Environment variable that dynamically changes when another variable is changed

youtoo · 02-16-2016, 10:27 AM

Bash Shell

I have a number of DBs on a server. I want to store paths to log files in .bash_profile as follows:

/mydir/mylowerdir/mydb/MYDB/mylog

mydb and MYDB will change when I change the ORACLE_SID parameter in memory. We set the ORACLE_SID using an ORACLE utility. We don't want to update the Oracle provided utility to also set paths.

Is there a way to have a parameter set in .bash_profile change automatically?

dr_agon · 02-16-2016, 11:13 AM

You cannot use environment variable for this, because it will be evaluated once (in .bash_profile) and keep the value.
But you can write bash function and use it every time you need.

Something like this in .bash_profile:

Code:

log_dir() { echo "/mydir/mylowerdir/${DB_NAME}/mylog"; }

Then use like this:

Code:

$ ls $(log_dir)

The function gets evaluated every time it is invoked, so if DB_NAME changes you will get different results.

grail · 02-16-2016, 12:00 PM

ORACLE_SID is itself an environment variable, so why not just relate your path to it?

youtoo · 02-16-2016, 01:01 PM

Cause the ORACLE_SID changes every time I set it.

the path would be

/mypath/$ORACLE_SID/lower($ORACLE_SID)

or something like that.

When I log in, bash_profile runs, but I may have 10 DBs on a server and I could change it later.

grail · 02-17-2016, 01:44 AM

Most Oracle environments I have worked on require sourcing of the environment variables for each DB as the SID is not the only thing to change.
So for each DB we have a env file and prior to working on said DB we do the following:

Code:

. DB_env

This sets our environment as required and away we go

If you actually mean you are changing the actual SID value regularly, which is dangerous for several reasons, you may wish to write a script to do the change and update the associated
env file