7.1 Joining lines

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 [email protected];
       spf=pass
Message-ID: <[email protected]>
From: John Doe <[email protected]>
To: Jane Smith <[email protected]>

$ 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 [email protected]; spf=pass
Message-ID: <[email protected]>
From: John Doe <[email protected]>
To: Jane Smith <[email protected]>

# A portable (non-gnu) variation:
#   sed -e :a -e '$!N;s/\n  */ /;ta' -e 'P;D'