Bash reference variable by string name

Question

I'm trying to write a simple function that checks whether a variables is declared and non empty or not.

my_var="dummy_val"

function validate_var(){
        ${1:?"Variable is not set"}
}

validate_var my_var
validate_var my_var2

I get the following error:

script: line n: my_var: command not found

Is there a simple way I could reference the variable from the function argument?

https://stackoverflow.com/questions/3601515/how-to-check-if-a-variable-is-set-in-bash -> https://stackoverflow.com/a/35412000/9072753 — KamilCuk, Dec 21 '21 at 12:55
Thanks, but I would like to solve the current issue, just for educational purposes. — Bogdan Prădatu, Dec 21 '21 at 13:05

score 1 · Answer 1 · answered Dec 21 '21 at 12:50

1

Pass the variable by prepending a $. Also I've added an echo (as an example), otherwise your validate_var function tries to execute it's argument:

my_var="dummy_val"

function validate_var(){
        echo ${1:?"Variable is not set"}
}

validate_var $my_var

answered Dec 21 '21 at 12:50

polo-language

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With this solution I get "invalid variable name", even if the variable is declared as empty string (e.g. my_var2=""). – Bogdan Prădatu Dec 21 '21 at 12:57
Also, this passes "dummy_val" to the function, instead of "my_var". – Bogdan Prădatu Dec 21 '21 at 13:05
It seems to work fine with an empty string: `$ my_var2="" $ validate_var $my_var2 bash: 1: Variable is not set ` – polo-language Dec 21 '21 at 13:08
You mean you want to pass a [variable by reference](https://stackoverflow.com/questions/540298/passing-arguments-by-reference)? – polo-language Dec 21 '21 at 13:08
Actually, you're right. It does work, I don't know what I was doing wrong. Anyway, @choroba's answer does exactly what I want. Thanks. – Bogdan Prădatu Dec 21 '21 at 13:16

score 1 · Accepted Answer · answered Dec 21 '21 at 13:14

1

This seems to work. It uses the ! variable indirection.

function validate_var(){
    : ${!1:?"$1 is not set"}
}

answered Dec 21 '21 at 13:14

choroba

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If `var` is set to empty string, it shows `bash: !1: var is not set` – Philippe Dec 21 '21 at 16:54
I don't know what the expected output is for that case. Maybe validate the input first. – choroba Dec 21 '21 at 17:02

Bash reference variable by string name

2 Answers2