fix(cli): refuse an --output path that already exists

CHANGELOG.md

@@ -13,6 +13,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/).
 
 ### Fixed
 
+- An explicit `--output` path that already exists (file, directory or symlink)
+  is now refused during validation instead of being silently truncated. Choose a
+  new path or remove the old file; scripts that reuse one path must do the same.
+  Auto-generated filenames are unaffected (#2050).
 - `--regions` now also narrows EC2 Instance Savings Plan recommendations to the listed
   regions by the plan's own region (previously only `--include-regions` did); a plan
   with no known region is dropped. Compute, SageMaker and Database plans stay

README.md

@@ -62,6 +62,8 @@ Export a reviewable report when you need to share results:
 ./cudly --services rds --profile default --output recommendations.csv
 ```
 
+The path must not already exist; the run is refused rather than overwriting it.
+
 All purchase operations can spend money. Check the account, region, quantity, and selected commitment before confirming - see [Safety Features](#safety-features) for what is and is not enforced today.
 
 ## Credentials and provider status

cmd/validators.go

@@ -293,6 +293,15 @@ func validateFilePaths() error {
 				return fmt.Errorf("output directory does not exist: %s", dir)
 			}
 		}
+		// Lstat so a symlink (even a dangling one) counts as existing; the
+		// later os.Create would otherwise follow it and truncate its target.
+		_, err := os.Lstat(toolCfg.CSVOutput)
+		if err == nil {
+			return fmt.Errorf("output path already exists: %s (it would be overwritten); choose a new --output path or remove the existing file", toolCfg.CSVOutput)
+		}
+		if !os.IsNotExist(err) {
+			return fmt.Errorf("cannot check output path %s: %w", toolCfg.CSVOutput, err)
+		}
 	}
 
 	// Validate CSV input path if provided

cmd/validators_test.go

@@ -347,6 +347,55 @@ func TestValidateFilePaths(t *testing.T) {
 			wantErr: true,
 			errMsg:  "output directory does not exist",
 		},
+		{
+			name: "CSV output file already exists",
+			setupFunc: func() func() {
+				p := filepath.Join(tmpDir, "existing.csv")
+				if err := os.WriteFile(p, []byte("keep"), 0644); err != nil {
+					t.Fatalf("Failed to create test file: %v", err)
+				}
+				toolCfg.CSVOutput = p
+				return func() { os.Remove(p) }
+			},
+			wantErr: true,
+			errMsg:  "output path already exists",
+		},
+		{
+			name: "CSV output path is an existing directory",
+			setupFunc: func() func() {
+				toolCfg.CSVOutput = tmpDir
+				return func() {}
+			},
+			wantErr: true,
+			errMsg:  "output path already exists",
+		},
+		{
+			name: "CSV output is a dangling symlink",
+			setupFunc: func() func() {
+				p := filepath.Join(tmpDir, "dangling.csv")
+				if err := os.Symlink(filepath.Join(tmpDir, "missing-target"), p); err != nil {
+					t.Skipf("symlinks unavailable: %v", err)
+				}
+				toolCfg.CSVOutput = p
+				return func() { os.Remove(p) }
+			},
+			wantErr: true,
+			errMsg:  "output path already exists",
+		},
+		{
+			name: "CSV input equals output",
+			setupFunc: func() func() {
+				p := filepath.Join(tmpDir, "same.csv")
+				if err := os.WriteFile(p, []byte("keep"), 0644); err != nil {
+					t.Fatalf("Failed to create test file: %v", err)
+				}
+				toolCfg.CSVOutput = p
+				toolCfg.CSVInput = p
+				return func() { os.Remove(p) }
+			},
+			wantErr: true,
+			errMsg:  "output path already exists",
+		},
 		{
 			name: "CSV input file does not exist",
 			setupFunc: func() func() {

docs/cli/README.md

@@ -87,7 +87,7 @@ All flags belong to the root command unless noted otherwise.
 
 | Flag | Short | Default | Description |
 |------|-------|---------|-------------|
-| `--output` | `-o` | auto-generated | Output CSV file path. Auto-name format: `ri-helper-dryrun-YYYYMMDD-HHMMSS.csv` or `ri-helper-purchase-YYYYMMDD-HHMMSS.csv`. |
+| `--output` | `-o` | auto-generated | Output CSV file path. Auto-name format: `ri-helper-dryrun-YYYYMMDD-HHMMSS.csv` or `ri-helper-purchase-YYYYMMDD-HHMMSS.csv`. An explicit path that already exists (file, directory or symlink) is refused instead of overwritten; choose a new path or remove the old file. |
 | `--input-csv` | `-i` | (none) | Read recommendations from an existing CSV instead of fetching from cloud APIs. The file must have a `.csv` extension and exist at the path given. The header must contain `Service`, `Region`, `ResourceType`, `Count`, `Term` and `PaymentOption`; a missing column is a load error that names it. `Term` must be `1yr` or `3yr` and `PaymentOption` must be `all-upfront`, `partial-upfront` or `no-upfront` on every row; a blank or unknown value fails the load with the row number. CSVs with only the older four columns are rejected. |
 
 ### Authentication