diff --git a/src/arion b/src/arion
index 32ec65d..41a1dea 100755
--- a/src/arion
+++ b/src/arion
@@ -46,6 +46,11 @@ while test $# != 0; do
             shift
             break
             ;;
+        exec)
+            command="$1"
+            shift
+            break
+            ;;
         docker-compose)
             command="docker-compose"
             shift
@@ -231,6 +236,45 @@ do_repl_cleanup() {
     rm -f $REPL_TMP
 }
 
+run_exec() {
+    case "${#docker_compose_args[@]}" in
+        0)
+            echo "As an argument to exec, please specify a service"
+            exit 1
+            ;;
+        1)
+            case "${docker_compose_args[0]}" in
+                -*|--*)
+                    echo "As an argument to exec, please specify a service"
+                    echo "Note that executing the default command currently does not support"
+                    echo "docker-compose options."
+                    # This requires parsing the options, in order to figure out
+                    # which service to invoke.
+                    exit 1
+                    ;;
+                *)
+                    serviceName="${docker_compose_args[0]}"
+                    do_eval
+                    default_args=()
+                    while read arg; do
+                        default_args+=("$arg")
+                    done < <(
+                        jq < "$docker_compose_yaml" \
+                            --arg serviceName "$serviceName" \
+                            -r \
+                            '.["x-arion"].serviceInfo.webserver.defaultExec | tostream | .[1] | select(.)'
+                        )
+                    docker-compose -f $docker_compose_yaml exec "$serviceName" "${default_args[@]}"
+                    ;;
+            esac
+            ;;
+        *)
+            do_eval
+            docker-compose -f $docker_compose_yaml exec "${docker_compose_args[@]}"
+            ;;
+    esac
+}
+
 case "$command" in
     cat)
         do_eval
@@ -239,6 +283,9 @@ case "$command" in
     repl)
         do_repl
         ;;
+    exec)
+        run_exec "$@"
+        ;;
     docker-compose)
         if [[ ${#docker_compose_args[@]} != 0
            && ${docker_compose_args[0]} != "help"
diff --git a/src/nix/eval-composition.nix b/src/nix/eval-composition.nix
index 7d10e99..b3fc78a 100644
--- a/src/nix/eval-composition.nix
+++ b/src/nix/eval-composition.nix
@@ -21,6 +21,7 @@ let
     ./modules/composition/docker-compose.nix
     ./modules/composition/host-environment.nix
     ./modules/composition/images.nix
+    ./modules/composition/service-info.nix
   ];
 
   argsModule = {
diff --git a/src/nix/eval-service.nix b/src/nix/eval-service.nix
index 887b419..26a498c 100644
--- a/src/nix/eval-service.nix
+++ b/src/nix/eval-service.nix
@@ -9,7 +9,9 @@ let
 
   builtinModules = [
     argsModule
+    ./modules/service/default-exec.nix
     ./modules/service/docker-compose-service.nix
+    ./modules/service/extended-info.nix
     ./modules/service/host-store.nix
     ./modules/service/host.nix
     ./modules/service/image.nix
diff --git a/src/nix/modules/composition/docker-compose.nix b/src/nix/modules/composition/docker-compose.nix
index 552c2e4..95855ff 100644
--- a/src/nix/modules/composition/docker-compose.nix
+++ b/src/nix/modules/composition/docker-compose.nix
@@ -26,7 +26,11 @@ in
     };
     docker-compose.raw = lib.mkOption {
       type = lib.types.attrs;
-      description = "Nested attribute set that will be turned into the docker-compose.yaml file, using Nix's toJSON builtin.";
+      description = "Attribute set that will be turned into the docker-compose.yaml file, using Nix's toJSON builtin.";
+    };
+    docker-compose.extended = lib.mkOption {
+      type = lib.types.attrs;
+      description = "Attribute set that will be turned into the x-arion section of the docker-compose.yaml file.";
     };
     docker-compose.services = lib.mkOption {
       default = {};
@@ -47,6 +51,7 @@ in
     docker-compose.raw = {
       version = "3.4";
       services = lib.mapAttrs (k: c: c.config.build.service) config.docker-compose.evaluatedServices;
+      x-arion = config.docker-compose.extended;
     };
   };
 }
diff --git a/src/nix/modules/composition/images.nix b/src/nix/modules/composition/images.nix
index 19035e8..9a9b04c 100644
--- a/src/nix/modules/composition/images.nix
+++ b/src/nix/modules/composition/images.nix
@@ -31,6 +31,6 @@ in
   };
   config = {
     build.imagesToLoad = lib.attrValues serviceImages;
-    docker-compose.raw.x-arion.images = config.build.imagesToLoad;
+    docker-compose.extended.images = config.build.imagesToLoad;
   };
 }
diff --git a/src/nix/modules/composition/service-info.nix b/src/nix/modules/composition/service-info.nix
new file mode 100644
index 0000000..1ed2989
--- /dev/null
+++ b/src/nix/modules/composition/service-info.nix
@@ -0,0 +1,24 @@
+/*
+   Adds extendedInfo from services to the Docker Compose file.
+
+   This contains fields that are not in Docker Compose schema.
+ */
+{ config, lib, ... }:
+let
+  serviceInfo =
+    lib.mapAttrs getInfo (
+      lib.filterAttrs filterFunction config.docker-compose.evaluatedServices
+    );
+
+  filterFunction = _serviceName: service:
+    # shallow equality suffices for emptiness test
+    builtins.attrNames service.config.build.extendedInfo != [];
+
+  getInfo = _serviceName: service: service.config.build.extendedInfo;
+
+in
+{
+  config = {
+    docker-compose.extended.serviceInfo = serviceInfo;
+  };
+}
diff --git a/src/nix/modules/nixos/default-shell.nix b/src/nix/modules/nixos/default-shell.nix
new file mode 100644
index 0000000..2eb4173
--- /dev/null
+++ b/src/nix/modules/nixos/default-shell.nix
@@ -0,0 +1,4 @@
+{ config, utils, ... }:
+{
+  config.system.build.x-arion-defaultShell = utils.toShellPath config.users.defaultUserShell;
+}
diff --git a/src/nix/modules/service/default-exec.nix b/src/nix/modules/service/default-exec.nix
new file mode 100644
index 0000000..299e083
--- /dev/null
+++ b/src/nix/modules/service/default-exec.nix
@@ -0,0 +1,19 @@
+{ config, lib, ... }:
+let
+  inherit (lib) types mkOption;
+in
+{
+  options = {
+    service.defaultExec = mkOption {
+      type = types.listOf types.str;
+      default = ["/bin/sh"];
+      description = ''
+        Container program and arguments to invoke when calling
+        <code>arion exec &lt;service.name></code> without further arguments.
+      '';
+    };
+  };
+  config = {
+    build.extendedInfo.defaultExec = config.service.defaultExec;
+  };
+}
\ No newline at end of file
diff --git a/src/nix/modules/service/extended-info.nix b/src/nix/modules/service/extended-info.nix
new file mode 100644
index 0000000..bb876c7
--- /dev/null
+++ b/src/nix/modules/service/extended-info.nix
@@ -0,0 +1,20 @@
+{ config, lib, ... }:
+let
+  inherit (lib) mkOption;
+  inherit (lib.types) attrsOf unspecified;
+in
+{
+  options = {
+    build.extendedInfo = mkOption {
+      type = attrsOf unspecified;
+      description = ''
+        Information about a service to include in the Docker Compose file,
+        but that will not be used by the <code>docker-compose</code> command
+        itself.
+
+        It will be inserted in <code>x-arion.serviceInfo.&lt;service.name></code>.
+      '';
+      default = {};
+    };
+  };
+}
\ No newline at end of file
diff --git a/src/nix/modules/service/nixos-init.nix b/src/nix/modules/service/nixos-init.nix
index 5c6da89..7521a09 100644
--- a/src/nix/modules/service/nixos-init.nix
+++ b/src/nix/modules/service/nixos-init.nix
@@ -21,6 +21,7 @@ in
   config = lib.mkIf (config.nixos.useSystemd) {
     nixos.configuration.imports = [
       ../nixos/container-systemd.nix
+      ../nixos/default-shell.nix
       (pkgs.path + "/nixos/modules/profiles/minimal.nix")
     ];
     image.command = [ "${config.nixos.build.toplevel}/init" ];
@@ -35,5 +36,6 @@ in
 
     service.stop_signal = "SIGRTMIN+3";
     service.tty = true;
+    service.defaultExec = [config.nixos.build.x-arion-defaultShell "-l"];
   };
 }
diff --git a/tests/arion-test/default.nix b/tests/arion-test/default.nix
index 4ad5aca..1020a1b 100644
--- a/tests/arion-test/default.nix
+++ b/tests/arion-test/default.nix
@@ -51,6 +51,8 @@ in
     subtest "full-nixos", sub {
       $machine->succeed("cp -r ${../../examples/full-nixos} work && cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion up -d");
       $machine->waitUntilSucceeds("curl localhost:8000");
+      # Also test exec with defaultExec
+      $machine->succeed("cd work && export NIX_PATH=nixpkgs='${pkgs.path}' && (echo 'nix run -f ~/h/arion arion -c arion exec webserver'; echo 'target=world; echo Hello \$target'; echo exit) | script /dev/null | grep 'Hello world'");
       $machine->succeed("cd work && NIX_PATH=nixpkgs='${pkgs.path}' arion down && rm -rf work");
       $machine->waitUntilFails("curl localhost:8000");
     };