Bash字符串替换让我“糟糕的替代”

我有一个变量,这是一个网址,我想在bash中替换该网址的一部分,但我不断得到“糟糕的替代”

URL="http://hostname/project/branches/Old_Branch/package"
SRC="Old_Branch"
REP="New_Branch"

echo ${$URL/$SRC/$REP};
# desired output is http://hostname/project/branches/New_Branch/package

不确定我哪里错了…

URL="http://hostname/project/branches/Old_Branch/package"
SRC="Old_Branch"
REP="New_Branch"

echo "${URL/$SRC/$REP}"

请注意,${}中的网址没有$sigill =)