Force line-buffering of stdout in a pipeline

you can try stdbuf

$ stdbuf --output=L ./a | tee output.txt

(big) part of the man page:

  -i, --input=MODE   adjust standard input stream buffering
  -o, --output=MODE  adjust standard output stream buffering
  -e, --error=MODE   adjust standard error stream buffering

If MODE is 'L' the corresponding stream will be line buffered.
This option is invalid with standard input.

If MODE is '0' the corresponding stream will be unbuffered.

Otherwise MODE is a number which may be followed by one of the following:
KB 1000, K 1024, MB 1000*1000, M 1024*1024, and so on for G, T, P, E, Z, Y.
In this case the corresponding stream will be fully buffered with the buffer
size set to MODE bytes.

keep this in mind, though:

NOTE: If COMMAND adjusts the buffering of its standard streams ('tee' does
for e.g.) then that will override corresponding settings changed by 'stdbuf'.
Also some filters (like 'dd' and 'cat' etc.) dont use streams for I/O,
and are thus unaffected by 'stdbuf' settings.

you are not running stdbuf on tee, you’re running it on a, so this shouldn’t affect you, unless you set the buffering of a‘s streams in a‘s source.

Also, stdbuf is not POSIX, but part of GNU-coreutils.

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