My code is this:

sed -n 'G;s/\n/&&/;/^\([ -~]*\n\).*\n\1/d;s/\n//; h; P' out.txt > out2.txt

I keep getting the error message.
sed: -e expression #2, char 50: Unmatched ( or \(

Can you tell me what I am doing wrong ?

You never bothered to go back to your other two threads to respond or show your effort, and you are now trying to take credit for something someone else wrote:
https://edoras.sdsu.edu/doc/sed-oneliners.html

May want to copy/paste the exact command with the proper spacing. Also may want to look at the uniq command, and try a further Internet search for "how to remove duplicate lines in linux".

3 members found this post helpful.

Why don't you just use uniq?

2 members found this post helpful.

Can I use:
$sort out.txt | uniq -u > out2.txt

to replace:
sed -n 'G;s/\n/&&/;/^\([ -~]*\n\).*\n\1/d;s/\n//; h; P' out.txt > out2.txt

You should know the answer.

Don't you have a test input file or two and know how the output should be modified?

Try your term out. It will remove duplicates and sort the file, but you should test to determine if it does exactly what you wanted for a result.

2 members found this post helpful.

@OP As a start please learn how to use [code] and [/code] tags.

2 members found this post helpful.

Why don't you try it and find out??? And again, you aren't even acknowledging your first two posts, where you just ask someone to do your homework for you, nor are you acknowledging the fact you have been caught claiming that 'you' wrote the above sed statement. You have been told about the uniq statement at least three times.

You also don't seem to want to respond to the advice to look in your textbook, or show what you have personally done/tried. Again, we are happy to HELP you, but we aren't going to do everything FOR YOU.

3 members found this post helpful.

Also awk can do this with an unsorted list and redirection:

1 members found this post helpful.

No, you will never get the same error message from sort.
Otherwise