Hi:
I use grep to search string in a message like this.
> [root@office tmp]# echo "axyz" | grep [abcd]xyz
> axyz
> [root@office tmp]# echo "cxyz" | grep [abcd]xyz
> cxyz
> [root@office tmp]# echo "bxyz" | grep [abcd]xyz
> bxyz
> [root@office tmp]# echo "dxyz" | grep [abcd]xyz
> dxyz
> [root@office tmp]# echo "oxyz" | grep [abcd]xyz
> [root@office tmp]#
>
but I find something strange ,when I create a file with special name in
current diretory, the command execute failed:
> [root@office tmp]# touch axyz
> [root@office tmp]# echo "bxyz" | grep [abcd]xyz
> [root@office tmp]# echo "cxyz" | grep [abcd]xyz
> [root@office tmp]# echo "dxyz" | grep [abcd]xyz
> [root@office tmp]# rm axyz
> rm: remove regular empty file 'axyz'? y
> [root@office tmp]# echo "bxyz" | grep [abcd]xyz
> bxyz
> [root@office tmp]# echo "cxyz" | grep [abcd]xyz
> cxyz
> [root@office tmp]# echo "dxyz" | grep [abcd]xyz
> dxyz
>
I dont know why? Is it a feature or a bug?
And another problem, when the files number is too big in directory, the
grep with bracket execute slow perfermance:
[root@office tmp]# cd greptest/
> [root@office greptest]# ls -l | wc -l #this dir has 50000
> files
> 50001
> [root@office greptest]# time echo "axyz" | grep [abcd]xyz
> axyz
>
> *real 0m0.014s*
> user 0m0.007s
> sys 0m0.008s
> [root@office greptest]# time echo "axyz" | grep [abcd]xyz
> axyz
>
> real 0m0.013s
> user 0m0.003s
> sys 0m0.010s
> [root@office greptest]# time echo "axyz" | grep [abcd]xyz
> axyz
>
> real 0m0.014s
> user 0m0.007s
> sys 0m0.008s
> [root@office greptest]# cd .. # go to upper dir with small number of
> files
> [root@office tmp]# time echo "axyz" | grep [abcd]xyz
> axyz
>
> *real 0m0.003s*
> user 0m0.001s
> sys 0m0.003s
> [root@office tmp]#
>
if I don't use bracket regex, the perfermance in dirs with different
number of files are similer.
in my mind, the cmd like ( echo message | grep [abcd]efg )
is uncorrelated with local filesystems
but I dont know what happened in this 2 cases. It seems grep do something
with filesystems
why and how to avoid it?
Thank you very very much
--
H.Huang
