How to properly expand a Bash variable that contains newlines on sed replacement (insertion) side - bash

Question

Bear with me at first, thank you. Suppose I have
$ echo $'foo\nbar'
foo
bar
Now when I assign the string to a Bash variable, Bash does not give the same vertical output anymore:
$ str='foo\nbar'
$
$ echo $str
foo\nbar
$
$ echo $'str'
str
Try printf:
$ printf "$str\n"
foo
bar
Those examples are for illustration purposes because I am looking for a way to expand the newline(s) inside the $str variable such that I can substitute the $str variable on sed replacement (insertion) side.
# this does not work:
sed -i.bak $'/<!-- insert here -->/i\\\n'$'str'$'\\\n' index.html
# this works as expected though:
sed -i.bak $'/<!-- insert here -->/i\\\n'foo$'\\\n'bar$'\\\n' index.html
I did several ways to hack this but none worked; here is one example:
# this does not work:
sed -i.bak $'/<!-- insert here -->/i\\\n'`printf 'foo\\x0Abar'`$'\\\n' index.html
Further tests, I realized that as long as the variable does not contain newlines, things work as expected:
# This works as long as str2 does not contain any newline.
str2='foo_bar'
sed -i.bak $'/<!-- insert here -->/i\\\n'$str2$'\\\n' index.html
The expected result is that sed will insert 2 liners in place before <!-- insert here --> of the index.html file.
foo
bar
<!-- insert here -->
I try to achieve this as one liner. I know I can break sed into the vertical, multi-line form, which will be easier for me; however, I want to explore if there is a one liner style.
Is this doable or not?
My system is macOS High Sierra 10.13.6
Bash version: 3.2.57(1)-release
BSD sed was last updated on May 10, 2005

Answer 1

Your examples have a few subtle error, so here are a few examples regarding quoting and newlines in strings in bash and sed.
How quoting works in general:
# bash converts escape-sequence '\n' to real newline (0x0a) before passing it to echo
$ echo $'foo\nbar'
foo
bar
# bash passes literal 8 characters 'foo\nbar' to echo and echo simply prints them
$ echo 'foo\nbar'
foo\nbar
# bash passes literal 8 characters 'foo\nbar' to echo and echo converts escape-sequence
$ echo -e 'foo\nbar'
foo
bar
# bash passes literal string 'foo\nbar' to echo (twice)
# then echo recombines both arguments using a single space
$ str='foo\nbar'
$ echo $str "$str"
foo\nbar foo\nbar
# bash interprets escape-sequences and stores result 'foo<0x0a>bar' in str,
# then passes two arguments 'foo' and 'bar' to echo, due to "word splitting"
# then echo recombines both arguments using a single space
$ str=$'foo\nbar'
$ echo $str
foo bar
# bash interprets escape-sequences and stores result 'foo<0x0a>bar' in str,
# then passes it as a single argument to echo, without "word splitting"
$ str=$'foo\nbar'
$ echo "$str"
foo
bar
How to apply shell quoting, when dealing with newlines in sed
# replace a character with newline, using newline's escape-sequence
# sed will convert '\n' to a literal newline (0x0a)
$ sed 's/-/foo\nbar/' <<< 'blah-blah'
# replace a character with newline, using newline's escape-sequence in a variable
# sed will convert '\n' to a literal newline (0x0a)
$ str='foo\nbar' # str contains the escape-sequence '\n' and not a literal newline
$ sed 's/-/'"$str"'/' <<< 'blah-blah'
# replace a character with newline, using a literal newline.
# note the line-continuation-mark \ after 'foo' before the literal newline,
# which is part of the sed script, since everything in-between '' is literal
$ sed 's/-/foo\
bar/' <<< 'blah-blah' # end-of-command
# replace a character with newline, using a newline in shell-escape-mode
# note the same line-continuation-mark \ before $'\n', which is part of the sed script
# note: the sed script is a single string composed of three parts '…\', $'\n' and '…',
$ sed 's/-/foo\'$'\n''bar/' <<< 'blah-blah'
# the same as above, but with a single shell-escape-mode string instead of 3 parts.
# note the required quoting of the line-continuation-mark with an additional \ escape
# i.e. after shell-escaping the sed script contains a single \ and a literal newline
$ sed $'s/-/foo\\\nbar/' <<< 'blah-blah'
# replace a character with newline, using a shell-escaped string in a variable
$ str=$'\n' # str contains a literal newline (0x0a) due to shell escaping
$ sed 's/-/foo\'"$str"'bar/' <<< 'blah-blah'
# same as above with the required (quoted) line-continuation inside the variable
# note, how the single \ from '…foo\' (previous example) became \\ inside $'\\…'
$ str=$'\\\n' # str contains \ and a literal newline (0x0a) due to shell escaping
$ sed 's/-/foo'"$str"'bar/' <<< 'blah-blah'
All the sed examples will print the same:
blahfoo
barblah
So, a newline in sed's replacement string must either be
(1) newline's escape-sequence (i.e. '\n'), so sed can replace it with a literal newline, or
(2) a literal newline preceded by a line-continuation-mark (i.e. $'\\\n' or '\'$'\n', which is NOT the same as '\\\n' or '\\n' or $'\\n').
This means you need to replace each literal newline <0x0a> with newline's escape-sequence \n or insert a line-continuation-mark before each literal newline inside your replacement string before double-quote-expanding it into sed's substitute replacement string.
Since there are many more caveats regarding escaping in sed, I recommend you use awk's gsub function instead passing your replacement string as a variable via -v, e.g.
$ str=$'foo\nbar'
$ awk -v REP="$str" -- '{gsub(/-/, REP); print}' <<< 'blah-blah'
blahfoo
barblah
PS: I don't know, if this answer is entirely true in your case, because your operating system uses an outdated version of bash.

Answer 2

echo -e $str
where -e is
enable interpretation of backslash escapes

Answer 3

Use sed command r to insert arbitrary text
str="abc\ndef"
tmp=$(mktemp)
(
echo
printf -- "$str"
echo
) > "$tmp"
sed -i.bak '/<!-- insert here -->/r '"$tmp" index.html
rm -r "$tmp"
sed interprets newline as command delimiter. The ; doesn't really is a seds command delimeter, only newline is. Don't append/suffix ; or } or spaces in the w command - it will be interpreted as part of the filename (yes, spaces also). sed commands like w or r are escaped by a newline.
If you want more flexibility, rather move to awk.