这可能对你有用(GNU sed):

This might work for you (GNU sed):

sed ':a;$!{N;ba};s/jack/'"$var"'/2' file

这会将文件拖入内存，然后将第二次出现的 jack 替换为 $var.

This slurps the file into memory and then substitutes the second occurence of jack for $var.