Golang Read File Line by Line

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We can use Golang “bufio” package along with the “os” package to read the contents of a file line by line.

The process to read a text file line by line include the following steps:

  • Use os.open() function to open the file.
  • Use bufio.NewScanner() function to create the file scanner.
  • Use bufio.ScanLines() function with the scanner to split the file into lines.
  • Then use the scanner Scan() function in a for loop to get each line and process it.

Here is the complete code to read a text file line by line and print in Golang.

package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {

	readFile, err := os.Open("data.txt")
 
	if err != nil {
		fmt.Println(err)
	}
	fileScanner := bufio.NewScanner(readFile)

	fileScanner.Split(bufio.ScanLines)
 
	for fileScanner.Scan() {
		fmt.Println(fileScanner.Text())
	}
 
	readFile.Close()
}

Note: The data.txt file should be in the same directory as the above program, else you will get an error printed as “open data.txt: no such file or directory”.

Reading File line by line to String Array

The above program has two issues:

  • The text file path is hardcoded, let’s read it from the command line argument.
  • We are processing the file lines, which is keeping our resources open for longer time. We can read the file data into a string array and then process it accordingly. This is not recommended for a very large file though.

Here is the updated code to read file path from command line and save its lines into a string array.

package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {

	filePath := os.Args[1]
	readFile, err := os.Open(filePath)
 
	if err != nil {
		fmt.Println(err)
	}
	fileScanner := bufio.NewScanner(readFile)
	fileScanner.Split(bufio.ScanLines)
	var fileLines []string
 
	for fileScanner.Scan() {
		fileLines = append(fileLines, fileScanner.Text())
	}
 
	readFile.Close()
 
	for _, line := range fileLines {
		fmt.Println(line)
	}
	
	fmt.Println(fileLines)
}

I have saved the above file as “read_file.go”. Let’s see how to run it from the command line.

 % go run read_file.go data.txt
Hello World
I love Go Programming
Bye Bye
[Hello World I love Go Programming Bye Bye]
%

Conclusion

It’s very easy to read a file in Go programming. We can read the file contents line by line and process them or we can store into a string array and process them later on. We should always close the file resources as soon as our work is done.

Further Readings: Golang os package, Golang command-line arguments, Golang Download File.