You only need to create a temp file for the checksum file, not the downloaded contents. In the below example, no file exists on disk with the contents of `$s`.

> $ s='1<space><space>2'

> $ printf %s\\n "$s" | shasum -a 256 > tmp.sum

> $ printf %s\\n "$s" | shasum --check tmp.sum

> -: OK

So you can just `printf '%s<space><space>-\n' "$c" > tmp.sum` and check with `printf %s\\n "$s" | shasum --check --status tmp.sum || { echo "checksum failed" > &2 ; exit 1 ; }`

Having to create temp files is a wrinkle (could probably avoid it by using process substitution if you want to give up on POSIX sh), but so is writing bash scripts in general.