Shell Programming and Scripting

I tried it, it does not make any difference.

The traditional way for a process to prevent multiple instances of itself is to create a lockfile, not ps | awk | sed | cut | grep | kitchen | sink.

Is it possible for you to do it as

Code:

cat tt11.ksh

#!/bin/ksh
ps | grep -c tt11.ksh
count=$(ps | grep -c tt11.ksh)
echo "value of count is $count"

In RHEL6

Code:

./tt11.ksh
1
value of count is 1

Tried it does not work in RHEL 5.

As said earlier, pipes do a fork/exec. And while the exec is not yet ready there is a cloned shell. This is a race condition.
To avoid the race conditions you must avoid sub-shells and pipes and maybe even calling external programs.
The best thing is pgrep. Unfortunately AIX does not have it.
cjcox and Aia solutions combined is maybe good enough:

Code:

!/bin/ksh
ps | grep -c "tt11[.]ksh"
count=$(ps | grep -c "tt11[.]ksh")
echo ""
echo "$count"
echo "----"

--
Someone mentioned the lockfile method.
If the purpose is to prevent multiple instances to run, then indeed the lockfile method seems better than the process method.

It's not that it "doesn't work". It's that every time you use a | or backticks, you're telling the shell to create new processes.

If you want a way to check if an instance of your process is running without using ps, I suggest a more traditional lock file.