Split an image evenly with optional margins and colored background to create an IG-style grid layout

gridify.sh

#!/bin/bash

file="$1"
background="white"
margin="0"
shift

while [ "$#" -gt 0 ]; do
    case "$1" in
        -background)
            background="$2"
            shift 2
            ;;
        -margin)
            margin="$2"
            shift 2
            ;;
        *)
            echo "Error: Unknown option $1"
            exit 1
            ;;
    esac
done

extent="%[fx:max(w,h)*(1+$margin/100)]x%[fx:max(w,h)*(1+$margin/100)]"
echo "Margin set to $margin%"
echo "Backgound set to $background"

magick "$file"\
    -gravity center\
    -background "$background"\
    -extent "$extent"\
    +gravity\
    -crop 3x3@\
    grid-%d.jpg