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From: Kaz Kylheku <kaz@kylheku.com>
Newsgroups: comp.unix.programmer,comp.programming
Subject: Re: Escaping of line breaks for text replacement
Followup-To: comp.unix.programmer
Date: Sun, 2 Mar 2014 18:11:59 +0000 (UTC)
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["Followup-To:" header set to comp.unix.programmer.]
On 2014-03-02, Markus Elfring <Markus.Elfring@web.de> wrote:
> Hello,
>
> I get the following result if I try out such a command on my openSUSE system.
>
> elfring@Sonne:~> $SHELL --version && sed --version && LINE='echo ---' && $LINE \
> && DRAFT='template1.txt' && cat $DRAFT && $LINE \
> && INPUT='replacement1.txt' && cat $INPUT && XY=$(< $INPUT) && $LINE \
> && LANG=C sed "s/# placeholder/$XY/" $DRAFT
> GNU bash, Version 4.2.45(1)-release (x86_64-suse-linux-gnu)
>    ...
> sed (GNU sed) 4.2.2
>    ...
> ---
> +++++++++++++
> # placeholder
> *************
> ---
> ABC
> XYZ
> ---
> sed: -e expression #1, char 19: unterminated `s' command

Unix already has a mechanism for interpolating variables into multi-line
text, called a "here document".

Here, it behooves us to use it, rather than reach for sed tricks
to do variable substitution.

Live demo:

$ cat template.txt 
my terminal is $TERM
the number of lines is $LINES
the number of colums is $COLUMNS

Now: dynamically wrap the here-doc syntax around the contents of template.txt
and evaluate it:

$ eval "cat <<EOF
$(cat template.txt)
EOF"
my terminal is xterm
the number of lines is 44
the number of colums is 80

You can have newlines in the variables that are substituted.

If you put the here-doc prologue and epilogue "cat <<EOF" and "EOF" the
template itself, then it's just a straight eval "$(cat template.txt)".

Remember to escape dollar signs in the here doc. It has to follow the
rules for a here doc.

If you ever do use sed for variable substitution have a clear delimiter on the
variables.  For instance, call it @REPLACEMENT@, not replacement,
for reasons which should be obvious.