| Reading and writing files are basic tasks needed for many Go programs. First we'll look at some examples of reading files. | ||
package main |
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import ( |
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"bufio" |
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"fmt" |
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"io" |
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"io/ioutil" |
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"os" |
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) |
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| Reading files requires checking most calls for errors. This helper will streamline our error checks below. | func check(e error) { |
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if e != nil { |
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panic(e) |
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} |
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} |
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func main() { |
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| Perhaps the most basic file reading task is slurping a file's entire contents into memory. | dat, err := ioutil.ReadFile("/tmp/dat") |
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check(err) |
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fmt.Print(string(dat)) |
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| 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") |
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check(err) |
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| 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) |
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n1, err := f.Read(b1) |
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check(err) |
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fmt.Printf("%d bytes: %s\n", n1, string(b1[:n1])) |
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| You can also `Seek` to a known location in the file and `Read` from there. | o2, err := f.Seek(6, 0) |
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check(err) |
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b2 := make([]byte, 2) |
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n2, err := f.Read(b2) |
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check(err) |
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fmt.Printf("%d bytes @ %d: ", n2, o2) |
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fmt.Printf("%v\n", string(b2[:n2])) |
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| 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, 0) |
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check(err) |
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b3 := make([]byte, 2) |
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n3, err := io.ReadAtLeast(f, b3, 2) |
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check(err) |
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fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3)) |
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| There is no built-in rewind, but `Seek(0, 0)` accomplishes this. | _, err = f.Seek(0, 0) |
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check(err) |
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| 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) |
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b4, err := r4.Peek(5) |
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check(err) |
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fmt.Printf("5 bytes: %s\n", string(b4)) |
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| Close the file when you're done (usually this would be scheduled immediately after `Open`ing with `defer`). | f.Close() |
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} |