I have two variables in bash that complete the name of another one, and I want to expand it but don't know how to do it I have:
echo $sala
a
echo $i
10
and I want to expand ${a10} in this form ${$sala$i} but apparently the {} scape the $ signs.
There are a few ways, with different advantages and disadvantages. The safest way is to save the complete parameter name in a single parameter, and then use indirection to expand it:
tmp="$sala$i" # sets $tmp to 'a10'
echo "${!tmp}" # prints the parameter named by $tmp, namely $a10
A slightly simpler way is a command like this:
eval echo \${$sala$i}
which will run eval with the arguments echo and ${a10}, and therefore run echo ${a10}. This way is less safe in general — its behavior depends a bit more chaotically on the values of the parameters — but it doesn't require a temporary variable.
$a10, $b7, and so on -- if that's what you have -- you can have an array named arr with indices like 'a10' and 'b7' and so on. That would let you write simply "${arr[$sala$i]}".
${arr[a10]} in version 4.0, and a lot of machines are still running 3.x versions.Use the eval.
eval "echo \${$sala$i}"
Put the value in another variable.
result=$(eval "echo \${$sala$i}")
The usual answer is eval:
sala=a
i=10
a10=37
eval echo "\$$sala$i"
This echoes 37. You can use "\${$sala$i}" if you prefer.
Beware of eval, especially if you need to preserve spaces in argument lists. It is vastly powerful, and vastly confusing. It will work with old shells as well as Bash, which may or may not be a merit in your eyes.
You can do it via indirection:
$ a10=blah
$ sala=a
$ i=10
$ ref="${sala}${i}"
$ echo $ref
a10
$ echo ${!ref}
blah
However, if you have indexes like that... an array might be more appropriate:
$ declare -a a
$ i=10
$ a[$i]="test"
$ echo ${a[$i]}
test
