如何连接匹配模式的行？

Question

我有一个大文件，其中大多数行很长，但有几行只是几个字：

Here is a long sentence.
This
is
a
short
line.
Here is a long sentence.
Here is a long sentence.
This
is
another
short
line.
Here is a long sentence.

所有短线都以独特的单词开头。在此示例中，短行以“This”开头，以“line”结尾。它们的行数完全相同。我需要一个命令来连接只文件中的短行，以便我得到

Here is a long sentence.
This is a short line.
Here is a long sentence.
Here is a long sentence.
This is another short line.
Here is a long sentence.

是否有命令可以完成这项工作？如果我不需要...我不想写脚本程序。

感谢。

Answer 1

perl -pe's/\n/ / if ($j ||= /^This$/) &&= !/^line\.$/'

用法：

perl -pe'...' file.in >file.out    # From file
perl -pe'...' <file.in >file.out   # From STDIN
perl -i~ -pe'...' file             # "In-place" with backup
perl -i -pe'...' file              # "In-place" without backup

Answer 2

sed -e :a -e '/line\.$/{p;d;}' -e '/^This/{N;s/\n/ /g;};ta' file.in > file.out