Diff for "BashFAQ/006"

Anchor(faq6)

How can I use variable variables (indirect variables, pointers, references) or associative arrays?

Before starting to use dynamically created variables, think again of a simpler approach. If it still seems to be the best thing to do, have a look at the following disadvantages:

1. It's hard to read and to maintain.

2. The variable names must match the regular expression ^[a-zA-Z_][a-zA-Z_0-9]* -- i.e., a variable name cannot contain arbitrary characters but only letters, digits, and underscores. We cannot have a variable's name contain Unix usernames; consider a user named hong-hu. A dash '-' cannot be a valid part of a variable name.

3. Quoting is hard to get right. If content strings (not variable name) can contain whitespace characters and quotes, it's hard to quote it right to preserve it.
4. If the program handles unsanitized user input, it can be [:BashFAQ#faq48:VERY dangerous]!

Bash (but not Korn shell, POSIX or Bourne shell) allows you to expand a parameter indirectly -- that is, one variable may contain the name of another variable:

•   realvariable=contents
    ref=realvariable
    echo "${!ref}"   # prints the contents of the real variable

This works for evaluating, but not for assigning a value. In order to assign a value "through" a reference (or pointer, or indirect variable, or whatever you want to call it -- I'm going to use "ref" from now on), you have to resort to tricks.

One such trick is to use read and Bash's here string syntax:

•   ref=realvariable
    read $ref <<< "contents"
    # realvariable now contains the string "contents"

This works equally well with Bash array variables too:

•   aref=realarray
    read -a $aref <<< "words go into array elements"
    echo "${realarray[1]}"   # prints "go"

Another is to use Bash's printf -v (only available in [:BashFAQ#faq61:recent versions]):

•   ref=realvariable
    printf -v $ref "contents"

The printf -v trick is handy if your contents aren't a constant string, but rather, something dynamically generated. You can use all of printf's formatting capabilities.

Yet another is Korn shell's typeset or Bash's declare. These are roughly equivalent to each other. Both of them cause a variable to become locally scoped to a function, if used inside a function; but if used outside a function, they can substitute for read in this case:

•   # Korn shell:
    typeset $ref="contents"
  
    # Bash:
    declare $ref="contents"

If you aren't using Bash or Korn shell, you can still do assignments to referenced variables using here document syntax:

•   # Portable code.
    ref=realvariable
    read $ref <<EOF
    contents
    EOF

Remember that, when using a here document, if the sentinel word (EOF in our example) is unquoted, then parameter expansions will be performed inside the body. If the sentinel is quoted, then parameter expansions are not performed. Use whichever is more convenient for your task.

Unfortunately, for shells other than Bash, there is no syntax for evaluating a referenced variable. You would have to use [:BashFAQ#faq48:eval], which means you would have to undergo extreme measures to sanitize your data to avoid catastrophe.

Sometimes it's convenient to have associative arrays, arrays indexed by a string. Awk has associative arrays. Perl calls them "hashes", while Tcl simply calls them "arrays". KornShell93 supports this kind of array:

•  # KornShell93 script - does not work with BASH
   typeset -A homedir             # Declare KornShell93 associative array
   homedir[jim]=/home/jim
   homedir[silvia]=/home/silvia
   homedir[alex]=/home/alex
   
   for user in ${!homedir[@]}     # Enumerate all indices (user names)
   do
       echo "Home directory of user $user is ${homedir[$user]}"
   done

BASH (including version 3.x) does not support them, unfortunately. Either use [:BashFAQ#faq48:eval] after sanitizing your data, or switch to awk, perl, ksh93, tcl, etc.