Re: ls -l|head seems to look at all files in directory

[Jim Meyering]

Kamil Dudka wrote:
> On Monday 25 of May 2009 15:43:50 Reuben Thomas wrote:
>> Hi,
>>
>> If I do "ls -l |head" on a directory with many files in it, it takes a long
>> time to complete, suggesting that it has read the entire directory. Since
>> no sorting is involved, why has it done this?
>
> If you want to turn off sorting, try the -U option.

That's a good start.
To do what he wants you have to know that ls -1U is the only
way to get one output entry per readdir call.

Reuben, you want to do it like this:

  ls -1U|head|xargs ls -l

Note the difference:
(#files, create time(s), time to list first 10(s))

$ for i in 10000 50000 100000 200000; do printf "$i "; mkdir $i &&
(cd $i && seq $i| env time --format %e xargs touch 2>&1 | tr '\n' ' ' ) &&
(cd $i && env time --format %e ls -lU|head>/dev/null) &&
(cd $i && seq $i|xargs rm -f); rmdir $i; done
10000 0.38 0.08
50000 1.92 0.37
100000 3.81 0.73
200000 7.60 1.47

$ for i in 10000 50000 100000 200000; do printf "$i "; mkdir $i &&
(cd $i && seq $i| env time --format %e xargs touch 2>&1 | tr '\n' ' ' &&
 ls -1U|head|env time --format %e xargs ls -l>/dev/null &&
 seq $i|xargs rm -f) && rmdir $i; done
10000 0.39 0.00
50000 1.91 0.00
100000 3.81 0.00
200000 7.63 0.00