openssl command line to verify the signature

I found two solutions to your problem.

You can use rsautl that way: (with private key: my.key and public key my-pub.pem)

$ openssl rsautl -sign -inkey my.key -out in.txt.rsa -in in.txt
Enter pass phrase for my.key:
$ openssl rsautl -verify -inkey my-pub.pem -in in.txt.rsa -pubin
Bonjour

With this method, all the document is included within the signature file and is outputted by the final command.

But in my case, my certificate says: Signature Algorithm: sha1WithRSAEncryption.
So I would recommend you to use the standard way of signing document in 4 steps: (This method is used for all asymmetric electronic signatures in order not to overcharge the signature file and/or CPU usage)

1. Create digest of document to sign (sender)
2. Sign digest with private key (sender)
3. Create digest of document to verify (recipient)
4. Verify signature with public key (recipient)

OpenSSL does this in two steps:

$ openssl dgst -sha256 -sign my.key -out in.txt.sha256 in.txt 
Enter pass phrase for my.key:
$ openssl dgst -sha256 -verify my-pub.pem -signature in.txt.sha256 in.txt  
Verified OK

With this method, you sent the recipient two documents: the original file plain text, the signature file signed digest. Attention: the signature file does not include the whole document! Only the digest.