How can I display unique words contained in a Bash string?

Question

I have a string that has duplicate words. I would like to display only the unique words. The string is:

variable="alpha bravo charlie alpha delta echo charlie"

I know several tools that can do this together. This is what I figured out:

echo $variable | tr " " "\n" | sort -u | tr "\n" " "

What is a more effective way to do this?

Answer 1

You may use xargs:

echo "$variable" | xargs -n 1 | sort -u | xargs

Answer 2

Use a Bash Substitution Expansion

The following shell parameter expansion will substitute spaces with newlines, and then pass the results into the sort utility to return only the unique words.

$ echo -e "${variable// /\\n}" | sort -u
alpha
bravo
charlie
delta
echo

This has the side-effect of sorting your words, as the sort and uniq utilities both require input to be sorted in order to detect duplicates. If that's not what you want, I also posted a Ruby solution that preserves the original word order.

Rejoining Words

If, as one commenter pointed out, you're trying to reassemble your unique words back into a single line, you can use command substitution to do this. For example:

$ echo $(echo -e "${variable// /\\n}" | sort -u)
alpha bravo charlie delta echo

The lack of quotes around the command substitution are intentional. If you quote it, the newlines will be preserved because Bash won't do word-splitting. Unquoted, the shell will return the results as a single line, however unintuitive that may seem.