唯一值的计算直接在Shell中有效，而在反引号中则无效

Question

I am trying to figure out why this works from the command line however does not when it is in a variable, I am trying to count the number of unique occurrences of a number,

$ echo 1,1,2,3,4,5 | tr , \\n | sort -u | wc -l
5
$ test=`echo 1,1,2,3,4,5 | tr , \\n | sort -u | wc -l`
$ echo $test
1

I am assuming that it is seen as one line when in a variable but unsure why and how to address it?

Thanks!

Answer 1

you may see what is happening by putting set -x :

$ test=`set -x; echo 1,1,2,3,4,5 | tr , \\n | sort -u | wc -l`
++ echo 1,1,2,3,4,5
++ tr , n
++ wc -l

here you see that the \\n became n

the escape of \\n fails; try

test=`echo 1,1,2,3,4,5 | tr , '\n' | sort -u | wc -l`

the quotes will prevent the `\\' to fade away

or even

 test=$(echo 1,1,2,3,4,5 | tr , '\n' | sort -u | wc -l)

as we usually prefere $() to backquotes that can cause some other kind of trouble

Answer 2

Using awk you can avoid all the piped commands:

count=$(echo 1,1,2,3,4,5 | awk -v RS=, '!seen[$1]++{c++} END{print c}')
echo $count
5