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    If your input has a huge number of lines that come in fast, you can speed this up by only updating the count every 10 lines (NR % 10 == 0 { printf ...}), and printing the exact count at the end. Even more fancy would be to print an update when a line comes in only if it's been 100 ms since the last print, maybe with an if() inside the rule. But +1, this is a good simple starting point that's sufficient for some use-cases, like commands that produce lines somewhat slowly, or if terminal updates aren't a bottleneck. Commented Nov 2, 2022 at 19:19
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    I’ve added those variants to the answer, thanks. AWK (even GNU) doesn’t deal with sub-second intervals AFAICT, but every second or even every n seconds should be sufficient for a long-running job. Commented Nov 4, 2022 at 10:57
  • Or print a result only when the current line count has increased by say 20%, so it will be fast to begin with and slow down over time. This would be useful for potentially huge inputs where one can't even estimate a reasonable size. Marking each line of output with a timestamp might be useful too. Commented Nov 4, 2022 at 20:09