What is the use of "-u" option in cat command? [closed]
Per the POSIX standard for cat:
SYNOPSIS
cat [-u] [file...]...
OPTIONS
...
The following option shall be supported:
-uWrite bytes from the input file to the standard output without delay as each is read.
That could be implemented by disabling buffering on the output.
The idea behind cat -u is indeed that the output should be unbuffered, so that even if cat is in a pipeline, the data will be written promptly as it is read.
It can matter when you use cat -u "$@" | … and the input is, in fact, coming from a keyboard.
The chances are that GNU cat effectively works without buffering (using direct read() and write() calls), so the -u option is irrelevant — it always works in 'unbuffered mode'.