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pipe output of one command to set /P

Posted on 2011-03-08
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Last Modified: 2012-05-11
I want the first line of a DIR output in an environment variable. I tried

dir *.bat /T:C /O:-D /B | set /P test= 

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That didn't set the variable. I got it running using the following command:

FOR /F "tokens=*" %I IN ('dir *.bat /T:C /O:D /B') DO SET test=%I) >NUL

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Since FOR will loop through the whole directory listing, I don't really like it and would prefer to use the piping instead. Going through an external file is not an option.
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Question by:cmnt
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9 Comments
 
LVL 23

Expert Comment

by:wdosanjos
ID: 35073546
You can try this. It basically inverts the sorting so the first line becomes the last for the FOR to work.

FOR /F "tokens=*" %I IN ('dir *.bat /T:C /O:-D /B') DO @SET test=%I

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Author Comment

by:cmnt
ID: 35073658
With the piping I intended to catch the first line of the DIR output. After running FOR the test variable will contain the last line. Therefore piping and FOR need to have opposite sorting order. The FOR works as expected. The piping does not work at all. I am looking for a solution using piping, so I don't need to go through all entries in the folder. The folder may contain several hundred entries and I am only concerned about the newest one.
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Author Comment

by:cmnt
ID: 35073883
BTW, I missed one character in the second code line in my question. The correct line needs to be

(FOR /F "tokens=*" %I IN ('dir *.bat /T:C /O:D /B') DO SET test=%I) >NUL

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Of course, I can leave out the outer parenthesis with the >NUL and just use @SET instead of SET as wdosanjos suggested. The sort order must be as stated in my question.
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LVL 16

Accepted Solution

by:
sjklein42 earned 300 total points
ID: 35074221
test.bat

Exits loop after first file found (and set)

usage:

call test.bat >nul:

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@FOR /F "tokens=*" %%I IN ('dir *.bat /T:C /O:D /B') DO (
@set test=%%I
@goto :EOF
)

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LVL 70

Assisted Solution

by:Qlemo
Qlemo earned 200 total points
ID: 35074833
To answer the original question: Piping into set /P never works. You can only redirect a file. The reason for that is unknown, but it has been tried what has to be one google times, I suppose, to no avail.

There is indeed no other way than to use a file, or process the command stream with FOR.
Just to add another one (which isn't better or worse - this is the non-batch version):
>nul (set test=& for /F "delims=" %F in ('dir *.bat /t:c /o:-d /b') do if not defined test set test=%F)

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sjklein42,
You need to use the reversed sort for that, as used in the very first code line shown by the asker.
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LVL 23

Expert Comment

by:wdosanjos
ID: 35075134
@cmnt,

> I am looking for a solution using piping, so I don't need to go through all entries in the folder.
> The folder may contain several hundred entries and I am only concerned about the newest one.

I think there is no way to avoid the IO required to iterate through all the entries in the folder.  First, dir has to find all entries that end with .bat, and then it needs retrieve the create dates for the sort.  By the time dir starts displaying the results all the IO on the folder is already done.
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LVL 70

Expert Comment

by:Qlemo
ID: 35075300
wdosanjos is correct about that. Unlike in UNIX or Linux, a process writing into a pipe is always executed completely before passing the output to the pipe consumer. The dir is performed completely.
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LVL 56

Expert Comment

by:Bill Prew
ID: 35075890
Not much different than what has already been discussed, but here's how I typically handle this in a batch file:

FOR /F "tokens=*" %%I IN ('dir *.bat /T:C /O:-D /B') DO SET test=%%I & goto DONE
:DONE

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But behind the scenes DOS still does have to read all the files in the directory, filter out just the BAT ones, and then sort by date.  There's no way to get the most recent one "directly".

~bp
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Author Comment

by:cmnt
ID: 35082795
Thanks everyone for their comments. I will go with the FOR...GOTO combination. Regarding the io issue I was a little bit unclear. I wasn't concerned about the Dir command, which I expected to go through the whole directory. I just didn't wanted to iterate through the whole result set.
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