命令行工具在输出后如何更改输出？ [英] How do command line tools change their output after outputting it?

问题描述

我注意到，许多命令行工具，例如，wget将显示进度作为一个数字或进度条，随着进程完成进度。虽然问题不是真正的语言特定的，我使用最常用的命令行工具（C ++，Node.js，Haskell）我没有看到一种方法这样做。

I've noticed that a lot of command line tools, wget for example, will show progress as a number or progress bar that advances as a process is completed. While the question isn't really language-specific, out of the languages I use most often for command line tools (C++, Node.js, Haskell) I haven't seen a way to do this.

下面是一个例子，一个单行终端的三个快照wget下载一个文件：

Here's an example, three snapshots of a single line of Terminal as wget downloads a file:

除了其他信息，wget会显示进度条（< =>）下载文件。到目前为止所下载的数据量（6363,179561,316053）和当前的下载速度（10.7KB / s，65.8KB / s，63.0KB / s）也被更新。

Along with other information, wget shows a progress bar (<=>) that advances as it downloads a file. The amount of data downloaded so far (6363, 179561, 316053) and the current download speed (10.7KB/s, 65.8KB/s, 63.0KB/s) update as well. How is this done?

理想情况下，请包含一种或多种上述三种语言的代码示例。

Ideally, please include a code sample from one or more of the three languages mentioned above.

推荐答案

只需打印一个CR（不带换行符）即可覆盖一行。下面是perl中的示例程序：

Just print a CR (without a newline) to overwrite a line. Here is an example program in perl:

#!/usr/bin/env perl

$| = 1;

for (1..10) {
  print "the count is: $_\r";
  sleep(1)
}

$ | = 1 ），以便打印命令立即将其输出发送到控制台，而不是缓冲。

I've also disabled output buffering ($| = 1) so that the print command sends its output to the console immediately instead of buffering it.

Haskell示例：

Haskell example:

import System.IO
import Control.Monad
import Control.Concurrent

main = do
  hSetBuffering stdout NoBuffering
  forM_ [1..10] $ \i -> do
    putStr $ "the count is: " ++ show i ++ "\r"
    threadDelay 1000000

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