Lazy Evaluation in Bash

One safe possibility is to use a function:

expand_pattern() {
    pattern="$x and $y"
}

That's all. Then use as follows:

x=1 y=1
expand_pattern
echo "$pattern"

You can even use x and y as environment variables (so that they are not set in the main scope):

x=1 y=1 expand_pattern
echo "$pattern"

You can use the command envsubst from gettext, for example:

$ pattern='x=$x and y=$y'
$ x=1 y=2 envsubst <<< $pattern
x=1 and y=2

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