The problem is that grep will run on each line, not the entire file. As long as the file is small enough to fit into memory (which should be the case in the vast majority of situations these days), you can use grep's -z flag to slurp the entire file:
-z, --null-data
Treat input and output data as sequences of
lines, each terminated by a zero byte (the ASCII NUL
character) instead of a newline. Like the -Z or --null option, this option can be used with commands
like sort -z to process arbitrary file names.
The next issue, is that if you pass grep something with newlines, it will treat it as a list of patterns to grep for:
$ string="1
> 2"
$ seq 10 | grep "$string"
1
2
10
"
Which means that I am afraid you will have to express the pattern as a proper regular expression:
\n\ntest1\n\ntest2\n\n
However, this also means you need the -P flag to enable perl-compatible regular expressions so the \n will work.
I created these two files to demonstrate:
$ cat file1
this is a test:
test1
test2
and another one
$ cat file2
this is a test:
test1
test2
and another one
Using those two files and the information above, you can do:
$ grep -Pz '\n\ntest1\n\ntest2\n\n' file1
$
$ grep -Pz '\n\ntest1\n\ntest2\n\n' file2
this is a test:
test1
test2
and another one
Putting all this together gives us:
string='\n\ntest1\n\ntest2\n\n'
if ! grep -Pzq "$string" test.txt; then
printf "$string" >> test.txt
fi
Or, as suggested by @steeldriver in a comment, you can use a variable and convert the newlines to \n on the fly:
$ string="
test1
test2
"
$ if ! grep -Pzq "${string//$'\n'/\\n}" test.txt; then
printf "$string" >> test.txt
fi
If your string contains special characters which have meanings in regular expressions, as you now show in your updated question, then that's a whole different situation. For the example you show, you would need something considerably more complicated. Like this:
searchString='\n\nif \[ -f ~/.script \]; then\s*\n\s*\.\s+~/\.script\s*\nfi\n\n'
printString='
if [ -f ~/.script ]; then
. ~/.script
fi
'
if ! grep -Pzq "$searchString" test.txt; then
printf "%s" "$printString" >> test.txt
fi
