Go – Truncate File with os.Truncate: Syntax, Examples & Safety (2025 Guide)

Introduction – Why Truncate a File in Go?

In Go, you can truncate (cut down) a file using the built-in os.Truncate() function. This is useful when you want to:

• Clear the contents of a log or temp file
• Limit file size for memory or disk usage
• Reset a file without deleting it

In this guide, you’ll learn:

• How to truncate a file in Go
• Set file size to 0 or custom length
• Handle permission and file-not-found errors
• Real-world use cases and safety tips

Example – Truncate File to Zero Bytes

package main

import (
    "fmt"
    "os"
)

func main() {
    err := os.Truncate("log.txt", 0)
    if err != nil {
        fmt.Println("Truncate failed:", err)
    } else {
        fmt.Println("File truncated successfully.")
    }
}

Output:

File truncated successfully.

This clears all content in log.txt by setting its size to 0.

Truncate File to Custom Size

err := os.Truncate("data.txt", 10)

Keeps the first 10 bytes of the file and discards the rest.
If the file is smaller than 10 bytes, it will be padded with null bytes.

Check If File Exists Before Truncating

if _, err := os.Stat("log.txt"); os.IsNotExist(err) {
    fmt.Println("File does not exist")
} else {
    os.Truncate("log.txt", 0)
}

Prevents unnecessary os.Truncate call on non-existent files.

Common Errors & Handling

Always validate path and permissions before truncating.

Alternative – Truncate via File Object

file, err := os.OpenFile("temp.txt", os.O_WRONLY, 0644)
if err != nil {
    panic(err)
}
defer file.Close()

err = file.Truncate(0)

Useful if you already have an os.File object open.

Best Practices

Summary – Recap & Next Steps

Go’s os.Truncate() offers a clean and efficient way to shrink or clear files. It’s great for log rotation, resetting cache, or cleaning up large output files without deleting them.

Key Takeaways:

• os.Truncate("file.txt", 0) clears the file
• You can keep the first N bytes with a custom size
• Handles padding or shrinking depending on target length
• Use error handling and file checks for safe operation

Next: Explore Appending to Files, File Rotation, or Managing Temp Files in Go.

FAQs – Go Truncate File

What does os.Truncate() do in Go?
It sets the file’s size to the specified number of bytes, trimming or padding as needed.

Will truncating a file delete it?
No. It clears the content, but the file still exists.

How can I reset a log file?
Use os.Truncate("log.txt", 0) to empty it.

Can I truncate a file that is being read by another process?
Yes, but be careful—other processes may crash or behave unpredictably.

What happens if the file doesn’t exist?
os.Truncate() returns an error. Use os.Stat() to check existence first.