Double and triple substitution in bash and zsh

Question

Follow-up to the background part in this question.

In bash I can use ${!FOO} for double substitution, in zsh ${(P)FOO}. In both, the old-school (hack-y) eval \$$FOO works.

So, the smartest and most logical thing for me would be ${${FOO}}, ${${${FOO}}}… for double/triple/n substitution. Why doesn’t this work as expected?

Second: What does the \ do in the eval statement? I reckon it’s an escape, making something like eval \$$$FOO impossible. How to do a triple/n substitution with that that works in every shell?

Answer 1

The \ must be used to prevent the expansion of $$ (current process id). For triple substitution, you need double eval, so also more escapes to avoid the unwanted expansions in each eval:

#! /bin/bash
l0=value
l1=l0
l2=l1
l3=l2
l4=l3

echo $l0
eval echo \$$l1
eval eval echo \\$\$$l2
eval eval eval echo \\\\$\\$\$$l3
eval eval eval eval echo \\\\\\\\$\\\\$\\$\$$l4

Answer 2

#!/bin/bash

hello=world
echo=hello

echo $echo ${!echo}

Answer 3

Supposing the value of FOO is a valid variable name (say BAR), eval \$$FOO splits the value of BAR into separate words, treats each word as a wildcard pattern, and executes the first word of the result as a command, passing the other words as arguments. The backslash in front of the dollar makes it be treated literally, so the argument passed to the eval builtin is the four-character string $BAR.

${${FOO}} is a syntax error. It doesn't do a “double substitution” because there's no such feature in any of the common shells (not with this syntax anyway). In zsh, ${${FOO}} is valid and is a double substitution, but it behaves differently from what you'd like: it performs two successive transformations on the value of FOO, both of which are the identity transformation, so it's just a fancy way of writing ${FOO}.

If you want to treat the value of a variable as a variable, be careful of quoting things properly. It's a lot easier if you set the result to a variable:

eval "value=\${$FOO}"

Answer 4

{ba,z}sh solution

Here's a function which works in both {ba,z}sh. I believe it's also POSIX compliant.

Without going mad with quoting, you can use it for many levels of indirection like so:

$ a=b
$ b=c
$ c=d
$ echo $(var_expand $(var_expand $a)
d

Or if you have more (!?!) levels of indirection you could use a loop.

It warns when given:

• Null input
• A variable name which isn't set

# Expand the variable named by $1 into its value. Works in both {ba,z}sh
# eg: a=HOME $(var_expand $a) == /home/me
var_expand() {
  if [ -z "${1-}" ] || [ $# -ne 1 ]; then
    printf 'var_expand: expected one argument\n' >&2;
    return 1;
  fi
  eval printf '%s' "\"\${$1?}\""
}

Answer 5

Why would you need to do that?

You can always do it in several steps like:

eval "l1=\${$var}"
eval "l2=\${$l1}"
...

Or use a function like:

deref() {
  if [ "$1" -le 0 ]; then
    eval "$3=\$2"
  else
    eval "deref $(($1 - 1)) \"\${$2}\" \"\$3\""
  fi
}

Then:

$ a=b b=c c=d d=e e=blah
$ deref 3 a res; echo "$res"
d
$ deref 5 a res; echo "$res"
blah

FWIW, in zsh:

$ echo ${(P)${(P)${(P)${(P)a}}}}
blah

Answer 6

POSIX solution

Without going mad with quoting, you can use this function for many levels of indirection like so:

$ a=b
$ b=c
$ c=d
$ echo $(var_expand $(var_expand $a)
d

Or if you have more(!?!) levels of indirection you could use a loop.

It warns when given:

• Null input
• More than one argument
• A variable name which isn't set

# Expand the variable named by $1 into its value. Works in both {ba,z}sh
# eg: a=HOME $(var_expand $a) == /home/me
function var_expand {
  if [ "$#" -ne 1 ] || [ -z "${1-}" ]; then
    printf 'var_expand: expected one argument\n' >&2;
    return 1;
  fi
  eval printf '%s' "\"\${$1?}\""
}

Answer 7

Just like ${$foo} doesn't work in place of ${(P)foo} in zsh, neither does ${${$foo}}. You just need to specify each level of indirection:

$ foo=bar
$ bar=baz
$ baz=3
$ echo $foo
bar
$ echo ${(P)foo}
baz
$ echo ${(P)${(P)foo}}
3

Of course, ${!${!foo}} doesn't work in bash, because bash doesn't allow nested substitutions.