Brace expansion with elements of an array

Question

In Bash we can already do this:

echo foo.{a,b,c}
# == foo.a foo.b foo.c

How do we get roughly:

arr=(a b c)
echo foo.{${arr[@]}}
# == foo.a foo.b foo.c

Answer 1

You can use parameter expansion

$ arr=(a b c)

$ echo "${arr[@]/#/foo.}"
foo.a foo.b foo.c

Answer 2

In case you don't have to use bash:

rc/es/akanga

(that's the default behaviour):

$ arr=(a b c)
$ echo foo.$arr
foo.a foo.b foo.c

zsh:

$ arr=(a b c)
$ echo foo.$^arr
foo.a foo.b foo.c

Or

$ set -o rcexpandparam
$ arr=(a b c)
$ echo foo.$arr
foo.a foo.b foo.c

(^ enables rcexpandparam for that one expansion, like = enables shwordsplit, or ~ globsubst)

fish

(also the default behaviour)

$ set arr a b c
$ echo foo.$arr
foo.a foo.b foo.c

---

All those shells have a better array design than bash's (itself copied from ksh)).

Note that zsh and fish expansion works like brace expansion. In rc, it differs when using echo $arr.$arr, which gives:

a.a b.b c.c

while in fish/zsh -o rcexpandparam, it gives the same as echo {a,b,c}.{a,b,c}, that is:

a.a a.b a.c b.a b.b b.c c.a c.b c.c