Append lines from stdin to a file, but only if they don't already appear in the file. Outputs new lines to stdout too, making it a bit like a tee -a that removes duplicates.

Here, a file called things.txt contains a list of numbers. newthings.txt contains a second list of numbers, some of which appear in things.txt and some of which do not. anew is used to append the latter to things.txt.

▶ cat things.txt
Zero
One
Two

▶ cat newthings.txt
One
Two
Three
Four

▶ cat newthings.txt | anew things.txt
Three
Four

▶ cat things.txt
Zero
One
Two
Three
Four

Note that the new lines added to things.txt are also sent to stdout, this allows for them to be redirected to another file:

▶ cat newthings.txt | anew things.txt > added-lines.txt
▶ cat added-lines.txt
Three
Four

• To view the output in stdout, but not append to the file, use the dry-run option -d.
• To append to the file, but not print anything to stdout, use quiet mode -q.

You can either install using go:

go install -v github.com/tomnomnom/anew@latest

Or download a binary release for your platform.