Generating a SHA-256 hash from the Linux command line

Question

I know the string \"foobar\" generates the SHA-256 hash c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2 using http://hash.online-convert.com/sh

Answer 1

echo produces a trailing newline character which is hashed too. Try:

/bin/echo -n foobar | sha256sum