Streaming Commands: Outputting Progress in Real-Time
In this scenario, you aim to stream the output of an executed command both to the parent process and to a log file. However, the standard cmd.StdoutPipe returns the final result as a string, which becomes a limitation for long-running processes.
Exploring the Solution
The provided code, which utilizes exec.Command, StdoutPipe, and bufio.NewScanner, works as intended. It effectively streams the output of the child process, printing it to the console and logging it in real-time. The live output of the child process is captured and displayed as it occurs.
Troubleshooting Tips
If the code doesn't work for you, consider the following reasons:
- Missing Newlines: The bufio.NewScanner reads entire lines, only returning data when encountering newline characters. If the command doesn't print newlines, the output may not be streamed immediately.
- Command Output Buffer: Processes have default buffer sizes for stdout and stderr. If the buffer is filled and not read, the child process may hang due to blocking writes. Therefore, always read both stdout and stderr to prevent this issue.
Possible Workarounds
If the command doesn't print newlines, there are alternative methods to stream the output:
- Read By Words: Set the scanner split function to bufio.ScanWords to read individual words instead of lines.
- Read By Characters: Set the scanner split function to bufio.ScanRunes to read characters one by one.
- Manual Reading: Read the stdout byte by byte or rune by rune, ensuring the UTF-8 encoding is handled correctly.
It's important to remember that if the child process writes to its default stdout and stderr streams, they will be discarded unless explicitly read. Reading these streams ensures that you don't miss any output or error messages, even if the child process doesn't typically output to these streams.
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