Using sed to replace to a var with special characters - sed

Question

I want to replace a placeholder on a file <<string>> in the example to the contents of a var that has several special characters.
file.txt
My string: <<string>>
script.sh
STRING="something-else;device=name.of.device;key=abcd1234/wtp="
sed -i "s/<<string>>/${STRING}/g" file.txt
I get this error:
sed: -e expression #1, char 165: unknown option to `s'
I already use this sed command for other vars that do not have special characters. Any way to escape the var ${STRING} entirely?

Answer 1

You can't do this job easily and robustly with sed, see Is it possible to escape regex metacharacters reliably with sed. Instead just use a tool like awk that understands literal strings:
$ string='~`!##$%^&*()-_+={[}]|\:;"'\''<,>.?/\1'
$ echo "$string"
~`!##$%^&*()-_+={[}]|\:;"'<,>.?/\1
$ string="$string" awk -i inplace 'match($0,/(.*)(<<string>>)(.*)/,a){ $0=a[1] ENVIRON["string"] a[3] } 1' file.txt
$ cat file.txt
My string: ~`!##$%^&*()-_+={[}]|\:;"'<,>.?/\1
That above will work for any characters (or backreference substrings like \1) that string might contain because it's simply using a literal string operation (concatenation) for the replacement.
It's using GNU awk for -i inplace just the same as your original script used GNU sed for -i.
Don't use all upper case for non-exported variable names by the way to avoid clashes with exported and built-in variables and not obfuscate your code by making it look like you're using exported variables, see Correct Bash and shell script variable capitalization.
Note that if you have multiple <<whatever>> placeholders you can easily parameterize the above, e.g.:
$ foo='Now is the Winter'
$ bar='Of our discontent'
$ cat file.txt
My foo string: <<foo>>
My bar string: <<bar>>
$ foo="$foo" bar="$bar" awk -i inplace 'match($0,/(.*)<<(\w+)>>(.*)/,a) && (a[2] in ENVIRON){ $0=a[1] ENVIRON[a[2]] a[3] } 1' file.txt
$ cat file.txt
My foo string: Now is the Winter
My bar string: Of our discontent
If you don't want to set foo and bar on the awk command line you can export them before it, or read them from a config file or a here-doc or ... - lots of options.

Answer 2

Since STRING contains a /, you should use a other delimiter, for example, you can use ^ like so:
sed 's^<<string>>^'"$STRING"'^g' file.txt
The quoting logic (''""'') is explained nicely on this SO answer.
Example on my locale machine:
$
$ cat file.txt
My string: <<string>>
$
$
$ STRING="something-else;device=name.of.device;key=abcd1234/wtp="
$
$
$ sed -i 's^<<string>>^'"$STRING"'^g' file.txt
$
$ cat file.txt
My string: something-else;device=name.of.device;key=abcd1234/wtp=
$
$