bash: using scp in cron job fails, but runs succesfully when run from command line

Question

I am trying to use scp in a bash script run by cron (I am running this on Ubuntu 10.0.4 LTS).

The script works fine (i.e. transfers and copies file1 and file2 to/from the remote server, when I run it from the command line. However, when I run the script as a cron job, it fails.

Th is is what the script looks like:

#!/bin/bash

cd /home/oompah/scripts/tests/
scp -P 12345 file1 [email protected]:~/uploads

if scp -P 12345 [email protected]:/path/to/file2.dat local.dat >&/dev/null ; then 
    echo "INFO: transfer OK" ; 
else 
    echo "ERROR: transfer failed" ; 
fi

The error message I get (redirected to a log file) when I run it as a cron job is:

ERROR: transfer failed

The error message I get sent to my mail inbox is:

Permission denied (publickey).
lost connection

Why is happening, and how may I fix it?.

[Edit]

I modified the 1st scp command with an -i command (as suggested by M Jenkins), i also added -v for debug messages. Here is the full debug message log. Hopefully, it can shed some light as to what is going on:

Executing: program /usr/bin/ssh host 12.34.56.78, user oompah, command scp -v -t ~/uploads
OpenSSH_5.3p1 Debian-3ubuntu6, OpenSSL 0.9.8k 25 Mar 2009
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 12.34.56.78 [12.34.56.78] port 12345.
debug1: Connection established.
debug1: identity file /home/oompah/.ssh/id_rsa type 1
debug1: Checking blacklist file /usr/share/ssh/blacklist.RSA-2048
debug1: Checking blacklist file /etc/ssh/blacklist.RSA-2048
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3p1 Debian-3ubuntu3
debug1: match: OpenSSH_5.3p1 Debian-3ubuntu3 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.3p1 Debian-3ubuntu6
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host '[12.34.56.78]:12345' is known and matches the RSA host key.
debug1: Found key in /home/oompah/.ssh/known_hosts:3
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey
debug1: Next authentication method: publickey
debug1: Offering public key: /home/oompah/.ssh/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 277
debug1: PEM_read_PrivateKey failed
debug1: read PEM private key done: type <unknown>
debug1: read_passphrase: can't open /dev/tty: No such device or address
debug1: No more authentication methods to try.
Permission denied (publickey).
lost connection
Permission denied (publickey).

Solution 1

My guess:

You have a password-protected SSH keypair, which is automatically loaded by GNOME Keyring when you login. However, cron does not have access to the keyring, and ssh cannot ask for a password either (due to lack of a tty).

To quote the ssh log you added:

debug1: Offering public key: /home/oompah/.ssh/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 277
debug1: PEM_read_PrivateKey failed
debug1: read PEM private key done: type
debug1: read_passphrase: can't open /dev/tty: No such device or address

Solution 2

What user is cron running as? It looks like that user doesn't have access to your public key.

Solution 3

It sounds like scp isn't picking up your public/private key pair from your ~/.ssh directory.

Try adding

HOME=/home/oompah

into the top of your crontab file (it should already be setting that anyway automatically)

You could also try adding

echo "DEBUG: My home dir is $HOME"

into your script to make sure it's getting the right value.

Another option is to specify the -i parameter to scp to force a specific key pair to use:

scp -i /home/oompah/.ssh/id_rsa ...

for example.

Comments

[oompahloompah]

I tried your suggesting (using the -i option). Please see my updated question

[oompahloompah]

@grawity: Thanks for the information. How do I fix the problem though?

[user1686]

@oompah: Remove the password from your keypair. (If you want, you can create a second one purely for automated use, and give it to scp -i.)

[oompahloompah]

@grawity: Thanks for the feedback. I am not comfortable removing a password from my Keypair (I wouldn't know how to do that in any case). You mention creating a "second one" - presumably you mean a second keypair - but this one with no password - so I can use it for automated logins. BTW, do you mean PASSPHRASE when you say password?

[user1686]

@oompah: If your CentOS machine is physically secure, it's okay. The second keypair suggestion will probably only be useful if you have the primary keypair on many systems. (OpenSSH calls it "passphrase", but not many people actually use an entire phrase for their keys.)

[oompahloompah]

I finally got this to work, after a lot of screwing around with keychain etc. In the end, I settled for you suggestion to create a passwordless keypair for cron and pass that to scp in the shell script. It shouldn't have to be this hard really ... SMH

[user1686]

@oompah: The thing is, at logon your primary SSH key is unlocked by the passphrase kept in GKeyring. The GKeyring keyring is unlocked by your login password, the one you enter. There's no way cron or cron>ssh could access any of these passwords.

[iafisher]

To remove the password from the SSH key, see stackoverflow.com/questions/112396