diff --git a/.envrc b/.envrc
new file mode 100644
index 0000000..3550a30
--- /dev/null
+++ b/.envrc
@@ -0,0 +1 @@
+use flake
diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 6e5d801..c915264 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -9,12 +9,22 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     name: Ruby ${{ matrix.ruby }}
     strategy:
+      fail-fast: false
       matrix:
-        ruby:
-          - '3.2.2'
+        include:
+          - ruby: '3.2.2'
+            os: ubuntu-latest
+            task: 'bundle exec rake'
+          - ruby: '2.6'
+            os: ubuntu-22.04
+            task: 'bundle exec rspec'
+            gemfile: 'gemfiles/ruby26.gemfile'
+
+    env:
+      BUNDLE_GEMFILE: ${{ github.workspace }}/${{ matrix.gemfile || 'Gemfile' }}
 
     steps:
     - uses: actions/checkout@v4
@@ -24,4 +34,4 @@ jobs:
         ruby-version: ${{ matrix.ruby }}
         bundler-cache: true
     - name: Run the default task
-      run: bundle exec rake
+      run: ${{ matrix.task }}
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 1e69962..a4fd59c 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
 
     container:
-      image: ghcr.io/tamatebako/tebako-ubuntu-20.04:latest
+      image: ghcr.io/tamatebako/tebako-ubuntu-20.04:0.12.6
 
     steps:
       - name: Checkout
diff --git a/.gitignore b/.gitignore
index c49c127..a1f33bf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -14,3 +14,6 @@
 .irbrc
 
 rubyshell-*.gem
+
+/.direnv/
+/gemfiles/*.lock
diff --git a/flake.lock b/flake.lock
new file mode 100644
index 0000000..e2a5570
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,61 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1789785513,
+        "narHash": "sha256-B44WL6h0XoLjJ41bUPJk0X5SDinLCII//6EcBLXKiJ0=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "20b1ddd1aa5ace70c9468305030aa4f9ef79671b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 0000000..fd68b59
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,33 @@
+{
+  description = "rubyshell development environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        ruby = pkgs.ruby_3_3;
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = [
+            ruby
+            pkgs.bundler
+            pkgs.libyaml
+            pkgs.gnumake
+            pkgs.gcc
+          ];
+
+          shellHook = ''
+            export BUNDLE_PATH="$PWD/.bundle/vendor"
+            export BUNDLE_BIN="$PWD/.bundle/bin"
+            export GEM_HOME="$BUNDLE_PATH"
+            export PATH="$BUNDLE_BIN:$PWD/bin:$PATH"
+          '';
+        };
+      });
+}
diff --git a/gemfiles/ruby26.gemfile b/gemfiles/ruby26.gemfile
new file mode 100644
index 0000000..103cfe9
--- /dev/null
+++ b/gemfiles/ruby26.gemfile
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+# Minimal dependency set for the Ruby 2.6 compatibility job.
+# The main Gemfile pulls in debug (needs >= 2.7) and rubocop, which do not
+# install on 2.6; this one only carries what the specs need.
+source "https://rubygems.org"
+
+gemspec path: ".."
+
+gem "rake", "~> 13.0"
+gem "rspec", "~> 3.0"
diff --git a/lib/rubyshell.rb b/lib/rubyshell.rb
index 0dc63e1..c6f8436 100644
--- a/lib/rubyshell.rb
+++ b/lib/rubyshell.rb
@@ -67,6 +67,10 @@ def config(kwargs)
 module Kernel
   def sh(command = nil, *args, **kwargs, &block)
     if command
+      # On Ruby 2.6 an empty **kwargs is still passed along as a positional
+      # empty hash, which breaks arity for methods like OverwritedCommands#cd.
+      return RubyShell::Executor.send(command, *args) if kwargs.empty?
+
       RubyShell::Executor.send(command, *args, **kwargs)
     elsif block.nil?
       RubyShell::Executor
diff --git a/lib/rubyshell/chainer.rb b/lib/rubyshell/chainer.rb
index 3097819..2ab7cf2 100644
--- a/lib/rubyshell/chainer.rb
+++ b/lib/rubyshell/chainer.rb
@@ -24,7 +24,7 @@ def shared_settings
     end
 
     def settings
-      @options.select { shared_settings.include?(_1.to_sym) }.transform_keys { :"_#{_1}" }
+      @options.select { |k, _v| shared_settings.include?(k.to_sym) }.transform_keys { |k| :"_#{k}" }
     end
 
     def method_missing(method_name, *args, &block)
diff --git a/lib/rubyshell/parallel_executor.rb b/lib/rubyshell/parallel_executor.rb
index ade812a..a699781 100644
--- a/lib/rubyshell/parallel_executor.rb
+++ b/lib/rubyshell/parallel_executor.rb
@@ -14,7 +14,7 @@ def shared_settings
     end
 
     def settings
-      @options.select { shared_settings.include?(_1.to_sym) }.transform_keys { :"_#{_1}" }
+      @options.select { |k, _v| shared_settings.include?(k.to_sym) }.transform_keys { |k| :"_#{k}" }
     end
 
     def _evaluate(&block)
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 1f606bb..3d54b1b 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,7 +1,9 @@
 # frozen_string_literal: true
 
 require "tmpdir"
-require "debug"
+# The debug gem requires Ruby >= 2.7. On 2.6 `require "debug"` would load the
+# stdlib debugger (rdb) instead, which takes over the run on the first raise.
+require "debug" if RUBY_VERSION >= "2.7"
 require_relative "../lib/rubyshell"
 
 RSpec.configure do |config|