UNIX for Dummies Questions & Answers

I want to incorportae a subscript in a job script which is used for loading purposes.
What i require is that before a job runs it should check for a flag ,if flag is not present then create it and the loading should start.
Once loading finishes it should delete the flag.
So if any other load comes while loading is happening it should wait until that flag is deleted by the previous load?

This is a "Dummy mutex" using the filesystem:

Code:

while [ -f /var/run/flag ]; do
  sleep 10; 
done
touch /var/run/flag
trap "rm -f /var/run/flag" 0    # works on most modern shells
... proceed

It is possible for a race condition to occur, so don't use this for anything critical.

If you have "flock" you can do this more securely:

Code:

#!/bin/sh
# script1.sh
exec flock /var/run/flag $PWD/script2.sh

Now script2.sh will only be run when the "flag" is not locked.

thank you....

suppose i want to create a log for the load file which waited {basically for how long it waited and all related info like time and all} while the previous file was waiting ..how do i proceed in the same script?

At the start of the script do:

Code:

{ echo "Starting process; date; } >somelogfile
# 
# <put wait code here>
#
{ echo "Finished waiting"; date; } >somelogfile

For best results, use a format with date that is both human and machine readable:

Code:

date "+%s %c"

The first field will be the number of seconds since 1970, so you can use that to easily and precisely calculate how long it was waiting. The second part of the line is the human-readable timestamp.