Shell Programming and Scripting

Hello All,

I want to run 2 shell scripts(bash shell) concurrently, both of which are of the form,

Code:

#######Script 1 & Script 2##########
while [ condition ]
do
      do something
      #some task T
      do something
done

I want when to perform task T in mutually exclusive manner.

I tried using Peterson's method and put loops like the one shown below to enter critical region(task T)

Code:

FLAG_1=1
TURN=0
while [[ $FLAG_2 -eq 1 && $TURN -eq 0 ]]
do
    DUMMY=1
done

I ran the scripts as

Code:

. ./script1.sh &
. ./script2.sh &

But later realized that this wont work !

Any ideas how I can do this?

You should create a flag file.
You should ensure the atomicity of this creation (this will ensure that even if the scripts are launched in a concurrent manner, only 1 will succeed in creating the flag).

You could set the noclobber option (depending on your OS set option may vary: man set and look for noclobber).
you could also use the mkdir command which is atomic but use it without the -p option, otherwise, the handling of error on creation may be messed up.

---------- Post updated at 01:47 AM ---------- Previous update was at 12:52 AM ----------

Read the good example here :
A Script Template and Useful Techniques for ksh Scripts - BigAdmin - wikis.sun.com

Thank you ctsgnb !

The information is really helpful

I used ln -s as given in the tutorial as an atomic operation.

In may case each script does an ln -s until it is successful in creating the link
After exiting the critical section it removes the symbolic link.

Thank you once again!

Watch out : as specified on the suggested link :

--------------------------------------------------

WPollock says:

Your createLockFile routine uses ln -s to create the lockfile. While it is common practice to do that, as far as I can tell from checking the POSIX docs at OpenGroup.org this is not guaranteed to create a file atomically.
Dave Korn taught me the correct way to do this:

Code:

set -C  # or: set -o noclobber
: > ${__LOCKFILE} 2>/dev/null
LN_RC=$?

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