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I have two arrays:

arrayA=(1 2 3)
arrayB=(a b c)

and I want to print out one of them using a command line argument, i.e., without any if else.

I tried a few variations on the syntax with no success. I am wanting to do something like this:

ARG="$1"

echo ${array${ARG}[@]}

but I get a "bad substitution" error. How can I achieve this?

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3 Answers 3

up vote 4 down vote accepted

Try doing this :

$ arrayA=(1 2 3)
$ x=A
$ var=array$x[@]
$ echo ${!var}
1 2 3

NOTE

  • from man bash (parameter expansion) :
    ${parameter}
           The value of parameter is substituted.
 The braces are required when parameter is a positional parameter with
  more than one

digit, or when parameter is followed by a character which is not to be interpreted as part of its name.
* 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.

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What exactly is the ! doing in front of the var variable? How does that work, it seemed to be history substitution on googling, but I couldn't see how that worked here. –  Aaron Jan 7 '13 at 22:44
    
See my edited post –  sputnick Jan 7 '13 at 22:53

no way :(

if your arrays are that simple, then use associative arrays

    declare -A array
    array[A]="1 2 3"
    array[B]="a b c"

unfortunately, if your arrays are more complicated ( for example array=( "a b" c ) ), that wouldn't work. Then, you need to think harder about another way to reach your goal.

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What is the reason for the downvote? The associative array provides a nice way of grouping everything, assuming that my elements will all contain no space. –  Aaron Jan 7 '13 at 23:45
    
@Aaron Assuming your elements don't contain spaces, that is a reasonable design. @watael I guess beginning the answer with “no way” when the primary focus of your question is clearly possible wasn't a good idea. –  Gilles Jan 8 '13 at 0:12

Use eval

arrayA=(1 2 3)
ARG=arrayA
eval echo \${$ARG[@]} # equivalent to eval echo \${arrayA[@]}
                      # note that we escape the first '$' to prevent from 
                      # its parameter expansion before passing it to echo
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