Hi,everybody.
I have a question now.
I write the code followed in test.sh:

#!/bin/bash

var tem_file=/tmp/cdb.$$
echo ${tem_file}
exit 0

When I execute sh test.sh.
It shows : /tmp/cdb.4639.
I execute it once more.
It shows : /tmp/cdb.4640.
and more and more.
They show: /tmp/cdb.4641. /tmp/cdb.4642....

Can anyone tell me why?
And I can't find /tmp/cdb.4639 and others file...
Thank you very much!

1. drop the 'var', this is bash not Pascal(?)
2. $$ is the pid of the current process. These are handed out by the kernel, and you have no control over them. Other processes are continuously being created that you don't notice.
They are a montonically increasing num that wraps after 4 or 5(?) digits, but never 2 processes with same pid(!)
HTH

you mean when I exec sh test.sh.
The kenel hand out a process ID to the program?

There's no mystery that the number increases. $$ expands to the PID of the current shell, and each time you run the script it's run into a new shell. So, it's simple maths.

I have no idea about what you are trying to do. Maybe if you explain that we can help you. If what you wanted is to access a file that's called literally 'scd.$$', then use single quotes around it so the $$ is not expanded.

EDITED:

When you run a shell script, it doesn't matter if you use "sh script.sh" or "./script.sh" to run it, a new shell is spawned. The script is not run in your current shell, but into a new session with it's own local environment. The new shell is a new process, with a new unique PID. The pid of this new shell is what $$ will return. For more info read the bash man page.