From: Joerg Bruehe (joerg@sql.de)
Date: Thu Jun 27 2002 - 05:35:31 EDT
Dear admins,
as usual on this list, I got several replies to my question -
thank you to all those (details further down).
The short summary:
The Tru64 "xargs" builds command lines no longer than 2 kB,
that applies to versions 4 as well as 5, and that behaviour
can not be changed (by the user / admin).
Now the details:
I had wanted to get a list of recently modified files (from
several levels in a directory hierarchy) by the command
find . -type f -mtime -3 -print | xargs ls -ltrd | more
but got my 551 files (size of "find" output: 13005 byte)
in seven sorted groups, due to seven separate calls of "ls".
Getting seven separate calls destroys the fuction I wanted,
because I do not get a date/time order of _all_ files.
I has asked why "xargs" generates these short lines when
the system supports longer ones (proof:
ls -ltrd `find . -type f -mtime -3 -print`
works perfect) and whether I had a chance to change that.
Fred Cassirer (of HP) proposed to let "find" produce the "ls"
output immediately, without "xargs", by
find . -type f -mtime 3 -ls
However, in that way I do not get any date/time sorting.
Mike ? ( mcaplin@miami.edu ) proposed to use "echo"
to see the command lines generated by "xargs"
(good idea, I could thus verify my theory of the seven calls)
and also pointed at the "-i" option for explicit placement of
the "xargs" input in the generated command line
(does not change the size, so the problem remains).
Lucien Hercaud used "truss" to trace "xargs", he found that it
reads the input pipe in 8 kB portions (the "PIPSIZ" value) and
"does output gathering of the args in blocks of -a bit- less
than 2k (2048). ... This behaviour is the same for version
4.0 and 5.1A of Tru64."
(So the problem will remain the same in the next version.)
Dr Thomas Blinn comments
"I can't tell you why there is such a small limit..
I'm sure there is a historical reason for it."
(with no change in version 5) and also proposes to perform the
task by writing a shell, C, or perl script / program.
Dr Tim Cutts comments "xargs is being conservative" and proposes
GNU xargs whose limit is 20 k (or a Perl script, which does not
have any such limit). If I run into this situation more often,
I will probably get and install the GNU version.
Let me repeat my personal point of view:
Given the fact that Tru64 supports much longer command lines
- in the OS (see '/usr/sys/include/sys/syslimits.h':
the ARG_MAX limit for "exec" calls is 38 kB = 38912) and
- in shells (the single command line
ls -ltrd `find . -type f -mtime -3 -print`
does work, where this "find" yields 13,005 bytes),
I am disappointed that "xargs" does impose such a low limit
(for which I see no technical reason).
I would rather accept / expect a shell with a line length limit
(due to it keeping history, providing command line edit, doing
variable substitution, getting interactive input or files where
the editor has a restriction, ...) than this one in "xargs".
Regards and thanks for all answers,
Joerg Bruehe
--
Joerg Bruehe, SQL Datenbanksysteme GmbH, Berlin, Germany
(speaking only for himself)
mailto: joerg@sql.de
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