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Old 06-09-2008, 05:57 AM   #1
lvprabhu
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Truncate from beginning of the file


Hi,

Is there any file utility in Linux to truncate from beginning of the file. I have a growing binary file and I need to delete part of it whenever it reaches pre-specified limit. I need to delete the old data. Please let me know how can I handle it.

Thanks,
lvp
 
Old 06-09-2008, 06:44 AM   #2
matthewg42
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what do you mean, "truncate from the beginning". Truncating just mean making th file empty, right?

Do you mean to remove data from the beginning of the file, but leave the last [some portion of data] in the file?

Not sure about that. Sounds like you want a special file which acts like a ring buffer.

It's more usual to write your program to accept a signal to re-open the log file, and use this to start a new log file periodically.

I'm not sure how you'd handle it with a binary file. I would expect you'd need to do this in the application itself.

Last edited by matthewg42; 06-09-2008 at 06:45 AM.
 
Old 06-09-2008, 07:43 AM   #3
theYinYeti
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There's a command to truncate a file by keeping the start of it only (and keeping the same inode): truncate.
There's a command to put the start/end of a file into another file: head/tail.

But I don't think there is any command to keep only the end of a file into the same inode…

Yves.
 
Old 06-10-2008, 03:06 PM   #4
schneidz
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this isnt elegant but try reversing the file , cutting off the end , and then revering it back:

Code:
cat file.txt | rev | cut -b -1024 | rev > file.out
edit:
oh wait i think cut can do it on its own
Code:
cut -b 1025- file.txt
will delete the first kilobyte of the file.

Last edited by schneidz; 06-10-2008 at 03:08 PM.
 
Old 06-10-2008, 06:12 PM   #5
chrism01
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Yet another alternative, if you know how big each field/rec is, is to overwrite that many bytes in binary mode.
 
Old 06-10-2008, 11:37 PM   #6
lwasserm
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Wouldn't you need to start your new file from some kind of record boundary in the original file? Anyway, how about tail -c xx >new file
where xx is the number of bytes you want to keep.
 
Old 06-11-2008, 01:26 AM   #7
icecoolcorey
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I have been using-- echo | cat fileName | command.....(what ever code) > samefilename. this will overwrite the same file will what ever altercations you made.
 
Old 06-11-2008, 07:51 PM   #8
chrism01
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"altercations"; that just has to be a subconscious slip http://dictionary.reference.com/sear...cation&x=0&y=0
 
Old 05-16-2010, 10:39 PM   #9
weebeng
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Quote:
Originally Posted by schneidz View Post
this isnt elegant but try reversing the file , cutting off the end , and then revering it back:

Code:
cat file.txt | rev | cut -b -1024 | rev > file.out
edit:
oh wait i think cut can do it on its own
Code:
cut -b 1025- file.txt
will delete the first kilobyte of the file.
Hi..I am in Linux.
I'd like to ask I got a file with following content

1234567890
ABCDEFGHIJKLMN

If I used cut command
cut -b 5- file.txt
It will deleted the 1st 4 bytes for 1st & 2nd line.

If I just want to delete the 1st line? How to do that?
Thank you
 
Old 05-16-2010, 11:42 PM   #10
AdnanShaheen
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Use tail command
 
1 members found this post helpful.
Old 05-16-2010, 11:53 PM   #11
mac.tieu
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Quote:
Originally Posted by weebeng View Post
...
If I just want to delete the 1st line? How to do that?
Thank you
Have a look at 'tail'
Code:
$tail -n +2 file.txt > out.txt
MT
 
1 members found this post helpful.
Old 05-17-2010, 02:30 AM   #12
grail
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How about raising your own question instead of bringing up one from the past.
 
Old 05-25-2010, 09:43 PM   #13
weebeng
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Quote:
Originally Posted by mac.tieu View Post
Have a look at 'tail'
Code:
$tail -n +2 file.txt > out.txt
MT
It is work. I used tail with -c to solve my problem. Thank you so much.
I'd like to ask, let said I have few Mbytes data in the file.txt, the 'tail' still can work to redirect output to out.txt (up to how many bytes tail can support)?
 
  


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