How to Stream Partial Output from a Long-Running Command in Go?

Streaming Command Output Progress

In scenarios where a long-running command is executed and its output needs to be continuously streamed both to the parent process and to a log file, the cmd.StdoutPipe() method can be utilized to capture the output.

cmd := exec.Command("sh", "-c", "some long runnig task")

stdout, _ := cmd.StdoutPipe()
cmd.Start()

scanner := bufio.NewScanner(stdout)
for scanner.Scan() {
    m := scanner.Text()
    fmt.Println(m)
    log.Printf(m)
}

cmd.Wait()

However, this approach only provides the final output as a single string, not streaming partial results.

Reasons for Limited Output

The bufio.NewScanner() function operates on lines, returning a complete line when a newline character is encountered. If the command being executed does not produce newline characters, the output will not be streamed immediately.

Possible Workarounds

To address this limitation, there are several approaches to consider:

1. Reading by Words or Characters:
   By setting a split function using Scanner.Split(), you can scan the input by words or characters, capturing output as it is produced.

   scanner := bufio.NewScanner(stdout)
   scanner.Split(bufio.ScanRunes)

2. Manual Reading:
   Reading byte-by-byte or rune-by-rune allows you to capture output as it is generated, without relying on newline characters.

   oneByte := make([]byte, 1)
   for {
       _, err := stdout.Read(oneByte)
       if err != nil {
           break
       }
       fmt.Printf("%c", oneByte[0])
   }
   
   oneRune := make([]byte, utf8.UTFMax)
   for {
       count, err := stdout.Read(oneRune)
       if err != nil {
           break
       }
       fmt.Printf("%s", oneRune[:count])
   }

Considerations

It is crucial to manage buffer sizes for standard output and error streams in child processes. By default, if these buffers are not read, they can cause the child process to hang. Therefore, it is recommended to always read both the stdout and stderr streams.

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