Imperative nix-env rewrite (so it becomes declarative)

imperative-env.sh

#!/usr/bin/env bash
statefile=~/.config/nixpkgs/declarative

action="$1"
package="$2"

mkdir -p $(dirname "$statefile")
touch "$statefile"
function update {
  envExpr=~/.config/nixpkgs/declarative-env.nix
  truncate -s 0 "$envExpr"
  for ch in $(ls /nix/var/nix/profiles/per-user/root/channels | grep -v -e manifest -e binary-caches) ; do
    echo "let $ch = import <$ch> { }; in" >> "$envExpr"
  done
  echo "let _pkgs = import <nixpkgs> { }; in" >> "$envExpr"
  echo "rec { _paths = [" >> "$envExpr"
  cat "$statefile" | sort -u >> "$envExpr"
  echo "             ]; " >> "$envExpr"
  echo "      env = _pkgs.buildEnv {" >> "$envExpr"
  echo "      name = ''declarative-collection''; " >> "$envExpr"
  echo "      paths = _paths;" >> "$envExpr"
  echo "}; }" >> "$envExpr"
  \nix-env -Q -if "$envExpr" -A env && echo "# Updated successfully!" >> "$envExpr"
}

case "$action" in
install|add)
    if (grep -Eq "^$package\$" "$statefile"); then
        echo "already installed, updating"
    else
        echo "scheduled install'n'update"
        echo "$package" >> "$statefile"
    fi
    # TODO: trap
    update && echo Success || sed -i "/^$package$/d" $statefile
;;
uninstall|remove|delete)
    if (grep -Eq "^$package\$" "$statefile"); then
        sed -i "/^$package\$/d" $statefile
        update && echo "Success"
    else
        echo Not installed
    fi
;;
list)
    cat "$statefile"
;;
*)
    echo Command not specified. Try $(basename "$0") install nixpkgs.htop
    echo Command list: install, uninstall, list
;;
esac

Reminder to whoever uses this script: alias nix-env=/path/to/script