How to rename a lot of files - bash

Question

I have directory of files called:
foo--ext1
foo--ext2
foo--ext3
...
I would like to rename them to:
foo-bar-ext1
foo-bar-ext2
foo-bar-ext3
....
How can I do this renaming in bash?
I have attempted to understand mywiki.wooledge.org/BashFAQ/030 but I can't work what should go in place of ${f%.foo}.bar"; in the first example.
So I have started with:
for f in foo--*; do echo mv -- "$f"
but what do I put next?

Answer 1

There are several ways to approach this.
I recommend bookmarking this page and referencing it often.
I would use this:
$: for f in foo--*; do mv "$f" "${f//foo--/foo-bar-}"; done
This uses string substitution in the current filename to construct the new name, replacing foo-- with foo-bar-.
Note the // in the replacement. This will replace every occurrence of foo-- with foo-bar- in each filename.
$: ls -l foo-*
-rw-r--r-- 1 paul 1049089 0 Jul 26 14:32 foo-bar-ext1
-rw-r--r-- 1 paul 1049089 0 Jul 26 14:32 foo-bar-ext2
-rw-r--r-- 1 paul 1049089 0 Jul 26 14:32 foo-bar-ext3
-rw-r--r-- 1 paul 1049089 0 Jul 26 14:33 foo-bar-foo-bar-etx4
Remove one of the leading slashes to make it only handle the first occurrence -
$: for f in foo--*; do mv "$f" "${f/foo--/foo-bar-}"; done
$: ls -l foo-*
-rw-r--r-- 1 paul 1049089 0 Jul 26 14:34 foo-bar-ext1
-rw-r--r-- 1 paul 1049089 0 Jul 26 14:34 foo-bar-ext2
-rw-r--r-- 1 paul 1049089 0 Jul 26 14:34 foo-bar-ext3
-rw-r--r-- 1 paul 1049089 0 Jul 26 14:34 foo-bar-foo--ext4

Answer 2

Another simple method avoiding the loop is to use rename (from the util-linux package). There all that is needed is:
rename "foo-" "foo-bar" foo--*
You can check what will be done before actually doing the rename with the -n (no-act) and -v (verbose) options. For your example files, that would be:
$ rename -nv "foo-" "foo-bar" foo--*
`foo--ext1' -> `foo-bar-ext1'
`foo--ext2' -> `foo-bar-ext2'
`foo--ext3' -> `foo-bar-ext3'
There are two versions of rename that you will find provided in Linux distributions, the rename above from the util-linux package and perl-rename, which some Linux distros use instead which will also install as rename. Both are capable of handling the rename, but the options will be different. Check which you have with rename --version before use.