How Can I Stream Partial Command Output in Real-Time During Execution?

Streaming Command Output Progress

In the process of executing commands, the need often arises to stream the output both to its designated output location and a log file in real-time. However, when dealing with lengthy processes, the cmd.StdoutPipe method only provides the final result as a string. This article delves into ways to enable streaming of partial output during the command execution process.

The example provided in the question utilizes bufio.NewScanner to read the standard output via a pipe. A common misconception is that it reads line by line. In reality, it reads input by newlines as defined by the bufio.ScanLines function. If the command executed does not produce newlines, the output won't be returned immediately.

Workarounds:

To overcome this limitation, consider alternative approaches:

1. bufio.Split(bufio.ScanRunes): This method allows reading by runes (characters), ensuring the return of output whenever a new rune becomes available.
2. Manual Byte/Rune Reading: It involves creating a buffer array and repeatedly reading either one byte or a byte slice representing a complete rune.

It's crucial to read both standard output and error to prevent potential blocking in the child process due to full buffers. By default, the child process's standard output and error streams are discarded. However, missing error messages may occur if the standard error stream is not read.

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