How can I 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)

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.

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'
}