missing.csail.mit.edu/2020/course…
第一节课比较简单，只是简单介绍了一下shell,作业也比较简单。
值得记录的只有这个exercise:
1. Use `|` and `>` to write the “last modified” date output by `semester` into a file called `last-modified.txt` in your home directory.
anwser:
./semester | grep "last-modified" > ~/last-modified.txt
grep 默认会输出包含你指定关键字的整行内容
