linux - Understanding bash brace expansion

Question

I read this post: https://superuser.com/questions/304807/brace-expansion-run-several-commands-instead-of-expanding-on-one-line and tried its eval echo way of expansion. Why are the two commands below different?

---

Command 1:

$ eval echo\ {a,b,c}\;

Result 1:

a
b
c

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Command 2:

$ eval echo\ {a,b,c} \;

Result 2:

a echo b echo c

Answer 1

这eval只是令人困惑的事情，所以摆脱它：

$ echo echo\ {a,b,c}\;
echo a; echo b; echo c;
$ echo echo\ {a,b,c} \;
echo a echo b echo c ;

这应该使正在发生的事情变得显而易见。在第一种情况下，eval 执行三个不同的 echo 命令。在第二种情况下，它只评估一个将字符串 'a echo b echo c' 作为参数的命令。