linux - 'N' 和 'D' 无法按预期使用 sed

Question

sed 'N; D' testfile

测试文件包含:

this is the first line
this is the second line
this is the third line
this is the fourth line

我正在使用 RHEL 6，输出如下:

this is the fourth line

根据我的理解，N 只是将下一行拉入模式空间，D 只是删除模式空间的第一行。因此，输出应该是:

this is the second line
this is the fourth line

有人可以解释为什么输出会像上面提到的那样吗？

Answer 1

根据 the documentation :

D

If pattern space contains no newline, start a normal new cycle as if the d command was issued. Otherwise, delete text in the pattern space up to the first newline, and restart cycle with the resultant pattern space, without reading a new line of input.

(强调我的。)

听起来这会从头开始重新启动您的 sed 程序，读取并删除行，直到用完输入，此时缓冲区中只剩下最后一行。