Bash - detect PNG, JPG, GIF file using file bytes (magic numbers in file)

[ "$(xxd -s 6 -l 4 -p "/path/to/image.jpg")" = "4a464946" ] && echo "is JPEG" || echo "is not JPEG"

[ "$(xxd -s 1 -l 3 -p "/path/to/image.png")" = "504e47" ] && echo "is PNG" || echo "is not PNG"

[ "$(xxd -s 0 -l 3 -p "/path/to/image.gif")" = "474946" ] && echo "is GIF" || echo "is not GIF"

#!/bin/bash

​

function is_jpg()  # args: file_path

{

    #                               J F I F  (as chars)

    #                               4a464946 (in hex)

    [ "$(xxd -s 6 -l 4 -p "$1")" = "4a464946" ] && return 0 || return 1

}

​

function is_png()  # args: file_path

{

    #                               P N G  (as chars)

    #                               504e47 (in hex)

    [ "$(xxd -s 1 -l 3 -p "$1")" = "504e47" ] && return 0 || return 1

}

​

function is_gif()  # args: file_path

{

    #                               G I F  (as chars)

    #                               474946 (in hex) 

    [ "$(xxd -s 0 -l 3 -p "$1")" = "474946" ] && return 0 || return 1

}

​

​

# Usage example:

​

echo -e "\n--------------------------------\nJPG files:\n";

for file_name in *.*

do

    is_jpg "$file_name" && echo "$file_name"

done

​

​

echo -e "\n--------------------------------\nPNG files:\n";

for file_name in *.*

do

    is_png "$file_name" && echo "$file_name"

done

​

​

echo -e "\n--------------------------------\nGIF files:\n";

for file_name in *.*

do

    is_gif "$file_name" && echo "$file_name"

done

​

​

echo -e "\n--------------------------------\n"

echo -e "DONE!\n"

./detect_image_type.sh

--------------------------------

JPG files:

​

picture.jpg

mountains.jpeg

river.tmp

sun.tmp

​

--------------------------------

PNG files:

​

ball.png

square.unknown

​

--------------------------------

GIF files:

​

animation.dat

​

--------------------------------

DONE!