Read input, save it to a dynamically-named variable and check if given input was empty

Question

Consider a generic ask() function that asks the user a question, reads the input and saves it in a variable named according to one of the function's arguments.

ask() {
    local question="$1"
    local varname="$2"

    echo "$question"
    read $varname
}

Suppose I want to ask the user what is his favourite pet and store the answer in a variable named $pet. Usage would be as follows:

ask "What is your favourite pet?" pet

What I want to do and need help with is check if the user's input was empty, and in that case set the user's input to some string. I would be able to do this easily if the name of the variable the user's input is stored in was constant, like so:

if [ -z "$pet" ]; then
    pet="foo"
fi

However the name of the variable I want to check whether or not is empty is whatever I pass in as the second argument. How can I check if the variable (named as per the value of $varname) containing the user's input is empty? The solution should be as portable and standard as possible, and must work under bash and zsh specifically.

This may be of help to you: http://stackoverflow.com/questions/1921279/how-to-get-a-variable-value-if-variable-name-is-stores-as-string — Noctua, May 31 '13 at 21:16

score 3 · Answer 1 · answered May 31 '13 at 21:30

3

In bash, ${!varname} gives you the value of the variable whose name is the value of $varname, but as far as I know, this syntax is not supported by zsh. If you want something that works in both bash and zsh, you may have to use the oldfashioned eval value=\${$varname} and then check $value. You should only use this if you know in advance that the value of $varname is a legal variable name; otherwise this is unsafe.

answered May 31 '13 at 21:30

Uwe

718
4
9

Thanks for the input. This made it a bit less confusing! – fabiomaia May 31 '13 at 21:46
1

In `zsh`, the syntax would be `foo=bar; bar=3; print ${(P)foo}`. `eval` should only be necessary for full POSIX compatibility. – chepner Jun 01 '13 at 14:12

score 2 · Answer 2 · answered May 31 '13 at 21:35

2

maybe:

ask() {
        name=$1;shift
        read -r -p "$@ >" var
        eval "$name='$var'"
}

ask pet "What is your favourite pet?"
pet=${pet:-foo}

echo "PET: $pet"

answered May 31 '13 at 21:35

clt60

62,119
17
107
194

fabiomaia · Accepted Answer · 2017-02-09T18:34:24.073

1

Based on the input thus far I managed to get a satisfying solution.

eval varname_tmp=\$$varname

if [ -z "$varname_tmp" ]; then
    eval "$varname=foo"
fi

edited Feb 09 '17 at 18:34

answered May 31 '13 at 21:50

fabiomaia

592
9
19

Read input, save it to a dynamically-named variable and check if given input was empty

3 Answers3