Removing lines from a file being used by another process using SHELL script Post: 302705573

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Top Forums Shell Programming and Scripting Removing lines from a file being used by another process using SHELL script Post 302705573 by Lem on Tuesday 25th of September 2012 06:47:37 AM

09-25-2012

Registered User

Are you in linux? Try flock command. Beware: file locks are NOT mandatory, so every command accessing the file must do it through flock.

Here is an example:

Code:

$ cat file
25/08/2012
31/08/2012
4/09/2012
5/10/2012

$ flock file -c "sed '1,/\/08\/2012/ d' <file >tmpfile; cat tmpfile >file" & flock file -c "echo yadayadayada |tee -a file >/dev/null"
[1] 22160
[1]+  Completato              flock file -c "sed '1,/\/08\/2012/ d' <file >tmpfile; cat tmpfile >file"

$ cat file
4/09/2012
5/10/2012
yadayadayada

To have a real feel of what's happening, reset the original file content and use some sleeps, as in:

Code:

flock file -c "sed '1,/\/08\/2012/ d' <file >tmpfile; cat tmpfile >file; sleep 10" & \
sleep 3; echo "sed has worked; cat has worked, here is the file:"; cat file; \
echo; echo "waiting the last 7 seconds for tee..."; \
flock file -c "echo yadayadayada |tee -a file >/dev/null"; \
echo "tee has been able to write just now. Here is the file:"; cat file

To sum it up:

a) to delete your old lines, use flock file -c "sed ... <file >tmpfile; cat tmpfile >file";

b) prepend flock file -c "... to your tee -a file command accessing the file.
--
Bye

Lem

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LEARN ABOUT OPENSOLARIS

script

script(1)							   User Commands							 script(1)

NAME

       script - make record of a terminal session

SYNOPSIS

       script [-a] [filename]

DESCRIPTION

       The  script  utility  makes a record of everything printed on your screen. The record is written to filename. If no file name is given, the
       record is saved in the file typescript. See WARNINGS.

       The script command forks and creates a sub-shell, according to the value of $SHELL, and records the text from this session. The script ends
       when the forked shell exits or when Control-d is typed.

OPTIONS

       The following option is supported:

       -a    Appends the session record to filename, rather than overwriting it.

NOTES

       script places everything that appears on the screen in filename, including prompts.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWcsu			   |
       +-----------------------------+-----------------------------+
       |CSI			     |Enabled			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       attributes(5)

WARNINGS

       script  can  pose  a  security risk when used in directories that are writable by other users (for example, /tmp), especially when run by a
       privileged user, that is, root. Be sure that typescript is not a link before running script.

SunOS 5.11							    30 Jan 2004 							 script(1)

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