Respect user-supplied quoting (#8)

Currently it's impossible to have 'lazily' evaluated environment
variables like

```
  FOO='${BAR}'
```

where `${BAR}` is evaluated at runtime.

This is because we strip any quotes (single or double) and replace them
with double quotes, and then `${BAR}` is expanded.

Instead, we can check if the value is already quoted, and only supply
our own double quotes if it's not.

dotenv.sh

@@ -56,11 +56,14 @@ export_envs() {
 			continue
 		fi
 
-		# Strip any existing quotes
-		temp="${temp%[\'\"]}";
-		temp="${temp#[\'\"]}";
-		# Add new double quotes for interpolation
-		value=$(eval echo "\"$temp\"");
+		# check if $temp doesn't start with with ' or ", and quote it if so
+		if [ "${temp%%[\"\']*}" = "${temp}" ]; then
+			temp="\"${temp}\""
+		fi
+
+		# We have already quoted if necessary, so we can safely eval it
+		# shellcheck disable=SC2086
+		value=$(eval echo ${temp});
 
 		eval export "$key='$value'";
 		echo "$key=$value" >> $GITHUB_ENV;

tests/.env

@@ -2,6 +2,10 @@
 TEST_UNQUOTED=a=1 b=2 c=3
 # Comment 2
 TEST_SINGLE_QUOTED='1 2 3 4'
+
+# This is single-quoted, so it should be treated as a literal
+TEST_SINGLE_QUOTE_NO_INTERPOLATE='${TEST}'
+
 # Comment 3
 TEST_DOUBLE_QUOTED="1 2 3 4"
 # Comment 4
@@ -15,5 +19,6 @@ TEST_EXISTING="new-value"
 TEST_DOTENV_OVERRIDES_DEFAULT="expected"
 # Test special characters are escaped
 TEST_SPECIAL_CHARACTER=special(character
+
 # Test that the last variable is parsed despite no trailing new line
 TEST_NO_NEWLINE="still there"

tests/dotenv-test.sh

@@ -16,6 +16,9 @@ main() {
     echo "Testing blank line parsing: ok" # i.e. didn't crash
     assert_equal "$TEST_UNQUOTED" 'a=1 b=2 c=3' 'Testing unquoted'
     assert_equal "$TEST_SINGLE_QUOTED" '1 2 3 4' 'Testing single quoted'
+    # We want to test it expands to this literal value
+    # shellcheck disable=SC2016
+    assert_equal "$TEST_SINGLE_QUOTE_NO_INTERPOLATE" '${TEST}' 'Testing single-quoted variables arent interpolated'
     assert_equal "$TEST_DOUBLE_QUOTED" '1 2 3 4' 'Testing double quoted'
     assert_equal "$TEST_INTERPOLATION" 'a=1 b=2 c=3 d=4' 'Testing interpolation'
     assert_equal "$TEST_EXISTING" 'new-value' 'Testing overwrite of existing variables'