sed: Joining lines

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1 7.1 Joining lines
1 =================
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1 This section uses 'N', 'D' and 'P' commands to process multiple lines,
11 and the 'b' and 't' commands for branching.  ⇒Multiline
 techniques and ⇒Branching and flow control.
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1    Join specific lines (e.g.  if lines 2 and 3 need to be joined):
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1      $ cat lines.txt
1      hello
1      hel
1      lo
1      hello
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1      $ sed '2{N;s/\n//;}' lines.txt
1      hello
1      hello
1      hello
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1    Join backslash-continued lines:
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1      $ cat 1.txt
1      this \
1      is \
1      a \
1      long \
1      line
1      and another \
1      line
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1      $ sed -e ':x /\\$/ { N; s/\\\n//g ; bx }'  1.txt
1      this is a long line
1      and another line
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1 
1      #TODO: The above requires gnu sed.
1      #      non-gnu seds need newlines after ':' and 'b'
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1    Join lines that start with whitespace (e.g SMTP headers):
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1      $ cat 2.txt
1      Subject: Hello
1          World
1      Content-Type: multipart/alternative;
1          boundary=94eb2c190cc6370f06054535da6a
1      Date: Tue, 3 Jan 2017 19:41:16 +0000 (GMT)
1      Authentication-Results: mx.gnu.org;
1             dkim=pass header.i=@gnu.org;
1             spf=pass
1      Message-ID: <abcdef@gnu.org>
1      From: John Doe <jdoe@gnu.org>
1      To: Jane Smith <jsmith@gnu.org>
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1      $ sed -E ':a ; $!N ; s/\n\s+/ / ; ta ; P ; D' 2.txt
1      Subject: Hello World
1      Content-Type: multipart/alternative; boundary=94eb2c190cc6370f06054535da6a
1      Date: Tue, 3 Jan 2017 19:41:16 +0000 (GMT)
1      Authentication-Results: mx.gnu.org; dkim=pass header.i=@gnu.org; spf=pass
1      Message-ID: <abcdef@gnu.org>
1      From: John Doe <jdoe@gnu.org>
1      To: Jane Smith <jsmith@gnu.org>
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1      # A portable (non-gnu) variation:
1      #   sed -e :a -e '$!N;s/\n  */ /;ta' -e 'P;D'
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