Why is “& ;” invalid syntax?

Question

I am trying to run a for loop on the terminal where I want to send each iteration to background process so that all of them run simultaneously.

Following is the command

Answer 1

& and ; both terminate the command that precedes them.

You can't write & ; any more than you could write ; ; or & &, because the language only allows a command to be terminated once (and doesn't permit a zero-word list as a command).

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Thus: for i in *.src; do fastq-dump --split-files "$i" & done is perfectly correct as-is, and does not require an additional ;.