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Old 12-01-2011, 11:34 AM   #1
ranjitcool
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What is this line doing?


Hey Guys,

I know the command mkfs but what is this line doing stuff like - <fd.data> and am not 100% on what dev null has been for.


mkfs -t ext3 /dev/sdb1 < fd.data > /dev/null 2> /dev/null

Thanks
RJ
 
Old 12-01-2011, 11:42 AM   #2
MensaWater
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< fd.data
Redirects in from a file called fd.data


> /dev/null 2 >/dev/null
Redirect standard output (a/k/a stdout which is file descriptor 1) to /dev/null AND standard error (a/k/a stderr which is file descriptor 2) to /dev/null. This could be simplified (and usually is) by writing:
> /dev/null 2>&1
The 2>&1 says to redirect stderr to stdout.

It is important when using this form that you define stdout before doing the 2>&1 because default is to the screen.
So if you did instead:
2>&1 >/dev/null
It would send stderr to the screen (because stdout was screen when you set stderr) but then send stdout to /dev/null because you defined it after the fact.

Last edited by MensaWater; 12-01-2011 at 11:44 AM.
 
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Old 12-01-2011, 11:54 AM   #3
ranjitcool
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Thanks for the reply.

I see the fd.data file has l p n w q all in a separate line. Now does the command pass all these switches in one shot or one after the other?

Also when i execute it I don't see any output while executing, only after its done (this is one line from a script that i picked).

So /dev/null is saying - if you get errors display on screen? I got confused with your explanation.

Thanks
RJ
 
Old 12-01-2011, 12:10 PM   #4
kbscores
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/dev/null is like a trashcan. Usually used for output. For instance if you do a find and you don't want to see errors you'd redirect errors to /dev/null.

So with your statement since it has both stdin and stderr going to /dev/null nothing will be shown.

As for the file -- it could be one of two things fs-options or a list of bad blocks.
 
Old 12-01-2011, 02:40 PM   #5
MensaWater
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As the prior poster indicated /dev/null means "nowhere" essentially. This is called the "bit bucket". Redirecting to /dev/null is just a way to prevent output from showing up anywhere else - you're simply discarding it by sending it to the bit bucket.

I was saying that UNLESS you do a redirect the default for most stdout and stderr is the screen. Almost all redirects to /dev/null are simply to prevent output from appearing on the screen (or in log files when run in the background).

Redirects can be done TO or FROM files. In UNIX/Linux nearly everything is called a "file" including "device files" such as /dev/null, /dev/sdb, /dev/st0. You could redirect into a "regular file" called /tmp/mkfs.log if you wanted by modifying your original line as:

mkfs -t ext3 /dev/sdb1 < fd.data > /tmp/mkfs.log 2> /tmp/mkfs.log
--OR--
mkfs -t ext3 /dev/sdb1 < fd.data > /tmp/mkfs.log 2>&1

After you ran the command you could type "cat /tmp/mkfs.log" to see what kind of output it had.

I don't know off the top of my head what the entries in your fd.data file are doing. It's possible it isn't doing anything because it doesn't understand what was sent to it. Running to a log might give you better insight. (Alternatively you could simple NOT do the redirects to stdout and stderr and it would go to the screen.)

However, since you don't know what /dev/null is it appears you really need to drop back and NOT muck with this. mkfs will overwrite your /dev/sdb1 partition if you have anything on it already.

You might want to do a search for "Linux tutorial" - there are many places on line to read for example this one:
http://www.ee.surrey.ac.uk/Teaching/Unix/

It is OK to ask questions but you really ought to work your way up from basics to more advanced topics and ask questions as you go along. You don't need to know everything all at once but if you don't know about /dev/null or redirects you DO need to go and learn about the basics before you do some damage to your system.
 
  


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