How to read/write SSH Private key (id_rsa)

Question

As part of an automated Cloud Init setup, I am auto generating public/private keys for users, reading them from file and then saving them in bash variables like so

public_key=$(cat /path/to/id_rsa.pub)
private_key=$(cat /path/to/id_rsa)

Then I write them to the target machine through Cloud Init like so

- sudo -H -u aryan bash -c 'echo "$public_key" > ~/.ssh/id_rsa.pub'
- sudo -H -u aryan bash -c 'echo "$private_key" > ~/.ssh/id_rsa'

The public key is written correctly, but the private key is written to a single line and then SSH complains that it is the wrong format. I am expecting this

-----BEGIN RSA PRIVATE KEY-----                                                                 
 MIICXAIBAAKBgQCpOipW5Xyjc9jLE6AX/0HktpZtyokJap9k5njJl3uw7VpcUITW               
 3UzHHZor4b4N1x8wp17Y0udPsrcPtfJR+pVSr0s6ZMkjX0B7J5jE64iPlVOkO+ww               
 b4CMlonViWeTJ/+gSLkfC2EDfSqPTEOWx44Vs7Mt2qi5Rvq/Po81NDrK2QIDAQAB               
 AoGAcklZ9r2dYzYFn4BtikdVVQUKqrMxwS5E33vW7y5i1qY1dErcq89g3shbKm+W               
 TvqNkeo23+/vT5++idmzATJeTQ+uhRidPrU6XTWd4I8LvHB6dYIGKpGuciv4NWG0               
 3CLDt5IRLpAJ8qAl3tmitWadZw7lJtGAWg+7zdbIoP07XhECQQD32vBAwePlB/ZY               
 CsRNeIDUWtAGaxqBvssmbh4wfCYZDH+3BJbyMG0AWycE0YLZLlACKzTUu5PC8CKu               
 zQBKb02LAkEArsnGdJ7ipDTZZWl1Q42M494SfQUA9+he12WU6O2o2BJqr8cVRG2V               
 BJHHXvdHB+xWRMpo0vxTiGdDIDqPwfPdqwJBAMBpkgvjuYSqur48lYpC21h/q3Dg               
 IrLIqDMMV5lyN61Ie7lb8cbQez5EhTUDZN4vSuN0IU5o1FwIShSDhw9B+uMCQFwN               
 UiJLJ0uZtcCOCL76BnBfnVcQUpE9ZO2FxyXhPGIHWP6YF6BBIhEVAW4HRvZqRojW               
 HNy5HPkigRyxGtLnrx8CQFWZtrGiBIrYRsrf9fwXv4DTB5z7sQLEf8x2dwvif34O               
 +bYMoDJPewr3ti88KJP4rubmIS9PTCAJxEfMBPkZHvE=                                   
 -----END RSA PRIVATE KEY-----

But instead I have this (truncated)

-----BEGIN RSA PRIVATE KEY-----MIICXAIBAAKBgQCpOipW5Xyjc9jLE6AX/0HktpZtyokJ...

Question: How do I correctly read a private key into a variable and then write it to a text file

Note: The easiest way would be to simply copy the file but I don't think Cloudinit supports cp/scp. If it does, please do let me know how

Answer 1

You need to quote your command substitution to preserve newlines:

private_key="$(cat /path/to/id_rsa)"