Shell escape dollar signs

But allow for shell expansion using curly braces e.g. ${PWD}

README.md

@@ -331,6 +331,21 @@ servers:
       my-label: "50"
 ```
 
+### Using shell expansion
+
+You can use shell expansion to interpolate values from the host machine into labels and env variables with the `${}` syntax.
+Anything within the curly braces will be executed on the host machine and the result will be interpolated into the label or env variable.
+
+```yaml
+labels:
+  host-machine: "${cat /etc/hostname}"
+
+env:
+  HOST_DEPLOYMENT_DIR: "${PWD}"
+```
+
+Note: Any other occurrence of `$` will be escaped to prevent unwanted shell expansion!
+
 ### Using container options
 
 You can specialize the options used to start containers using the `options` definitions:

lib/mrsk/utils.rb

@@ -1,6 +1,8 @@
 module Mrsk::Utils
   extend self
 
+  DOLLAR_SIGN_WITHOUT_SHELL_EXPANSION_REGEX = /\$(?!{[^\}]*\})/
+
   # Return a list of escaped shell arguments using the same named argument against the passed attributes (hash or array).
   def argumentize(argument, attributes, sensitive: false)
     Array(attributes).flat_map do |key, value|
@@ -75,7 +77,9 @@ module Mrsk::Utils
 
   # Escape a value to make it safe for shell use.
   def escape_shell_value(value)
-    value.to_s.dump.gsub(/`/, '\\\\`')
+    value.to_s.dump
+      .gsub(/`/, '\\\\`')
+      .gsub(DOLLAR_SIGN_WITHOUT_SHELL_EXPANSION_REGEX, '\$')
   end
 
   # Abbreviate a git revhash for concise display

test/utils_test.rb

@@ -49,5 +49,16 @@ class UtilsTest < ActiveSupport::TestCase
   test "escape_shell_value" do
     assert_equal "\"foo\"", Mrsk::Utils.escape_shell_value("foo")
     assert_equal "\"\\`foo\\`\"", Mrsk::Utils.escape_shell_value("`foo`")
+
+    assert_equal "\"${PWD}\"", Mrsk::Utils.escape_shell_value("${PWD}")
+    assert_equal "\"${cat /etc/hostname}\"", Mrsk::Utils.escape_shell_value("${cat /etc/hostname}")
+    assert_equal "\"\\${PWD]\"", Mrsk::Utils.escape_shell_value("${PWD]")
+    assert_equal "\"\\$(PWD)\"", Mrsk::Utils.escape_shell_value("$(PWD)")
+    assert_equal "\"\\$PWD\"", Mrsk::Utils.escape_shell_value("$PWD")
+
+    assert_equal "\"^(https?://)www.example.com/(.*)\\$\"",
+      Mrsk::Utils.escape_shell_value("^(https?://)www.example.com/(.*)$")
+    assert_equal "\"https://example.com/\\$2\"",
+      Mrsk::Utils.escape_shell_value("https://example.com/$2")
   end
 end