Reading and writing files are basic tasks needed for
many Go programs. First we’ll look at some examples of
reading files. | |
|  package main
|
| import (
"bufio"
"fmt"
"io"
"os"
)
|
Reading files requires checking most calls for errors.
This helper will streamline our error checks below. | func check(e error) {
if e != nil {
panic(e)
}
}
|
| func main() {
|
Perhaps the most basic file reading task is
slurping a file’s entire contents into memory. | dat, err := os.ReadFile("/tmp/dat")
check(err)
fmt.Print(string(dat))
|
You’ll often want more control over how and what
parts of a file are read. For these tasks, start
by
Open
ing a file to obtain an
os.File
value. | f, err := os.Open("/tmp/dat")
check(err)
|
Read some bytes from the beginning of the file.
Allow up to 5 to be read but also note how many
actually were read. | b1 := make([]byte, 5)
n1, err := f.Read(b1)
check(err)
fmt.Printf("%d bytes: %s\n", n1, string(b1[:n1]))
|
You can also
Seek
to a known location in the file
and
Read
from there. | o2, err := f.Seek(6, io.SeekStart)
check(err)
b2 := make([]byte, 2)
n2, err := f.Read(b2)
check(err)
fmt.Printf("%d bytes @ %d: ", n2, o2)
fmt.Printf("%v\n", string(b2[:n2]))
|
Other methods of seeking are relative to the
current cursor position, | _, err = f.Seek(4, io.SeekCurrent)
check(err)
|
and relative to the end of the file. | _, err = f.Seek(-10, io.SeekEnd)
check(err)
|
The
io
package provides some functions that may
be helpful for file reading. For example, reads
like the ones above can be more robustly
implemented with
ReadAtLeast
. | o3, err := f.Seek(6, io.SeekStart)
check(err)
b3 := make([]byte, 2)
n3, err := io.ReadAtLeast(f, b3, 2)
check(err)
fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))
|
There is no built-in rewind, but
Seek(0, io.SeekStart)
accomplishes this. | _, err = f.Seek(0, io.SeekStart)
check(err)
|
The
bufio
package implements a buffered
reader that may be useful both for its efficiency
with many small reads and because of the additional
reading methods it provides. | r4 := bufio.NewReader(f)
b4, err := r4.Peek(5)
check(err)
fmt.Printf("5 bytes: %s\n", string(b4))
|
Close the file when you’re done (usually this would
be scheduled immediately after
Open
ing with
defer
). | f.Close()
}
|