Working with Files

Read, write, and manage files safely — the foundation of any tool that persists data to disk.

8 min readProgramming Foundations (Go)Code

Reading Files

For small files, os.ReadFile loads the entire content into memory in one call. For large files, open with os.Open and use a bufio.Scanner or io.Copy to stream the data.

Goreading-a-whole-file-vs-streaming.go

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`os.IsNotExist(err)` distinguishes 'file not there yet' from a real read error. `defer f.Close()` must appear immediately after a successful `os.Open` — if the open fails, there is nothing to close.

Writing Files

os.WriteFile writes a byte slice to a file, creating it if it does not exist and truncating it if it does. Use os.OpenFile with flags when you need to append or control permissions explicitly.

Gowriting-and-appending-to-files.go

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File permissions `0644` mean owner can read/write, group and others can read. `os.O_APPEND|os.O_WRONLY` opens for appending without truncating. `fmt.Fprintln(f, ...)` writes to any `io.Writer`, including a file.

File Paths and Directories

filepath.Join builds paths correctly for the current platform. os.MkdirAll creates nested directories. os.UserHomeDir and os.UserCacheDir give portable locations for config and data files.

Goportable-paths-and-directory-creation.go

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`os.MkdirAll` is idempotent — it creates all missing directories and does not error if they already exist. `filepath.Join` uses the OS-specific separator (`/` on Unix, `\` on Windows) automatically.

Atomic Writes

Writing to a file directly risks corruption if the program is interrupted mid-write. The safe pattern is: write to a temporary file, then rename it over the target. Rename is atomic on all modern filesystems.

Goatomic-file-replacement.go

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Creating the temp file in the same directory as the target ensures `os.Rename` is an in-place rename on the same filesystem — which is atomic. A cross-filesystem rename copies the file, which is not atomic.

Use atomic writes for any persistent data file

Power failures, OS crashes, and signal interrupts can terminate a program mid-write. A partially written JSON file is often unrecoverable. Atomic writes make your tool resilient to these events at essentially zero extra cost.

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