reading from a file and writing contents back to it

Posted on 2011-09-04
Last Modified: 2012-05-12
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?

Question by:ambuli
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LVL 15

Expert Comment

by:Minh Võ Công
ID: 36482193
You can open file with r+ or w+ to read, write and modify content.
and fseek() func
LVL 38

Expert Comment

ID: 36482200
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

Author Comment

ID: 36482234
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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LVL 38

Expert Comment

ID: 36482246
> 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

Accepted Solution

xeltek earned 200 total points
ID: 36482259
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:
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?

Author Comment

ID: 36482312
I know it sounds silly, but I need to remove the data and write it back.  This is for a test.
LVL 38

Assisted Solution

wesly_chen earned 100 total points
ID: 36482358
#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 " )
LVL 34

Assisted Solution

sarabande earned 200 total points
ID: 36484238
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.


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