4

I have a variable VARNAME which contains a name of another variable. I'd like to assign to this another variable without using eval. How can I do that?

Th reason I don't want to use eval is the following. Assume first a function for prepending foo variable:

% prepend_foo() { foo=("$@" $foo) }
% foo=(1 2)
% print -l $foo
1
2
% prepend_foo x 'a b c' y
% print -l $foo
x
a b c
y
1
2
%

Now consider a generalized function for appending to any variable:

% prepend() { var=$1; shift; eval "$var=($@ ${(P)var})" }
% foo=(1 2)
% print -l $foo
1
2
% prepend foo x 'a b c' y
% print -l $foo
x
a
b
c
y
1
2
%

As you can see, variables with spaces are split into several array items. I couldn't properly combine quote to achieve desired thing.

On IRC, someone suggested using ${name::=word}, but that doesn't work well with arrays:

21:23 < someone> > b=(bar baz); a=b; : ${(P)a::=(foo ${(P)a})}; typeset -p b
21:23 < machabot> someone: typeset b='(foo bar baz)'
21:23 < someone> dammit that's a string
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2 Answers 2

Reset to default
6

To prepend elements to an array, you can just do:

a[1,0]=(more elements)

Or you can do:

a=(more elements "$a[@]")

Note that doing:

a=(more elements $a)

would remove the empty elements in $a.

Now, to make a function for that, that's what eval is for, but you have to get the syntax right:

prepend() {
  eval "${1}[1,0]"='("${@[2,-1]}")'
}

See how the ("${@[2,-1]}") is single-quoted so it is passed literally to eval.

Or the longer way, but wouldn't work for prepend var more elements (when the variable name is var):

prepend() {
  local var=$1; shift
  eval "$var"='("$@" "${'"$var"'[@]}")'
}

The code you want to eval evaluate upon prepend varname ... is:

 varname=("$@" "${varname[@]")

You don't want "$@" to be expanded before being passed to eval. Only $var needs to be expanded there.

Note that the P variable expansion flag is as dangerous as eval when used with unsanitized data.

var='x[$(uname>&2)0]'
echo "${(P)var}"

Will run that uname command.

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2

Your prepend function can be written without eval, if you loop over the elements to unshift them one at a time.

$ prepend () {
then> local i=$#*
then> while [[ $i > 1 ]]; do
then>   typeset -g "${1}[1,0]=$*[$i]"
then>   i=$((i - 1))
then> done
then> }
$ foo=(a 'b c' d)
$ prepend foo m 'n o p' q r
$ print -l $foo
m
n o p
q
r
a
b c
d
$

Note that -g is needed for typeset to accept setting a non-local array element, but doesn't mean that the array referenced by $1 can't be local to the calling function. It just means "it's OK to look up the function call stack to find this array".

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