time identify *.png
-->  0.56user 0.22system 0:06.77elapsed 11%CPU (0avgtext+0avgdata 25648maxresident)k
     34256inputs+0outputs (119major+2115minor)pagefaults 0swaps

By reading only the bytes necessary, this operation can be significantly sped up.

     $ /usr/bin/time ./pngsize *.png
-->  0.00user 0.00system 0:00.03elapsed 12%CPU (0avgtext+0avgdata 1904maxresident)k
     0inputs+0outputs (0major+160minor)pagefaults 0swaps

Here is pngsize:

#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <err.h>
#define oops(syscall) { printf("error processing %s: ", argv[i]); 
        fflush(0); perror(syscall"()"); continue; }
int main(int argc, char **argv) {
    int fd, i;
    uint32_t h, w;
    if (argc < 2) { printf("%s <pngfile> [pngfile ...]n", argv[0]); exit(0); }
    for (i = 1; i < argc; i++) {
        if (argc > 2) printf("%s: ", argv[i]);
        if ((fd = open(argv[i], O_RDONLY)) == -1) oops("open");
        if (lseek(fd, 16, SEEK_SET) == -1) oops("lseek");
        if (read(fd, &w, 4) < 1) oops("read");
        if (read(fd, &h, 4) < 1) oops("read");
        printf("%dx%dn", htonl(w), htonl(h));
        if (close(fd) == -1) oops("close");
    }
    return 0;
}

This method is much faster than using a library which loads the PNG forwards, backwards and sideways just to get the image size :P (Consider the code carefully before feeding it a directory full of arbitrary PNGs of course.)

The code uses inet.h for htonl() to de-endian-ize the header byte ordering.

you can just use the command "file" to get the informations you need:

~# file cha_2.png 
cha_2.png: PNG image data, 656 x 464, 8-bit/color RGB, non-interlaced

found a solution: identify, part of the imagemagick package, does exactly what I need

$ identify color.jpg 
> color.jpg JPEG 1980x650 1980x650+0+0 8-bit DirectClass 231KB 0.000u 0:00.000