Date: Sat, 05 Jun 2010 13:12:42 -0700 From: Bakul Shah <bakul@bitblocks.com> To: freebsd-hackers@freebsd.org Subject: head behaviour Message-ID: <20100605201242.C79345B52@mail.bitblocks.com>
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Consider:
$ yes | cat -n | (read a; echo $a; head -1)
1 y
2 y
$ yes | cat -n | (head -1; read a; echo $a)
1 y
456 y
As you can see, head reads far more than it should. This is
fine most of the time but often it results in surprising
output:
# print ps header and all lines with sh in it
$ ps|(head -1; grep sh)
PID TT STAT TIME COMMAND
# print first and last two lines
$ look xa | (head -2; tail -2)
xanthaline
xanthamic
Not quite what you expected, right?
Yes, you can use read and echo N times but this is not
as convenient as using head:
$ look xa | (read a; echo $a; read a; echo $a; tail -2)
xanthaline
xanthamic
xarque
Xaverian
The "fix" is to make sure head reads no more than $N bytes
where $N is the number of *remaining* lines to be read.
Yes this slows head down some but makes it more useful.
[Ideally all commands that quit after partially reading
their input ought to behave like this but that would slow
down their common use far too much]
Comments?
Thanks to Rob Warnock for pointing out the head problem.
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