diff --git a/context_test.go b/context_test.go
index 397b8e2fb..4106020bb 100644
--- a/context_test.go
+++ b/context_test.go
@@ -96,21 +96,32 @@ func TestEnv(t *testing.T) {
 
 func TestEnvsubst(t *testing.T) {
 	c := Context{}
+	c.Setenv("REPLACE", "me")
+	for s, expected := range map[string]string{
+		"BEFORE${REPLACE}AFTER":  "BEFOREmeAFTER",
+		"BEFORE${!REPLACE}AFTER": "BEFORE${REPLACE}AFTER",
 
-	if s, err := c.Envsubst("start${example}end"); s != "startend" || err != nil {
-		t.Fail()
-	}
+		"BEFORE${REPLACE:-default}AFTER":  "BEFOREmeAFTER",
+		"BEFORE${!REPLACE:-default}AFTER": "BEFORE${REPLACE:-default}AFTER",
 
-	if s, err := c.Envsubst("start${example:-default}end"); s != "startdefaultend" || err != nil {
-		t.Fail()
-	}
+		"BEFORE${REPLACE/me/you}AFTER":  "BEFOREyouAFTER",
+		"BEFORE${!REPLACE/me/you}AFTER": "BEFORE${REPLACE/me/you}AFTER",
 
-	c.Setenv("example", "value")
-	if s, err := c.Envsubst("start${example}end"); s != "startvalueend" || err != nil {
-		t.Fail()
-	}
+		// TODO support curly brackets
+		// "BEFORE${REPLACE/me/with\\}curly}AFTER":  "BEFOREwith}curlyAFTER",
+		// "BEFORE${!REPLACE/me/with\\}curly}AFTER": "BEFORE${REPLACE/me/with\\}curly}AFTER",
 
-	if s, err := c.Envsubst("start${example:-default}end"); s != "startvalueend" || err != nil {
-		t.Fail()
+		"BEFORE${UNSET:-default}AFTER":  "BEFOREdefaultAFTER",
+		"BEFORE${!UNSET:-default}AFTER": "BEFORE${UNSET:-default}AFTER",
+	} {
+		t.Run(s, func(t *testing.T) {
+			actual, err := c.Envsubst(s)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if actual != expected {
+				t.Fatalf("invalid replacement\nexpected: %#v\nactual  : %#v", expected, actual)
+			}
+		})
 	}
 }
diff --git a/docs/src/carapace/context/envSubst.md b/docs/src/carapace/context/envSubst.md
index 6d5650dc4..e0b5d3400 100644
--- a/docs/src/carapace/context/envSubst.md
+++ b/docs/src/carapace/context/envSubst.md
@@ -1 +1,32 @@
 # Envsubst
+
+Expands variables in a string using [drone/envsubst].
+
+> Expressions are ignored if the variable name has a `!` prefix (which is stripped).
+
+## Supported Functions
+
+| Expression                    | Meaning                                                             |
+| -----------------             | --------------                                                      |
+| `${var}`                      | Value of `$var`                                                     |
+| `${#var}`                     | String length of `$var`                                             |
+| `${var^}`                     | Uppercase first character of `$var`                                 |
+| `${var^^}`                    | Uppercase all characters in `$var`                                  |
+| `${var,}`                     | Lowercase first character of `$var`                                 |
+| `${var,,}`                    | Lowercase all characters in `$var`                                  |
+| `${var:n}`                    | Offset `$var` `n` characters from start                             |
+| `${var:n:len}`                | Offset `$var` `n` characters with max length of `len`               |
+| `${var#pattern}`              | Strip shortest `pattern` match from start                           |
+| `${var##pattern}`             | Strip longest `pattern` match from start                            |
+| `${var%pattern}`              | Strip shortest `pattern` match from end                             |
+| `${var%%pattern}`             | Strip longest `pattern` match from end                              |
+| `${var-default`               | If `$var` is not set, evaluate expression as `$default`             |
+| `${var:-default`              | If `$var` is not set or is empty, evaluate expression as `$default` |
+| `${var=default`               | If `$var` is not set, evaluate expression as `$default`             |
+| `${var:=default`              | If `$var` is not set or is empty, evaluate expression as `$default` |
+| `${var/pattern/replacement}`  | Replace as few `pattern` matches as possible with `replacement`     |
+| `${var//pattern/replacement}` | Replace as many `pattern` matches as possible with `replacement`    |
+| `${var/#pattern/replacement}` | Replace `pattern` match with `replacement` from `$var` start        |
+| `${var/%pattern/replacement}` | Replace `pattern` match with `replacement` from `$var` end          |
+
+[drone/envsubst]:https://github.com/drone/envsubst
diff --git a/third_party/github.com/drone/envsubst/parse/parse.go b/third_party/github.com/drone/envsubst/parse/parse.go
index bc418fbfb..47b9c7415 100644
--- a/third_party/github.com/drone/envsubst/parse/parse.go
+++ b/third_party/github.com/drone/envsubst/parse/parse.go
@@ -90,6 +90,8 @@ func (t *Tree) parseFunc() (Node, error) {
 	// Turn on all escape characters
 	t.scanner.escapeChars = escapeAll
 	switch t.scanner.peek() {
+	case '!':
+		return t.parseIgnoredFunc()
 	case '#':
 		return t.parseLenFunc()
 	}
@@ -355,6 +357,29 @@ func (t *Tree) parseCasingFunc(name string) (Node, error) {
 	return node, t.consumeRbrack()
 }
 
+// parses the ignored ${!...} function
+func (t *Tree) parseIgnoredFunc() (Node, error) {
+	node := new(TextNode)
+
+	t.scanner.accept = acceptOneExclamationMark
+	t.scanner.mode = scanIdent
+	switch t.scanner.scan() {
+	case tokenIdent:
+	default:
+		return nil, ErrBadSubstitution
+	}
+
+	t.scanner.accept = acceptNotClosing
+	t.scanner.mode = scanIdent
+	switch t.scanner.scan() {
+	case tokenIdent:
+		node.Value = "${" + t.scanner.string() + "}"
+	default:
+		return nil, ErrBadSubstitution
+	}
+	return node, t.consumeRbrack()
+}
+
 // parses the ${#param} string function
 func (t *Tree) parseLenFunc() (Node, error) {
 	node := new(FuncNode)
diff --git a/third_party/github.com/drone/envsubst/parse/scan.go b/third_party/github.com/drone/envsubst/parse/scan.go
index 2710879ac..c30c500c3 100644
--- a/third_party/github.com/drone/envsubst/parse/scan.go
+++ b/third_party/github.com/drone/envsubst/parse/scan.go
@@ -227,6 +227,10 @@ func acceptColon(r rune, i int) bool {
 	return r == ':'
 }
 
+func acceptOneExclamationMark(r rune, i int) bool {
+	return r == '!' && i == 1
+}
+
 func acceptOneHash(r rune, i int) bool {
 	return r == '#' && i == 1
 }