A couple of years ago webp was the defacto standard for images on websites. However, it is slowly going to be replaced by avif to serve images and graphics on websites. Converting a single image is easy using avifenc and therefore it can easily be used in a shell script.

Inline it could be done using the following:

for file in *.png; do if [ -f "$file" ]; then avifenc --lossless $file "${file%.*}.avif" ; fi; done

for file in *.jpg; do if [ -f "$file" ]; then avifenc --speed 2 $file  "${file%.*}.avif" ; fi; done

If a bit more control is needed, a proper bash script is the solution:

#!/bin/bash

speed=2
valid_img_file_extensions=("png" "tif" "tiff" "jpg" "jpeg")
folder_path=""
lossless_mode=false

for i in "$@"
do
    case $i in
        -p=*|--folderpath=*)
            folder_path="${i#*=}"
            ;;
        -ext=*|--file-extension=*)
	    valid_img_file_extensions=("${i#*=}")
            ;;
        -q=*|--compression-quality=*)
	    speed="${i#*=}"
	    ;;
	-l*|--lossless*)
	    lossless_mode=true
	    ;;
    esac
done

for file in "$folder_path"*
do
    if [ -f $file ]
    then
	    ext=${file#*.}
	    if [[ $valid_img_file_extensions =~ (^|[[:space:]])"$ext"($|[[:space:]]) ]]
	    then
	        output_fn="${file%.*}.avif"
	        if  $lossless_mode 
            then
		        avifenc --lossless $file $output_fn
	        else
		        avifenc --speed $speed $file $output_fn
	        fi
	    fi	    
    fi
done

The shell script can also be found here.