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