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Full Discussion: sed question - change 2d to last line in file
Top Forums UNIX for Dummies Questions & Answers sed question - change 2d to last line in file Post 302525947 by agama on Sunday 29th of May 2011 09:11:36 PM
Old 05-29-2011
Here's a single pass sed that should work. I've put each command on a separate line to make it more clear.

Code:
sed '
$ b last
/^$/ b blank
H
d
b
:blank
x
p
d
x
b
:last
x
s/This is line 4/New Line Four/
p
x
' tdata

I tested this with GNU sed version 4.1.5.

I've not added an explanation of the logic on the off chance that you'd like to figure it out; I'd be happy to if you'd like.

There is a big assumption that you know the line 4 text to put into a substitute command. If it's not known, this won't work, but from your description I believe you do know it.

The one liner is below:

Code:
' $ b last; /^$/ b blank; H; d; b; :blank x; p; d; x; b; : last x; s/This is line 4/new line 4/; p; x' tdata

 

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LEARN ABOUT REDHAT

line

line(1)                                                            User Commands                                                           line(1)

NAME

       line - read one line

SYNOPSIS

       line

DESCRIPTION

       The  line utility copies one line (up to and including a new-line) from the standard input and writes it on the standard output. It returns
       an exit status of 1 on EOF and always prints at least a new-line. It is often used within shell files to read from the user's terminal.

EXIT STATUS

       Exit status is:

       0        Successful completion

       >0       End-of-file on input.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       +-----------------------------+-----------------------------+
       |Availability                 |SUNWcsu                      |
       +-----------------------------+-----------------------------+

SEE ALSO

       sh(1), read(2), attributes(5)

SunOS 5.10                                                          1 Feb 1995                                                             line(1)
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