Golang: find string in file and show line number

Question

read, err := ioutil.ReadFile(path)
if err != nil {
    return err
}

if strings.Contains(string(read), "STRING") {
    // display line number?
    // what if many occurrences of the string
    // how to display for each the line number?
}

I'm trying to search files for a specific string and display the line number at which the string is located.

Answer 1

Use scanner to iterate over the file, line-by-line, incrementing your line count on each loop.

e.g.

f, err := os.Open(path)
if err != nil {
    return 0, err
}
defer f.Close()

// Splits on newlines by default.
scanner := bufio.NewScanner(f)

line := 1
// https://golang.org/pkg/bufio/#Scanner.Scan
for scanner.Scan() {
    if strings.Contains(scanner.Text(), "yourstring") {
        return line, nil
    }

    line++
}

if err := scanner.Err(); err != nil {
    // Handle the error
}

Update: if you need to do this across 'thousands of files' (as per the comment on another answer), then you would wrap this approach in a worker pool and run this concurrently.

Answer 2

Hi Try this out.

    /* ioutil.ReadFile returns []byte, error */
    data, err := ioutil.ReadFile("output.txt")
    /* ... omitted error check..and please add ... */
    /* find index of newline */
    file := string(data)
    line := 0
    /* func Split(s, sep string) []string */
    temp := strings.Split(file, "\n")

    for _, item := range temp {
        fmt.Println("[",line,"]\t",item)
        line++
    }

Basically, it will read file using ioutil package and find out newline from the contents. Strings packages is very useful when you find a certain character or newlines and so on. Take a look at GO's official webpage for further note and examples. (https://golang.org/pkg/strings/#Split).

I made a "output.txt" from (https://golang.org/pkg/strings/#Split) above and expected output should be

[ 0 ]    
[ 1 ]    Examples
[ 2 ]    
[ 3 ]        Contains
[ 4 ]        ContainsAny
[ 5 ]        Count
[ 6 ]        EqualFold
[ 7 ]        Fields
[ 8 ]        FieldsFunc
[ 9 ]        HasPrefix
[ 10 ]       HasSuffix
...

Regarding the format, you can modify fmt.Println("[",line,"]\t",item) with fmt.Printf(