[SOLVED] Bash: Image dimensions

graphicsmanx1 · 11-01-2012, 02:32 PM

ok... Tell me if Im understanding whats going on.

Code:

maxsize="3000000"

you are declaring the variable maxsize with a value of 2M

Code:

while read -r width height fname

Im lost here. I understand its a simple do while loop but Im lost with the read -r width height fname. read is read but whats -r doing exactly??? I thought -r was read, too. Im stuck with what fname is doing are, are you running the whole line and setting it as a variable fname?

Code:

((width*height>=maxsize)) && echo "$fname"

you are doing "this line" and the width*height>=maxsize AND echoing the variable fname. it will do...... How are you declaring the variable fname??

Code:

 < <(identify -format "%w %h %f\n" *jpg)

im stuck with whats going on with < <. Im understanding that the identity format is what is used with ImageMagick and %w and %h are pulling the width and height. Whats %f doing? \n prints a new line and *jpg is all images.

Code:

>> results.csv

prints to CSV file with the name results

EDIT:
how would you also extend *jpg to also check for other file types like .png and .jpeg

graphicsmanx1 · 11-01-2012, 05:04 PM

FYI.. if anyone reads this to be able to do all types of image formats you dont need a comparision operator it just needs to look like this:

Code:

(identify -format "%w %h %f\n"  *png *jpg *jpeg *tif *gif *bmp)>> results.csv

millgates · 11-01-2012, 06:20 PM

Code:

#!/bin/bash

maxsize="2000000"
while read -r width height fname; do 
    ((width*height>maxsize)) && echo "$fname";
done < <(identify -format "%w %h %f\n" *jpg)

Ok. The variable declaration is clear. To explain the rest, I would, if I may, start from the end:

Code:

identify -format "%w %h %f\n" *jpg

This is the command to get the size of the image, as proposed by druuna. I took the liberty of slightly rearranging the output format. I put the width and height to the front because they will never contain spaces. So I know that the first word is width, the second one is height and the rest of the line is filename.
Now, the <( ) is bash process substitution. It makes the output of the command inside look like a file to the shell script. The < in front then redirects this "file" to the while loop.

Code:

read -r width height fname

this part in the loop will read the lines one at a time, so that first word of each line will be stored in variable width, the second one in height and the rest of the line (the file name) will go to fname. -r means that read will not interpret backslashes as metacharacters.

Code:

((width*height>=maxsize)) && echo "$fname"

Here we have the bash arithmetic (( )). Whatever is inside will be evaluated as a mathematical expression. So, it computes width * height and does the comparison. The result of the operation is either true or false.
&& is the bash 'and' operator. It runs the second command only if the first command was successful (that is, if width * height >= maxsize is true. Alternatively, you could write

Code:

if ((width*height>=maxsize)); then
    echo "$fname"
fi

To achieve the same effect.
That's it. If you have any questions, feel free to ask.

graphicsmanx1 · 11-02-2012, 09:42 AM

for some reason the process substitution is throwing me off and Im trying to look at it from a simplified manner.

millgates · 11-02-2012, 10:06 AM

Quote:

Originally Posted by graphicsmanx1

for some reason the process substitution is throwing me off and Im trying to look at it from a simplified manner.

Sorry, a typo in the link. It should work now

graphicsmanx1 · 11-06-2012, 03:41 PM

im sorry can you explain how this would be done with a simple loop?? Im still lost in understanding how the read and process substitution is working. That is throwing me off. So the statement is starting with

Code:

(identify -format "%w %h %f\n" *jpg)

from what I am reading.

millgates · 11-07-2012, 02:12 AM

Well, that was a simple loop. But if I did this without read and process substitution, it could look like this:

Code:

#!/bin/bash

# define the max size of the image
maxsize="2000000"

# iterate over all jpg images in the working dir
for image in *.jpg; do

    # use identify to get the size in format "width height"
    size="$(identify -format "%w %h" "$image")"

    # extract width from the string by 'cutting off' everything
    # from the first space on (or up to the first space(see string manipulations)
    width="${size%% *}"

    # extract height
    height="${size##* }" 

    # use bash math to compare the numbers
    if ((width*height>maxsize)); then
        echo "$fname";
    fi
done