Updating shell functions and aliases through environment variables

Question

Say we define the following shell function and alias in .zshenv:

export VARIABLE_A='original'

function my_function()
{
    print $VARIABLE_A
}

export VARIABLE_B='original'

alias my_alias="print $VARIABLE_B"

Now, say that we start a new interactive shell, and we do the following:

$ my_function
$ export VARIABLE_A='new'
$ my_function


$ my_alias
$ export VARIABLE_B='new'
$ my_alias

The commands above print:

original
new
original
original

It looks like the function is updated, but the alias isn't. Is this behavior expected? And if so, why?

Answer 1

Yes, the behavior is expected. The reason being simply that you used double quotes ("…") when defining the alias, allowing for parameter substitution. This led to $VARIABLE_B being substituted at the time of the definition of the alias instead of its execution.

Essentially, when writing

VARIABLE_B="original"
alias my_alias="print $VARIABLE_B"

zsh will replace $VARIABLE_B before running the alias command like so:

alias my_alias="print original"

Therefore the alias my_alias will always be replaced with print original before execution.

If you want to substitute a parameter at execution time you have to quote it, either with single quotes ('…') or by quoting $ with a \:

alias my_alias='print $VARIABLE_B'
alias my_alias="print \$VARIABLE_B"

This will set the alias so, that the variable name remains part of the command and the alias my_alias is actually replaced with print $VARIABLE_B before execution.

---

In contrast parameters inside a function definition do not need to be quoted explicitly, they are always only substituted at runtime.

So when writing

my_function () {
{
    print $VARIABLE_A
}

this is exactly what will be stored. When running my_function $VARIABLE_A will be substituted with the value it has at that time.

Answer 2

Say we define the following shell function and alias in .zshenv:

Aliases are evaluated on execution of a script. So $VARIABLE_B is evaluated and replaced with its value at the moment when your .zshenv is processed. Whatever value you will assign to VARIABLE_B later is not going to affect the alias.

On the contrary functions do not evaluate enviroment variables when they are processed. Enviroment variables in function are evaluted when a function is called.