Read and Write Files

Overview

Reading and writing files is one of the most common I/O tasks: loading configuration, processing logs, exporting reports, or persisting state. Doing it safely means handling encoding correctly and always closing the file handle.

This recipe shows the idiomatic way to read and write text files in Python, JavaScript (Node.js), and Bash.

When to Use

Use this recipe when:

• Loading configuration or data files at startup
• Generating reports, exports, or logs
• Processing text line by line (CSV, logs, fixtures)
• Persisting small amounts of state without a database

Solution

Python

# Write
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("Hello, file!\n")

# Read
with open("notes.txt", "r", encoding="utf-8") as f:
    content = f.read()
print(content)

JavaScript

import { readFile, writeFile } from "node:fs/promises";

await writeFile("notes.txt", "Hello, file!\n", "utf-8");

const content = await readFile("notes.txt", "utf-8");
console.log(content);

Bash

# Write (overwrite) and append
echo "Hello, file!" > notes.txt
echo "Another line" >> notes.txt

# Read
cat notes.txt

Explanation

• Python uses the with statement (context manager) so the file is always closed, even on error. Always specify encoding="utf-8".
• JavaScript uses the promise-based fs/promises API. Prefer it over the synchronous readFileSync/writeFileSync, which block the event loop.
• Bash uses redirection: > overwrites, >> appends. cat prints the contents.

To turn file contents into structured data, see Parse JSON.

Variants

Language	Read	Write	Append
Python	open(p).read()	open(p, "w")	open(p, "a")
JavaScript	readFile(p)	writeFile(p, data)	appendFile(p, data)
Bash	cat p	> p	>> p

Best Practices

• Always set encoding: explicit utf-8 avoids platform-dependent defaults.
• Use context managers / async APIs: with in Python, fs/promises in Node, to avoid leaks and blocking.
• Check the path exists: handle missing files gracefully rather than crashing.
• Stream large files: read line by line instead of loading gigabytes into memory.
• Write atomically: write to a temp file then rename, to avoid corrupting data on crash.

Common Mistakes

• Forgetting to close the handle: leaks file descriptors; use with or try/finally.
• Blocking the event loop in Node: avoid readFileSync in request handlers.
• Wrong encoding: reading UTF-8 as ASCII corrupts non-English characters.
• Overwriting with >: using > instead of >> in Bash silently erases the file.
• Ignoring errors: a missing file or permission error should be handled, not swallowed.

Frequently Asked Questions

Q: How do I append instead of overwrite? A: Open in append mode: open(p, "a") in Python, appendFile in Node, or >> in Bash.

Q: Why should I avoid readFileSync in Node.js? A: It blocks the single-threaded event loop, freezing all other requests until the read completes. Use fs/promises instead.

Q: How do I read a large file without running out of memory? A: Stream it line by line — for line in f in Python, a readable stream in Node, or while read line in Bash.