New subject: head/sed/tail (was The Unix shell: a 50-year view)

16 Jul 2021

...
 > -r is weird because it enables backwards reading,
but only as
> limited by count. Better would be a program, say revfile, that simply
> reads backwards by lines. Then tail p  has an elegant implementation:
>      revfile p | head | revfile 
...
  tail -n can be smarter in that it can simply read the
last K bytes
 and see if there are n lines. If not, it can read back further.
 revfile would have to read the whole file, which could be a lot
 more than n lines! tail -n < /dev/tty may never terminate but it
 will use a small finite amount of memory. 
Revfile would work the same way. When head has seen enough
and terminates, revfile will get SIGPIPE and stop. I agree that,
depending on scheduling and buffer management, revfile might
read more than tail -n,  but  it wouldn't read the whole of a
humongous file.

Doug

Re: [TUHS] head/sed/tail (was The Unix shell: a 50-year view)