Question

I try send a mail with mail command, with the output of other command in the subject

subj="hello from $(hostname -s) $(date)" 
echo "data" | mail -s $subj mail@mail

but I only get the first part of subject (hello from).

why?

Answer 1

You need to quote your subject, like this:

echo "data" | mail -s "$subj" mail@mail

If you don’t quote it, the mail program will not know where your subject ends and will take the first “word” (hello) as the subject and everything else as an address.

In general, it is good practice to always quote your variables.