fix(shellenv): preserve literal newlines in exported env values (ambiguous redirect)

internal/devbox/envvars.go

@@ -89,11 +89,17 @@ func exportify(w io.Writer, vars map[string]string) string {
 			strb.WriteString("export ")
 			strb.WriteString(key)
 			strb.WriteString(`="`)
+			// Escape the characters that are special inside double quotes:
+			// https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_02_03
+			//
+			// A newline is NOT special inside double quotes and must not be
+			// escaped. Writing a backslash before a newline produces a line
+			// continuation, which the shell removes entirely, joining adjacent
+			// lines together (e.g. a multi-line PROMPT_COMMAND would be silently
+			// mangled). Leaving the newline unescaped preserves it literally.
 			for _, r := range vars[key] {
 				switch r {
-				// Special characters inside double quotes:
-				// https://pubs.opengroup.org/onlinepubs/009604499/utilities/xcu_chap02.html#tag_02_02_03
-				case '$', '`', '"', '\\', '\n':
+				case '$', '`', '"', '\\':
 					strb.WriteRune('\\')
 				}
 				strb.WriteRune(r)

internal/devbox/envvars_test.go

@@ -9,6 +9,44 @@ import (
 	"testing"
 )
 
+func TestExportify(t *testing.T) {
+	tests := []struct {
+		name string
+		vars map[string]string
+		want string
+	}{
+		{
+			name: "simple value",
+			vars: map[string]string{"FOO": "bar"},
+			want: `export FOO="bar";`,
+		},
+		{
+			name: "escapes shell-special characters",
+			vars: map[string]string{"FOO": "a$b`c\"d\\e"},
+			want: `export FOO="a\$b\` + "`" + `c\"d\\e";`,
+		},
+		{
+			// Regression test for #2814: a multi-line value (e.g. a
+			// PROMPT_COMMAND set by bash-preexec) must keep its newlines
+			// literal. Escaping a newline with a backslash produces a line
+			// continuation that the shell removes, concatenating the lines and
+			// corrupting the value.
+			name: "preserves embedded newlines without escaping",
+			vars: map[string]string{"PROMPT_COMMAND": "__bp_precmd_invoke_cmd\ndbus-send >/dev/null 2>&1\n__bp_interactive_mode"},
+			want: "export PROMPT_COMMAND=\"__bp_precmd_invoke_cmd\ndbus-send >/dev/null 2>&1\n__bp_interactive_mode\";",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := exportify(io.Discard, tt.vars)
+			if got != tt.want {
+				t.Errorf("exportify() =\n%q\nwant\n%q", got, tt.want)
+			}
+		})
+	}
+}
+
 func TestIsValidEnvName(t *testing.T) {
 	valid := []string{"FOO", "_foo", "foo_BAR_123", "a", "_"}
 	for _, name := range valid {