.. highlight:: go

catn.go
=======

::

    // catn.go

    /*

    In the following code, panic us used instead of os.Exit to quit the
    program. Calling panic ensures deferred functions are called. Calling
    os.Exit skips calling deferred functions.

    See https://groups.google.com/forum/#!topic/golang-nuts/_Twwb5ULStM for
    discussion of how to get both error codes and deferred functions to work
    correctly.

    */
    package main

    import (
        "bufio"
        "fmt"
        "os"
        "strings"
    )

    // Split a file name string like "conf.py" into strings "conf" and "py".
    func splitAtLastDot(name string) (string, string) {
        dotLoc := strings.LastIndex(name, ".")
        if dotLoc == -1 {
            return name, ""
        }
        return name[:dotLoc], name[dotLoc+1:]
    }

    func main() {
        //
        // check number of command-line arguments
        //
        nargs := len(os.Args) // os.Args contains command-line arguments
        if nargs != 2 {
            fmt.Println("Error: exactly one filename argument must be provided")
            panic("exited")
        }
        inname := os.Args[1]

        //
        // open the input file
        //
        infile, err := os.Open(inname)
        defer infile.Close() // run this when the function is done

        if err != nil {
            fmt.Printf("Error: cannot open file \"%s\" (%s)", inname, err)
            panic("exited")
        }

        //
        // open the output file
        //
        n, ext := splitAtLastDot(inname)
        outname := n + "-lines." + ext
        outfile, err := os.Create(outname) // deletes existing file with same name
        defer outfile.Close() // run this when the function is done
        if err != nil {
            fmt.Printf("Error: cannot create file \"%s\" (%s)", outname, err)
            panic("exited")
        }

        //
        // create the writer
        //
        writer := bufio.NewWriter(outfile)
        defer writer.Flush() // run this when the function is done

        //
        // process the files line by line with a scanner
        //
        scanner := bufio.NewScanner(infile)
        num := 1
        for scanner.Scan() {
            line := scanner.Text()
            // fmt.Printf("%5d  %s\n", num, line) // uncomment to print to screen
            fmt.Fprintf(writer, "%5d  %s\n", num, line)
            num++
        }
        fmt.Println("Results written to " + outname + ".")
    }