---
name: idempotent-file-replace-237c62cd9eaf
description: Read-then-diff before writing a file to ensure the write is skipped when the content already matches, preventing unnecessary modifications and downstream side effects.
---

# Idempotent File Replace

When tasked with **completely replacing** a file with specific content, always
perform a read-and-compare step before writing. Only write when the file
actually differs from the desired content.

## Why This Matters

| Risk of blind writes | Benefit of idempotent check |
|---|---|
| Triggers filesystem watchers / hot-reload loops | No spurious rebuild when content is unchanged |
| Wastes a tool call on a no-op | One extra read saves a write + all downstream costs |
| Pollutes VCS history with empty diffs | Clean commit history |
| Breaks CI caching layers | Stable mtimes keep caches valid |

---

## Procedure

### Step 1 — Read the current file

Always read the file first, even when you are confident about the replacement.

```
read_file(path="<target_file>")
```

If the file does **not yet exist**, treat its current content as an empty
string and proceed to Step 3.

---

### Step 2 — Compare current content to desired content

Perform an exact string comparison (whitespace-sensitive).

```python
current  = <content returned by read_file>
desired  = <full replacement content>

needs_write = (current.strip() != desired.strip())
# Use .strip() to tolerate a single trailing newline difference,
# or compare verbatim if byte-exact output is required.
```

Alternatively, a unified diff gives a human-readable explanation of what
would change and is useful for logging:

```python
import difflib, sys

diff = list(difflib.unified_diff(
    current.splitlines(keepends=True),
    desired.splitlines(keepends=True),
    fromfile="current",
    tofile="desired",
))

if not diff:
    print("Files are identical — skipping write.")
    needs_write = False
else:
    print("Diff detected:\n" + "".join(diff))
    needs_write = True
```

---

### Step 3 — Conditional write

```python
if needs_write:
    write_file(path="<target_file>", content=desired)
    print("File written.")
else:
    # Report completion immediately — no write needed.
    print("File already matches desired content. No action taken.")
```

---

## Decision Flowchart

```
Task: "Replace <file> with <content>"
        │
        ▼
   read_file(<file>)
        │
        ▼
  content == desired?
   ┌────┴────┐
  YES       NO
   │         │
   ▼         ▼
 SKIP     write_file(<file>, desired)
 write        │
   │          ▼
   └──► Report COMPLETE
```

---

## Complete Example

```python
TARGET = "src/components/Panel.ts"
DESIRED_CONTENT = """// Auto-generated — do not edit
export const Panel = () => { ... };
"""

# Step 1: read
current = read_file(path=TARGET)          # tool call

# Step 2: compare
if current.strip() == DESIRED_CONTENT.strip():
    print(f"{TARGET} already matches. Skipping write.")
    # → COMPLETE
else:
    # Step 3: write
    write_file(path=TARGET, content=DESIRED_CONTENT)   # tool call
    print(f"{TARGET} updated.")
    # → COMPLETE
```

---

## Edge Cases

| Situation | Handling |
|---|---|
| File does not exist | `read_file` raises / returns empty → treat as `""` → always write |
| Encoding differences | Normalize to UTF-8 before comparison |
| Line-ending differences (CRLF vs LF) | Normalize with `.replace("\r\n", "\n")` before comparing |
| Byte-exact requirement | Skip `.strip()` and compare verbatim |
| Large files | Compute SHA-256 hash of both strings for efficiency before full diff |

---

## Key Principle

> **Never assume a file needs to be written. One cheap read prevents an
> expensive (and potentially harmful) write.**

This pattern applies to any "replace entire file" task regardless of file
type: source code, configuration, templates, lock files, generated assets,
etc.

---
Source attribution: AG064
Original project: https://github.com/AG064/OpenSpace/blob/ff671ef61ed6587c49bd54c81cdc751c0b99e363/showcase/skills/idempotent-file-replace/SKILL.md
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Mega AI Hub.

## Upstream license notice

MIT License

Copyright (c) 2026 ✨Data Intelligence Lab@HKU✨

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Mega AI Hub resource metadata

Creator / publisher: AG064
Original project: https://github.com/AG064/OpenSpace/blob/ff671ef61ed6587c49bd54c81cdc751c0b99e363/showcase/skills/idempotent-file-replace/SKILL.md
Attribution basis: Source repository record

Fields of work: Development & Engineering
Roles: Backend Developer, DevOps / SRE, QA / Test Engineer
Specialties: Backend
Tools: Filesystem
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: 1f5b79b8a753eb35655222da084425b5742d8889e7eb0c93e0c8558cfcf74547