關於美元符號：美元符號內的美元符號？

假設我們有：

echo $A
abc

echo $B
def

echo $abcdef
yesyes

如何使用 A 和 B 獲得“yesyes”？我正在嘗試類似的東西：

${$A$B}        
$`echo "$A"$B`

但失敗了。有什麼建議嗎？

如果您使用的是 Bash shell，那麼只要您引入一個中間變數，您就可以使用間接：

$ echo $A
abc
$ echo $B
def
$ echo $abcdef
yesyes

然後

$ AB=$A$B
$ echo "${!AB}"
yesyes

變數間接在 Bash 手冊 ( ) 的 *Parameter Expansion 部分中進行了描述man bash：

   If the first character of parameter is an  exclamation  point  (!),  it
   introduces a level of variable indirection.  Bash uses the value of the
   variable formed from the rest of parameter as the name of the variable;
   this  variable  is  then expanded and that value is used in the rest of
   the substitution, rather than the value of parameter itself.   This  is
   known as indirect expansion.  The exceptions to this are the expansions
   of ${!prefix*} and ${!name[@]} described below.  The exclamation  point
   must  immediately  follow the left brace in order to introduce indirec‐
   tion.

引用自：https://unix.stackexchange.com/questions/453123