Randall R Schulz wrote:
On Tuesday 14 November 2006 07:16, Geoffrey wrote:
Randall R Schulz wrote:
On Tuesday 14 November 2006 06:48, Geoffrey wrote:
TheOldWiseKing wrote:
S A
I found a good solution, here is the command:
ls > file1.txt | cat file1.txt > file2.txt
Not so wise OldWiseKing. :)
I don't know what the pipe is for, but I don't think that will
work. file2.txt will likely end up empty.
No, it won't. When cat is given command-line file name arguments it
ignores the standard input.
Yes, sometimes it will. Do you think I posted without testing it
myself?
What I said was true. As was the result you report.
No, you said "No, it won't", my results indicate it will. See your own
words above.
Results are unreliable. I tried this 10 times, twice file2.txt was
empty, the other 8 it contained the same data as file1.txt. Cut and
paste from my latest attempt:
Had I stopped to think about it a moment longer, I'd have seen the
problem.
The shell launches every command in a pipeline concurrently. That means
the cat command in that pipeline can easily run before the ls completes
and thus get thing from total ls output to partial output to none at
all.
rhws -> ls > file1.txt | cat file1.txt > file2.txt
rhws -> ls -l file?.txt
-rw-r--r-- 1 esoteric esoteric 14168 Nov 14 10:11 file1.txt
-rw-r--r-- 1 esoteric esoteric 14168 Nov 14 10:11 file2.txt
I'll not explain why this happens,
But I did, above.
but it has to do with the improper
use of the pipe symbol. If you research how the command line is
parsed you will understand why the results is unrealiable. The
bottom line is, you don't want that solution, tee is your friend
here.
It's not so much the parsing as it is the execution. And it's not
an "improper" use, just an unuseful one.
Come on, there will be no output to standard out by 'ls > file1.txt' so
it's not proper usage. It can produce unwanted results. What more do
you need to say it's improper usage? Argue for the sake of arguing?
--
Until later, Geoffrey
Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety.
- Benjamin Franklin
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