cut: pipe reads fully buffered where GNU allows line-based buffering

(This has been forwarded from Launchpad Bug #2142228)

Problem

Piping output using stdubf -oL and cut leads to the output being read fully buffered in uutils, whereas GNU allows line-based buffering for cut.

Reproduce

Run a command producing output with a given interval, pipe it to line-buffered stdbuf and cut, then pipe this further to cat.

GNU:

while sleep 1; do echo hello,world; done | stdbuf -oL gnucut -f1 -d, | cat

A line saying "hello" is output every second.

uutils:

while sleep 1; do echo hello,world; done | stdbuf -oL cut -f1 -d, | cat

No output is produced until the buffer is full (takes up to a few minutes), whereupon a large number of lines with "hello" are output.

Note: the sleep time can be reduced to very low numbers (0.001) to produce output quicker and confirm the type of buffering used.

System

• OS release: Ubuntu 26.04 (Development branch)
• uutils coreutils version: 0.7.0-0ubuntu1

Start by reproducing the GNU and uutils commands in the issue, using the reduced sleep interval to observe buffering. Trace the cut command's input and output handling, then verify that piping through stdbuf -oL produces one "hello" line per second rather than waiting for a full buffer; the issue names no files or tests.