Shell Programming and Scripting

Hi All,

I am new new to unix.com, I have a question related to shell scripting.
We have a Oracle database backup shell script, which can be used for taking full, incremental & archive log backup based on the parameters passed.

Within the script we export a variable as

Code:

export gv_shell_id=$(date "+%y%m%d_%H%M%S_")$$

So for every execution of backup, we will have a unique gv_shell_id. We store the gv_shell_id along with other backup details in a database table for reporting purpose.

If we have executed a full backup, it will set a gv_shell_id and if simultaneously some one kicks off archive log backup, it will reset the gv_shell_id with different value.

And this is causing a wrong entry of gv_shell_id for full backup in the table.

Basically i want to make the variable itself unique for every execution to avoid such problem.
I tried using gv_shell_id_$$=$(date "+%y%m%d_%H%M%S_")$$

but it throws below error.

Code:

gv_shell_id_$$=$(date "+%y%m%d_%H%M%S_")$$
-bash: gv_shell_id_24357=140712_073709_24357: command not found

Can you please help me in generating a unique variable so that it doesn't get overwirtten by any other execution of same script.

Welcome to the forums.
You can not use the $ on the left hand side.

Code:

gv_shell_id=$(date "+%y%m%d_%H%M%S_").$$

Should work.

Using CygWin at the moment but you could try:-

Code:

#!/bin/bash
export eval "gv_shell_id_$$=$(date "+%y%m%d_%H%M%S_")$$"
set

Results as part of the environment after running:-

Code:

WINDIR='C:\Windows'
_=gv_shell_id_3904
gv_shell_id_3904=140712_161911_3904
temp='C:\Users\Tami\AppData\Local\Temp'
tmp='C:\Users\Tami\AppData\Local\Temp'
AMIGA:/tmp> _

I'm missing something very basic here. The command:

Code:

export gv_shell_id=$(date "+%y%m%d_%H%M%S_")$$

will assign a distinct value to gv_shell_id each time your backup script is invoked.

Why isn't your backup script initiating the appropriate type of backup and writing the results of that backup into your database using the value of gv_shell_id it assigned? What is getting confused and writing the wrong value of gv_shell_id into your database?

I can only emphasize what Don Cragun has already said. You didn't post your script therefore, instead of offering detailed advice, i will only make some general remarks. You are invited to post your complete script if you want to get better and more specific help.


I think we have here a (common) misunderstanding about what "export" does, so let us address that first.

Every process has its own set of variables (and their respective values), which form the so-called "process environment". Processes form hierarchies, where one process (the "child process") is called by the other (the "parent process") and it would be good to have a mechanism which passes the process environment of the parent on to the child.

Exactly this does the "export" keyword. Issuing an "export <variablename>" flags the variable to be inherited by an eventual child process if one is invoked. As long as you stay inside the process there is no difference. You also do not need to "export" the variable again if its value changes. A variable is either "marked for being inherited by the child process" or it is not, but the inheritance takes place at the time the child process starts and with the value the exported variable has by then.


No. If your script is called twice these two will be (maybe simultaneously running) instances, which will - from an OS point of view - not interfere with each other. They will have two separate environments and will run alongside each other.

Of course, just because they are indenpendent in the eyes of the operating system that doesn't mean they couldn't interfere at all. If they use the same resources - they probably issue commands against the same database, which will cause the DB to change its state - they can cause problems for each other. But this has nothing to do with their process environments, but rather with a competition for common resources, where the "common resource" in this case is the database or some vital part of it.

My suggestion is to make sure the script is executed only once at a time. Usually you do this by creating a PID-file at the begin of the script and removing it at the end. See the following sketch:

Code:

#! /bin/sh

typeset fRun="/var/run/myscript.pid"

if [ -e "$fRun" ] ; then
     echo "ERROR: can only be run once" >&2
     exit 2
fi
echo "$$" > "$fRun"

# ... rest of your script goes here ...

rm -f "$fRun"
exit 0

Depending on your shell you may put the removal of the PID file into a service routine for "trap 0", which will enhance the runtime security, but not change the basic, underlying logic.

I hope this helps.

bakunin

thanks alot bakunin for such a detailed expalination on process variables. It really cleared my doubts.
Regarding my original problem statement,i will get back to you guys after doing somemore homework.

Thank you everyone for trying to resolve my issue.

Regards,
Veeresham