Dynamically Change Environment Variable

[Tom Dawson]

Hi,

In the Korn Shell, is there a method to dynamically change an environment variable based on another variable's changing?

I'm trying to use this:

BUD=/oravol/u3/dbbackup_${ORACLE_SID}/backups
export BUD

Where BUD would change values if ORACLE_SID changed. i.e. BUD needs to be re-evaluated each time it is referenced.

The single quotes make this happen for the PS1 environment variable:

PS1='${MYHOST} ${PWD} $ '
export PS1

I suspect that is a special case.

Any ideas?

TIA,
Tom

[Michael Steele_2]

Use:

BUD=/oravol/u3/dbbackup_$ORACLE_SID/backups

No parens.

[James R. Ferguson · Accepted solution]

Hi Tom:

You can do something like the following. I'll use the shell's SECONDS variable in this example:

# BUD='echo /oravol/u3/dbbackup_${SECONDS}/backups'

# eval ${BUD}

Those are single quotes in the variable assignent. Every time the 'eval' is executed, the current number of seconds (since the shell began executing) is displayed.

Regards!

...JRF...

[Tom Dawson]

Michael,

Thanks. But that's what I had been trying and it didn't work.

James,

Thanks, that works. The "eval" could be a little cumbersome for our developers. I'm going to leave the Bunny off for a while to see if I can get any other suggestions.

Thanks again,
Tom

[James R. Ferguson]

Hi Tom:

To be clear, it is best to get in the habit of using curly braces when doing parameter substitution!

Consider:

# VAR=tom
# echo $VAR_was_here
sh: VAR_was_here: Parameter not set.

Now:

# VAR=tom
# echo ${VAR}_was_here
tom_was_here

As noted in the manpages for 'sh-posix', "Braces are required when parameter is followed by a letter, digit, or underscore that should not be interpreted as part of its name or when a named parameter is subscripted."

Regards!

...JRF...

[Tom Dawson]

James,

Thanks.

Yea, I almost always use the curley braces. I find myself forgetting occaisionally, but I've been doing that for 10 - 15 years.

Thanks again,
Tom

[James R. Ferguson]

Hi Tom:

My pointer regarding the use of curly braces was based upon the fact that you gave apparent credence to dropping them as a potential solution to your original query. The use of curly braces in parameter substitution is a very valuable habit to develop, in my opinion.

Regards!

...JRF...