BASH - How to access a variablized variable?

konsolebox · 12-17-2009, 04:00 AM

Just want to add some more points as well. Using eval or ! is not bad in all forms. Sometimes there's no choice other than use them like when emulating multi-dimentional arrays.

We know that this is not possible:

Code:

a[0][1]=2
echo ${a[0][1]}

so sometimes we have to do things like this:

Code:

a=(a0 a1 a2 a3 a4)
a0[1]=2
eval echo \${${a[0]}}

we probably can make a cleaner version to that with associative arrays like

Code:

a["0,1"]=2
echo ${a["0,1"]}
i=0 j=1
echo ${a["$i,$j"]}   # correct me if the syntax of the index is incorrect.

but we know in associative arrays, we cannot associate many values as once as in indexed arrays and order of indices in associative arrays is never a guarantee. I'm not sure if it's sorted by default in bash.

Code:

a0=(value1 value2 value3 value4)
bvalues=<input from somewhere with values: value5 value6 value7 value8 in order>
b0=(${bvalues})
for A in ${a0[@]}; do ...

There are also many other situations where only indexed arrays can be applicable.

GrapefruiTgirl · 12-17-2009, 11:01 AM

Further to the above, simulating arrays, even multi-dimentional ones, *can* be done like this:

Code:

a[0][1][2]

would become:

a012 or a_0_1_2 (or however you want to write it). To put a value in there:

shell$ a012="element 0,1,2"

To do it using some arbitrary index:

shell$ x=2 # index
shell$ eval a01${x}="'element 0,1,2'"

to get an indexed value out:

shell$ eval echo \$a01${x}

further example with two indexes:

shell$ y=5;x=2
shell$ eval a0${y}${x}="'element 0,5,2'" # assign a value
shell$ eval echo \$a0${y}${x} # read the value

further example with 3 indexes:

shell$ z=6
shell$ eval a${z}${y}${x}="'element 6,5,2'" # assign a value
shell$ eval echo \$a${z}${y}${x} # read the value

NOTE: Be careful to use enough "quotes" when assigning multi-part strings to the 'arrays' this way. Unquoted multi-part strings/values will give an error when assigned like this.

Sasha

DevonB · 12-18-2009, 01:59 PM

Quote:

Originally Posted by GrapefruiTgirl

Hopefully this will help you:

Code:

sasha@reactor:~$ STRING1=MyName
sasha@reactor:~$ SOME_VALUE=Devon
sasha@reactor:~$ newvar=$(eval echo \${STRING2_$STRING1_REST_OF_STRING=$SOME_VALUE})
sasha@reactor:~$ echo $newvar
Devon
sasha@reactor:~$
sasha@reactor:~$ eval echo \${STRING2_$STRING1_REST_OF_STRING}             
Devon
sasha@reactor:~$

Sasha

...and it worked perfectly, thank you so much.

Devon