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reading from a file and writing contents back to it

Hi Experts,

I want to wait for a file to be written with some data and remove the contents from that file and write back to it.  How can I do it?

Thanks
0
ambuli
Asked:
ambuli
3 Solutions
 
Minh Võ CôngCommented:
You can open file with r+ or w+ to read, write and modify content.
http://www.go4expert.com/forums/showthread.php?t=2977
and fseek() func
http://www.cplusplus.com/reference/clibrary/cstdio/fseek/
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wesly_chenCommented:
1. If you know the unique pattern of the content, then you can do
grep   <PATTERN>   file
   to check the output is the content that you want to remove.
  If the output is exactly those content you want to remove, then
sed '/PATTERN/d'   file > new_file
mv   new_file   file


2. If you know the unique pattern in the first line of the content to the end of file, then you can do
sed '/PATTERN/,$d'   file   > new_file
mv  new_file   file


3. If you know the line number of the content you want to remove, say line 21 ~ 25, then you can do
sed   '21,25d'   file  > new_file
mv  new_file   file
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ambuliAuthor Commented:
Thanks all. Sorry for not being clear. Actually I am trying to code what
the following commands do

#exec 3<> /ambuli/myfilel
#echo "msg:some-message" >&3; cat <&3

My code has to wait until something is written to the file and then remove the contents and write back to it.
Thank you.
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wesly_chenCommented:
> My code has to wait until something is written to the file and then remove the contents and write back to it.
Then don't write to it. Take out
echo "msg:some-message" >&3
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xeltekCommented:
I am confused about the "remove the contents and write back to it."
1. you need to monitor the file is written.
[you ere under linux ,you can use inotify:
http://www.ibm.com/developerworks/linux/library/l-ubuntu-inotify/
]
2. you want to read the content
[
when you get notified that file content is changed, you can open and read it.
]
3. you will write back what you read?
[
seek to the position where you read, and write back?
]
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ambuliAuthor Commented:
I know it sounds silly, but I need to remove the data and write it back.  This is for a test.
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wesly_chenCommented:
#exec 3<> /ambuli/myfilel
# cp  /ambuli/myfilel   /ambuli/myfilel_backup
#echo "msg:some-message" >&3; cat <&3
# cp  /ambuli/myfilel_backup   /ambuli/myfilel    (or replace "cp" with  "/bin/mv -f " )
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sarabandeCommented:
if it is a text file it is not possible to simply remove data. on the best you could open the file for both reading and writing; move to the file position where the part is which needs to be removed, remind that position and read the rest of the file into dynamic memory. finally set the file position and write the rest from memory to file. close the file. it now has all old contents beside of the removed part. to insert the removed part again, open the file for reading and writing again. set the file position agqin and write the removed part prior to the rest.

Sara
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