Update every game's proton version.

update_proton.sh

#!/bin/sh

set -eu

steam_dir="$HOME/.steam"
proton=
list=false

eval set -- "$(getopt -o d:p:l -l directory,proton,list -- "$@")"
while true; do
	case "$1" in
		-d|--directory)
			steam_dir="$2"
			shift 2;;
		-p|--proton)
			proton="$2"
			shift 2;;
		-l|--list)
			list=true
			shift;;
		--)
			shift
			break;;
		*)
			echo "Invalid option: $1"
			exit 1
	esac
done

# Grab all the library folders
dirs=$(grep -o '/.*' "$steam_dir/steam/steamapps/libraryfolders.vdf" | tr -d '"')

(IFS='
'
for dir in $dirs; do
	printf "Found library at '%s'\n" "$dir"
	if [ "$proton" ]
	then
		sed -i "1s/.*/$proton/" "$dir"/steamapps/compatdata/*/version
	fi
	if [ "$list" = true ]
	then
		head -1 "$dir"/steamapps/compatdata/*/version
	fi
done)

Great explanation of the structure of the script:
https://unix.stackexchange.com/a/383864