Expanding a variable within a variable

larrycohen28 · 01-14-2014, 06:24 PM

I've looked around for days and have not been able to find the exact answer I'm looking for.

Here's what I'm trying to do. I have a list which I've put into an array.

list=(a b c d)

I want to be able to use the values in this list in other variables. For example

${${list[0]}_test}=0

I want this to expand to a_test=0

I know this is the incorrect syntax, but I've tried everything I know including eval, but I'm not sure that is applicable here, or if what I am trying to do is even possible.

Would someone be able to either give me the correct syntax or help me find the correct way to do this?

Thanks!

Larry

sag47 · 01-14-2014, 11:18 PM

This is called indirect expansion but it is not quite as powerful as you would like.

See bash man page Parameter Expansion.

Quote:

Originally Posted by bash man page

If the first character of parameter is an exclamation point (!), a level of variable indirection is introduced. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion. The exceptions to this are the expansions of ${!prefix} and ${!name[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection.

So basically you can substitute a variable in a variable. Except only the whole value of that variable can be the replacement. Commands I type start with a $ otherwise it is output.

e.g.

Code:

$ a="b"
$ b="c"
$ echo "${a}"
b
$ echo "${!a}"
c
$ d=(x y z)
$ x="hello"
$ echo "${d[0]}"
x
$ echo "${!d[0]}"
hello
$ echo "${!d[@]}"
0 1 2

However if you attempt echo "${!a_test}" there will be a null value for the variable indirect expansion because a_test as a whole is not a variable.

SAM

grail · 01-15-2014, 01:51 AM

It appears to me you need to create 'test' as an associative array so you can use text as indices:

Code:

declare -A test

test["x"]=blah

echo ${test["x"]}

I have to say, and this no indictment of the OP, but I am amused how many people want to join the output of one variable with other text to make a new variable.
There may be other languages that can do this but to date I have not come across one. Plus for the rest of your code you would need to call the original variable every time you wish
to refer to this new variable ... not sure I see the upside?? (just a thought)

pan64 · 01-15-2014, 02:10 AM

this will work:

Code:

v=${list[1]}_test
eval ${v}="new value"
echo $b_test

but as it was mentioned already several times usage of eval can be unsafe, therefore not suggested.
I agree with grail, you need to find another construct, for example associative array instead