Question

I’m getting totally crazy with the following script.

The following command works as expected :

echo a | sed 's/a/b/'

Output :

b

But this script doesn’t :

test="'s/a/b/'"
echo a | sed $test

Output :

sed: -e expression #1, char 1: unknown command : `''

I should really be stupid, but I don’t see what I am missing.

Thanks,

Answer 1

This is because your double wrapping your string. test="'s/a/b'". Sed then gets 's/a/b/' as literal string. You only want sed to receive s/a/b/.
You only need to wrap the string in one set of quotes, otherwise the inner set of quotes will be interpreted as part of the argument.