I noticed, that when trying to generate some hashes after copy-pasting them into a file with vim, the hash is not as it is supposed to be. Same when file was opened and written out. Same behavior with nano, so there must be something I am mising.
$ echo -n "foo" | openssl dgst -sha256
2c26...e7ae
$ echo -n "foo" > hash.txt
$ openssl dgst -sha256 hash.txt
SHA256(hash.txt)= 2c26...e7ae
But when I open hash.txt with nano or vim and quit without inserting anything, I subsequently get the following hash: b5bb...944c
I also noticed that without opening the file and writing out I do not see the output when using cat or head. Was the encoding changed?
Most text editors, including Vim, save the file with a newline at the end when you quit. That's because according to POSIX, a text file is either empty or ends with a newline, so most users expect and want that behavior.
As you've noticed, adding a newline at the end changes the hash, and the hash you're getting is consistent with the sequence foo\n, where \n is a newline.
If you have a recent enough Vim, you can control the behavior mentioned above by setting nofixeol, which will preserve the existing line ending, or lack thereof, on the last line.