Question

I am scratching my head on this one, can’t find the bash reference talking about it.

In the code below

host_color=${uphost}_host_color
host_color=${!host_color}

What is the second line doing ? what does the !operator do in this case ?

Answer 1

That is a short form for indirect references.

$ foo=bar
$ bar=bas
$ echo ${!foo}
bas

The seemingly similar construction ${!foo*} expands to the names of all variables whose name begin with foo:

$ foo1=x
$ foo2=y
$ echo ${!foo*}
foo1 foo2