bash scripting need help getting a variable of a variable

dovo · 10-27-2008, 05:24 PM

I'm not sure what the best way is to do this, but here's my best example

PORT:~$ set JOHN SAM BILL
PORT:~$ name=$3
PORT:~$ echo $name
BILL
PORT:~$ number=3
PORT:~$ name="$"${number}""
PORT:~$ echo $name
$3

If it will make a difference inside of a bash script of not I'm unsure

the first method gives me the answer I want but the number is being randomly generated, and I need to be able to get the script to pull that number from another variable if there is a better way to do this I'm all ears. I also realize that this is probably the worst application for a bash script, but my question is whether it is or is not possible. I know that C C++ would be better code

Thanks for any suggestions

chrism01 · 10-27-2008, 05:36 PM

I think you need to read this page: http://www.tldp.org/LDP/abs/html/arrays.html
In fact, bookmark the whole doc.
see also
http://rute.2038bug.com/index.html.gz

dovo · 10-28-2008, 03:52 PM

pcarrick@PORT:~$ set JOHN SAM BILL
pcarrick@PORT:~$ name=$3
pcarrick@PORT:~$ echo $name
BILL
pcarrick@PORT:~$ number=3
pcarrick@PORT:~$ name="$"$number""
pcarrick@PORT:~$ echo $name
$3

maybe I wasn't clear enough

regardless of where the variable is I want to use the value of $number to retrieve $3, I'm not sure if its possible

jcookeman · 10-28-2008, 04:02 PM

???

Code:

$ set JOHN SAM BILL
$ name=$3
$ echo $name
BILL
$ number=3
$ name=$(eval "echo \${$number}")
$ echo $name
BILL

gnashley · 10-28-2008, 04:18 PM

Or you can use indirect substitution:

Code:

bash-3.1$ set JOHN SAM BILL
bash-3.1$ echo $3
BILL
bash-3.1$ name=$3
bash-3.1$ echo $name
BILL
bash-3.1$ number=3
bash-3.1$ echo ${!number}
BILL

This is very handy when you are writing a script and are afraid of what might happen by using 'eval' if the input is unknown.

dovo · 10-29-2008, 01:24 PM

Thanks that worked perfect.

Two questions is this variables by reference, something else, what is it called?

The other is do the {! symbols work together or are they two separate commands?

Thanks again

colucix · 10-29-2008, 01:30 PM

The correct name is "indirect variable reference". The two symbols work together. See here for further details.