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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
Old 09-25-2012
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
 

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SHELL-QUOTE(1)						User Contributed Perl Documentation					    SHELL-QUOTE(1)

NAME
shell-quote - quote arguments for safe use, unmodified in a shell command SYNOPSIS
shell-quote [switch]... arg... DESCRIPTION
shell-quote lets you pass arbitrary strings through the shell so that they won't be changed by the shell. This lets you process commands or files with embedded white space or shell globbing characters safely. Here are a few examples. EXAMPLES
ssh preserving args When running a remote command with ssh, ssh doesn't preserve the separate arguments it receives. It just joins them with spaces and passes them to "$SHELL -c". This doesn't work as intended: ssh host touch 'hi there' # fails It creates 2 files, hi and there. Instead, do this: cmd=`shell-quote touch 'hi there'` ssh host "$cmd" This gives you just 1 file, hi there. process find output It's not ordinarily possible to process an arbitrary list of files output by find with a shell script. Anything you put in $IFS to split up the output could legitimately be in a file's name. Here's how you can do it using shell-quote: eval set -- `find -type f -print0 | xargs -0 shell-quote --` debug shell scripts shell-quote is better than echo for debugging shell scripts. debug() { [ -z "$debug" ] || shell-quote "debug:" "$@" } With echo you can't tell the difference between "debug 'foo bar'" and "debug foo bar", but with shell-quote you can. save a command for later shell-quote can be used to build up a shell command to run later. Say you want the user to be able to give you switches for a command you're going to run. If you don't want the switches to be re-evaluated by the shell (which is usually a good idea, else there are things the user can't pass through), you can do something like this: user_switches= while [ $# != 0 ] do case x$1 in x--pass-through) [ $# -gt 1 ] || die "need an argument for $1" user_switches="$user_switches "`shell-quote -- "$2"` shift;; # process other switches esac shift done # later eval "shell-quote some-command $user_switches my args" OPTIONS
--debug Turn debugging on. --help Show the usage message and die. --version Show the version number and exit. AVAILABILITY
The code is licensed under the GNU GPL. Check http://www.argon.org/~roderick/ or CPAN for updated versions. AUTHOR
Roderick Schertler <roderick@argon.org> perl v5.16.3 2010-06-11 SHELL-QUOTE(1)
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