How can I read a whole file into a string variable

I have lots of small files, I don't want to read them line by line.

Is there a function in Go that will read a whole file into a string variable?

Use ioutil.ReadFile:

func ReadFile(filename string) ([]byte, error)

> ReadFile reads the file named by filename and returns the contents. A successful call
> returns err == nil, not err == EOF. Because ReadFile reads the whole file, it does not treat
> an EOF from Read as an error to be reported.

You will get a []byte instead of a string. It can be converted if really necessary:

s := string(buf)

Edit: the ioutil package is now deprecated: "Deprecated: As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be preferred in new code. See the specific function documentation for details." Because of Go's compatibility promise, ioutil.ReadMe is safe, but @openwonk's updated answer is better for new code.

If you just want the content as string, then the simple solution is to use the ReadFile function from the io/ioutil package. This function returns a slice of bytes which you can easily convert to a string.

Go 1.16 or later

Replace ioutil with os for this example.

package main

import (
    "fmt"
    "os"
)

func main() {
    b, err := os.ReadFile("file.txt") // just pass the file name
    if err != nil {
        fmt.Print(err)
    }

    fmt.Println(b) // print the content as 'bytes'

    str := string(b) // convert content to a 'string'

    fmt.Println(str) // print the content as a 'string'
}

Go 1.15 or earlier

package main

import (
    "fmt"
    "io/ioutil"
)

func main() {
    b, err := ioutil.ReadFile("file.txt") // just pass the file name
    if err != nil {
        fmt.Print(err)
    }

    fmt.Println(b) // print the content as 'bytes'

    str := string(b) // convert content to a 'string'

    fmt.Println(str) // print the content as a 'string'
}

I think the best thing to do, if you're really concerned about the efficiency of concatenating all of these files, is to copy them all into the same bytes buffer.

buf := bytes.NewBuffer(nil)
for _, filename := range filenames {
  f, _ := os.Open(filename) // Error handling elided for brevity.
  io.Copy(buf, f)           // Error handling elided for brevity.
  f.Close()
}
s := string(buf.Bytes())

This opens each file, copies its contents into buf, then closes the file. Depending on your situation you may not actually need to convert it, the last line is just to show that buf.Bytes() has the data you're looking for.

This is how I did it:

package main

import (
  "fmt"
  "os"
  "bytes"
  "log"
)

func main() {
   filerc, err := os.Open("filename")
   if err != nil{
     log.Fatal(err)
   }
   defer filerc.Close()
   
   buf := new(bytes.Buffer)
   buf.ReadFrom(filerc)
   contents := buf.String()

   fmt.Print(contents) 
   
}    

You can use strings.Builder:

package main

import (
   "io"
   "os"
   "strings"
)

func main() {
   f, err := os.Open("file.txt")
   if err != nil {
      panic(err)
   }
   defer f.Close()
   b := new(strings.Builder)
   io.Copy(b, f)
   print(b.String())
}

Or if you don't mind []byte, you can use
os.ReadFile:

package main
import "os"

func main() {
   b, err := os.ReadFile("file.txt")
   if err != nil {
      panic(err)
   }
   os.Stdout.Write(b)
}

For Go 1.16 or later you can read file at compilation time.

Use the //go:embed directive and the embed package in Go 1.16

For example:

package main

import (
    "fmt"
    _ "embed"
)

//go:embed file.txt
var s string

func main() {
    fmt.Println(s) // print the content as a 'string'
}