diff --git a/DESCRIPTION b/DESCRIPTION
index 05c9ac2..67bf448 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -20,5 +20,5 @@ Suggests:
     rmarkdown
 Encoding: UTF-8
 LazyData: true
-RoxygenNote: 6.1.0
+RoxygenNote: 7.1.0
 VignetteBuilder: knitr
diff --git a/NEWS.md b/NEWS.md
index 2ef645f..0c57b08 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,6 +1,7 @@
 # dockerfiler 0.1.3.9000
 
 * Corrected bug in `rthis()`
+* `add_arg` can now receive an optional default value (@antoine-sachet, #8)
 
 # dockerfiler 0.1.3
 
diff --git a/R/add.R b/R/add.R
index d5bf57e..9081ad7 100644
--- a/R/add.R
+++ b/R/add.R
@@ -94,8 +94,9 @@ add_user <- function(user){
   glue("USER {user}")
 }
 
-add_arg <- function(arg){
-  glue("ARG {arg}")
+add_arg <- function(arg, default = NULL){
+  default <- if (!is.null(default)) glue('="{default}"') else ""
+  glue("ARG {arg}{default}")
 }
 
 add_onbuild <- function(cmd){
diff --git a/R/dockerfile.R b/R/dockerfile.R
index 31319a4..8434ef7 100644
--- a/R/dockerfile.R
+++ b/R/dockerfile.R
@@ -5,15 +5,15 @@
 #' @section Methods:
 #' \describe{
 #'   \item{\code{RUN}}{add a RUN command}
-#'   \item{\code{ADD}}{add a ADD command}
+#'   \item{\code{ADD}}{add an ADD command with optional default value}
 #'   \item{\code{COPY}}{add a COPY command}
 #'   \item{\code{WORKDIR}}{add a WORKDIR command}
 #'   \item{\code{EXPOSE}}{add an EXPOSE command}
 #'   \item{\code{VOLUME}}{add a VOLUME command}
 #'   \item{\code{CMD}}{add a CMD command}
 #'   \item{\code{LABEL}}{add a LABEL command}
-#'   \item{\code{ENV}}{add a ENV command}
-#'   \item{\code{ENTRYPOINT}}{add a ENTRYPOINT command}
+#'   \item{\code{ENV}}{add an ENV command}
+#'   \item{\code{ENTRYPOINT}}{add an ENTRYPOINT command}
 #'   \item{\code{VOLUME}}{add a VOLUME command}
 #'   \item{\code{USER}}{add a USER command}
 #'   \item{\code{ARG}}{add an ARG command}
@@ -75,11 +75,11 @@ Dockerfile <- R6::R6Class("Dockerfile",
                         USER = function(user){
                           self$Dockerfile <- c(self$Dockerfile, add_user(user))
                         },
-                        ARG = function(arg, ahead = FALSE){
+                        ARG = function(arg, ahead = FALSE, default = NULL){
                           if (ahead) {
-                            self$Dockerfile <- c(add_arg(arg), self$Dockerfile)
+                            self$Dockerfile <- c(add_arg(arg, default = default), self$Dockerfile)
                           } else {
-                            self$Dockerfile <- c(self$Dockerfile,add_arg(arg))
+                            self$Dockerfile <- c(self$Dockerfile, add_arg(arg, default = default))
                           }
                         },
                         ONBUILD = function(cmd){
diff --git a/man/Dockerfile.Rd b/man/Dockerfile.Rd
index fef954a..775c986 100644
--- a/man/Dockerfile.Rd
+++ b/man/Dockerfile.Rd
@@ -1,32 +1,29 @@
 % Generated by roxygen2: do not edit by hand
 % Please edit documentation in R/dockerfile.R
-\docType{data}
 \name{Dockerfile}
 \alias{Dockerfile}
 \title{A Dockerfile template}
-\format{An object of class \code{R6ClassGenerator} of length 24.}
-\usage{
-Dockerfile
-}
 \value{
 A dockerfile template
 }
 \description{
+A Dockerfile template
+
 A Dockerfile template
 }
 \section{Methods}{
 
 \describe{
   \item{\code{RUN}}{add a RUN command}
-  \item{\code{ADD}}{add a ADD command}
+  \item{\code{ADD}}{add an ADD command with optional default value}
   \item{\code{COPY}}{add a COPY command}
   \item{\code{WORKDIR}}{add a WORKDIR command}
   \item{\code{EXPOSE}}{add an EXPOSE command}
   \item{\code{VOLUME}}{add a VOLUME command}
   \item{\code{CMD}}{add a CMD command}
   \item{\code{LABEL}}{add a LABEL command}
-  \item{\code{ENV}}{add a ENV command}
-  \item{\code{ENTRYPOINT}}{add a ENTRYPOINT command}
+  \item{\code{ENV}}{add an ENV command}
+  \item{\code{ENTRYPOINT}}{add an ENTRYPOINT command}
   \item{\code{VOLUME}}{add a VOLUME command}
   \item{\code{USER}}{add a USER command}
   \item{\code{ARG}}{add an ARG command}
@@ -46,4 +43,267 @@ A Dockerfile template
 \examples{
 my_dock <- Dockerfile$new()
 }
-\keyword{datasets}
+\section{Methods}{
+\subsection{Public methods}{
+\itemize{
+\item \href{#method-new}{\code{Dockerfile$new()}}
+\item \href{#method-RUN}{\code{Dockerfile$RUN()}}
+\item \href{#method-ADD}{\code{Dockerfile$ADD()}}
+\item \href{#method-COPY}{\code{Dockerfile$COPY()}}
+\item \href{#method-WORKDIR}{\code{Dockerfile$WORKDIR()}}
+\item \href{#method-EXPOSE}{\code{Dockerfile$EXPOSE()}}
+\item \href{#method-VOLUME}{\code{Dockerfile$VOLUME()}}
+\item \href{#method-CMD}{\code{Dockerfile$CMD()}}
+\item \href{#method-LABEL}{\code{Dockerfile$LABEL()}}
+\item \href{#method-ENV}{\code{Dockerfile$ENV()}}
+\item \href{#method-ENTRYPOINT}{\code{Dockerfile$ENTRYPOINT()}}
+\item \href{#method-USER}{\code{Dockerfile$USER()}}
+\item \href{#method-ARG}{\code{Dockerfile$ARG()}}
+\item \href{#method-ONBUILD}{\code{Dockerfile$ONBUILD()}}
+\item \href{#method-STOPSIGNAL}{\code{Dockerfile$STOPSIGNAL()}}
+\item \href{#method-HEALTHCHECK}{\code{Dockerfile$HEALTHCHECK()}}
+\item \href{#method-SHELL}{\code{Dockerfile$SHELL()}}
+\item \href{#method-MAINTAINER}{\code{Dockerfile$MAINTAINER()}}
+\item \href{#method-custom}{\code{Dockerfile$custom()}}
+\item \href{#method-print}{\code{Dockerfile$print()}}
+\item \href{#method-write}{\code{Dockerfile$write()}}
+\item \href{#method-switch_cmd}{\code{Dockerfile$switch_cmd()}}
+\item \href{#method-remove_cmd}{\code{Dockerfile$remove_cmd()}}
+\item \href{#method-add_after}{\code{Dockerfile$add_after()}}
+\item \href{#method-clone}{\code{Dockerfile$clone()}}
+}
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-new"></a>}}
+\if{latex}{\out{\hypertarget{method-new}{}}}
+\subsection{Method \code{new()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$new(FROM = "rocker/r-base", AS = NULL)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-RUN"></a>}}
+\if{latex}{\out{\hypertarget{method-RUN}{}}}
+\subsection{Method \code{RUN()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$RUN(cmd)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-ADD"></a>}}
+\if{latex}{\out{\hypertarget{method-ADD}{}}}
+\subsection{Method \code{ADD()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$ADD(from, to, force = TRUE)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-COPY"></a>}}
+\if{latex}{\out{\hypertarget{method-COPY}{}}}
+\subsection{Method \code{COPY()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$COPY(from, to, force = TRUE)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-WORKDIR"></a>}}
+\if{latex}{\out{\hypertarget{method-WORKDIR}{}}}
+\subsection{Method \code{WORKDIR()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$WORKDIR(where)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-EXPOSE"></a>}}
+\if{latex}{\out{\hypertarget{method-EXPOSE}{}}}
+\subsection{Method \code{EXPOSE()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$EXPOSE(port)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-VOLUME"></a>}}
+\if{latex}{\out{\hypertarget{method-VOLUME}{}}}
+\subsection{Method \code{VOLUME()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$VOLUME(volume)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-CMD"></a>}}
+\if{latex}{\out{\hypertarget{method-CMD}{}}}
+\subsection{Method \code{CMD()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$CMD(cmd)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-LABEL"></a>}}
+\if{latex}{\out{\hypertarget{method-LABEL}{}}}
+\subsection{Method \code{LABEL()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$LABEL(key, value)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-ENV"></a>}}
+\if{latex}{\out{\hypertarget{method-ENV}{}}}
+\subsection{Method \code{ENV()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$ENV(key, value)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-ENTRYPOINT"></a>}}
+\if{latex}{\out{\hypertarget{method-ENTRYPOINT}{}}}
+\subsection{Method \code{ENTRYPOINT()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$ENTRYPOINT(cmd)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-USER"></a>}}
+\if{latex}{\out{\hypertarget{method-USER}{}}}
+\subsection{Method \code{USER()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$USER(user)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-ARG"></a>}}
+\if{latex}{\out{\hypertarget{method-ARG}{}}}
+\subsection{Method \code{ARG()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$ARG(arg, ahead = FALSE, default = NULL)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-ONBUILD"></a>}}
+\if{latex}{\out{\hypertarget{method-ONBUILD}{}}}
+\subsection{Method \code{ONBUILD()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$ONBUILD(cmd)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-STOPSIGNAL"></a>}}
+\if{latex}{\out{\hypertarget{method-STOPSIGNAL}{}}}
+\subsection{Method \code{STOPSIGNAL()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$STOPSIGNAL(signal)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-HEALTHCHECK"></a>}}
+\if{latex}{\out{\hypertarget{method-HEALTHCHECK}{}}}
+\subsection{Method \code{HEALTHCHECK()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$HEALTHCHECK(check)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-SHELL"></a>}}
+\if{latex}{\out{\hypertarget{method-SHELL}{}}}
+\subsection{Method \code{SHELL()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$SHELL(shell)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-MAINTAINER"></a>}}
+\if{latex}{\out{\hypertarget{method-MAINTAINER}{}}}
+\subsection{Method \code{MAINTAINER()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$MAINTAINER(name, email)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-custom"></a>}}
+\if{latex}{\out{\hypertarget{method-custom}{}}}
+\subsection{Method \code{custom()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$custom(base, cmd)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-print"></a>}}
+\if{latex}{\out{\hypertarget{method-print}{}}}
+\subsection{Method \code{print()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$print()}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-write"></a>}}
+\if{latex}{\out{\hypertarget{method-write}{}}}
+\subsection{Method \code{write()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$write(as = "Dockerfile")}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-switch_cmd"></a>}}
+\if{latex}{\out{\hypertarget{method-switch_cmd}{}}}
+\subsection{Method \code{switch_cmd()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$switch_cmd(a, b)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-remove_cmd"></a>}}
+\if{latex}{\out{\hypertarget{method-remove_cmd}{}}}
+\subsection{Method \code{remove_cmd()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$remove_cmd(where)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-add_after"></a>}}
+\if{latex}{\out{\hypertarget{method-add_after}{}}}
+\subsection{Method \code{add_after()}}{
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$add_after(cmd, after)}\if{html}{\out{</div>}}
+}
+
+}
+\if{html}{\out{<hr>}}
+\if{html}{\out{<a id="method-clone"></a>}}
+\if{latex}{\out{\hypertarget{method-clone}{}}}
+\subsection{Method \code{clone()}}{
+The objects of this class are cloneable with this method.
+\subsection{Usage}{
+\if{html}{\out{<div class="r">}}\preformatted{Dockerfile$clone(deep = FALSE)}\if{html}{\out{</div>}}
+}
+
+\subsection{Arguments}{
+\if{html}{\out{<div class="arguments">}}
+\describe{
+\item{\code{deep}}{Whether to make a deep clone.}
+}
+\if{html}{\out{</div>}}
+}
+}
+}
diff --git a/man/dock_from_desc.Rd b/man/dock_from_desc.Rd
index c8f9f8e..c8ec692 100644
--- a/man/dock_from_desc.Rd
+++ b/man/dock_from_desc.Rd
@@ -4,8 +4,7 @@
 \alias{dock_from_desc}
 \title{Docker file from DESCRIPTION}
 \usage{
-dock_from_desc(path = "DESCRIPTION", FROM = "rocker/r-base",
-  AS = NULL)
+dock_from_desc(path = "DESCRIPTION", FROM = "rocker/r-base", AS = NULL)
 }
 \arguments{
 \item{path}{Path to DESCRIPTION}
@@ -35,7 +34,7 @@ my_dock <- dock_from_desc("DESCRIPTION")
 my_dock
 my_dock$CMD(r(library(dockerfiler)))
 my_dock$add_after(
-cmd = "RUN R -e 'remotes::install_cran(\\"rlang\\")'",
+cmd = "RUN R -e 'remotes::install_cran(\"rlang\")'",
 after = 3
 )
 }
diff --git a/tests/testthat/test-r6.R b/tests/testthat/test-r6.R
index 30f08de..d4fefd2 100644
--- a/tests/testthat/test-r6.R
+++ b/tests/testthat/test-r6.R
@@ -30,7 +30,7 @@ test_that("R6 creation works", {
   expect_captured_length(my_dock, 11)
   my_dock$USER("plop  /usr/scripts")
   expect_captured_length(my_dock, 12)
-  my_dock$ARG("plop /usr/scripts")
+  my_dock$ARG("plop")
   expect_captured_length(my_dock, 13)
   my_dock$ONBUILD("plop /usr/scripts")
   expect_captured_length(my_dock, 14)
@@ -46,6 +46,8 @@ test_that("R6 creation works", {
   expect_captured_length(my_dock, 17)
   my_dock$switch_cmd(5,6)
   expect_captured_length(my_dock, 17)
+  my_dock$ARG("plop", default = "pouet")
+  expect_captured_length(my_dock, 18)
   my_dock <- Dockerfile$new(FROM = "plop")
   expect_match(my_dock$Dockerfile, "plop")
   my_dock <- Dockerfile$new(FROM = "plop", AS = "pouet")