Question

I need to execute several shell commands using python, but I couldn’t resolve one of the problems. When I scp to another machine, usually it prompts and asks whether to add this machine to known host. I want the program to input “yes” automatically, but I couldn’t get it to work. My program so far looks like this:

from subprocess import Popen, PIPE, STDOUT

def auto():
  user = "abc"
  inst_dns = "example.com"    
  private_key = "sample.sem"
  capFile = "/home/ubuntu/*.cap"

  temp = "%s@%s:~" %(user, inst_dns)
  scp_cmd = ["scp", "-i", private_key, capFile, temp]

  print ( "The scp command is: %s" %" ".join(scp_cmd) )
  scpExec = Popen(scp_cmd, shell=False, stdin=PIPE, stdout=PIPE)
  # this is the place I tried to write "yes" 
  # but doesn't work
  scpExec.stdin.write("yes\n")
  scpExec.stdin.flush()
  while True:
    output = scpExec.stdout.readline()
    print ("output: %s" %output)
    if output == "": 
      break

If I run this program, it still prompt and ask for input. How can I response to the prompt automatically? Thanks.

Answer 1

You’re being prompted to add the host key to your know hosts file because ssh is configured for StrictHostKeyChecking. From the man page:

StrictHostKeyChecking

If this flag is set to “yes”, ssh(1) will never automatically add host keys to the ~/.ssh/known_hosts
file, and refuses to connect to hosts whose host key has changed. This provides maximum protection
against trojan horse attacks, though it can be annoying when the /etc/ssh/ssh_known_hosts file is
poorly maintained or when connections to new hosts are frequently made. This option forces the user
to manually add all new hosts. If this flag is set to “no”, ssh will automatically add new host keys
to the user known hosts files. If this flag is set to “ask”, new host keys will be added to the user
known host files only after the user has confirmed that is what they really want to do, and ssh will

You can set StrictHostKeyChecking to “no” if you want ssh/scp to automatically accept new keys without prompting. On the command line:

scp -o StrictHostKeyChecking=no ...

You can also enable batch mode:

BatchMode

If set to “yes”, passphrase/password querying will be disabled. This option is useful in scripts and
other batch jobs where no user is present to supply the password. The argument must be “yes” or
“no”. The default is “no”.

With BatchMode=yes, ssh/scp will fail instead of prompting (which is often an improvement for scripts).