linux 中 sed = 选项在每一行之前插入编号

001、

(base) root@PC1:/home/test4# ls
a.txt
(base) root@PC1:/home/test4# cat a.txt        ## 测试数据
This is 1
This is 2
This is 3
This is 4
This is 5
(base) root@PC1:/home/test4# sed = a.txt      ## 在每一行之前插入编号
1
This is 1
2
This is 2
3
This is 3
4
This is 4
5
This is 5