This is a classic buffering issue in Linux pipelines. The problem occurs because pv is writing to a pipe, and by default, pipes are fully buffered (typically 64KB or more). Since /dev/zero is an infinite stream, pv keeps filling the buffer without flushing, so you don't see any progress until the buffer is full or the process is interrupted.

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Solutions

1. Force pv to use line buffering

Use the --line-mode (-l) flag to make pv flush its output after each line (or block, depending on your version). This is the simplest and most reliable fix:

bash
Copy
diff <(pv -l /dev/zero) /dev/zero

2. Use stdbuf to override buffering

If pv doesn't support --line-mode or you want more control, use stdbuf to force line buffering for pv:

bash
Copy
diff <(stdbuf -oL pv /dev/zero) /dev/zero

• -oL sets line buffering for stdout.

3. Use unbuffer (from expect)

If stdbuf is unavailable (e.g., on older systems), install expect and use unbuffer:

bash
Copy
diff <(unbuffer pv /dev/zero) /dev/zero

4. Use socat as a buffer flusher

If you have socat installed, you can use it to force unbuffered output:

bash
Copy
diff <(pv /dev/zero | socat -u - STDIO) /dev/zero

• -u disables buffering in socat.

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Why This Happens

• By default, pv inherits the buffering behavior of the pipe.
• diff reads both inputs in chunks, so pv's output is stuck in the buffer until diff reads it or the buffer fills up.
• Forcing line buffering (-l or stdbuf -oL) ensures pv flushes its output frequently, so you see progress in real time.

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Try the first solution (pv -l) first—it’s the cleanest and most portable. Let me know if you need further adjustments!