bashで2つのコマンドの実行結果のdiffを見る

$ diff <(echo a) <(echo b)
1c1
< a
---
> b

$ diff <(echo a) <(echo b)

1c1

< a

---

> b

これを実行すると２つのechoが実行され、その結果をパイプとしてdiffに渡す。

$ diff `echo a` `echo b`
diff: a: No such file or directory
diff: b: No such file or directory
$

$ diff `echo a` `echo b`

diff: a: No such file or directory

diff: b: No such file or directory

これだと、diff a bを実行したことと同じになってしまう。