Those are not standard single quotes, but instead come from the keyboard key that normally sits above the Tab key. We can do exactly the same thing with bash's alternate command substitution syntax
那不是标准的单引号，而是键盘中通常位于 Tab 键之上的单引号。
2
It will not return an error to standard output if the file is not found. One way to test the return of the command is to use substitution, like shown in Listing 8.
即使找不到文件，它也不会向标准输出返回错误。
3
This system is still based on The Standard of Estimating Skeletal Maturity for Chinese (Method of CHN Scoring), uses the digitized' image substitution tradion X light film.