SCP doesn't work when echo in .bashrc?

问题:

I have two users in Fedora:

1. Wani
2. root (quite obvious!)

My contents of .bashrc of user Wani are:

# .bashrc
echo "Hello"
# Source global definitions
if [ -f /etc/bashrc ]; then
    . /etc/bashrc
fi

# User specific aliases and functions

Now after logging into root, I type the following commands:

[root@Dell Wani]# touch try.txt
[root@Dell Wani]# service sshd start
[root@Dell Wani]# scp try.txt Wani@localhost:~/
Wani@localhost's password: 
Hello
[root@Dell Wani]# 

Now I log into Wani, and type:

[Wani@Dell ~]$ cat try.txt
cat: try.txt: No such file or directory
[Wani@Dell ~]$ 

Now I again log into root and type the same command with -v:

[root@Dell Wani]# scp -v morph.log Wani@localhost:
Executing: program /usr/bin/ssh host localhost, user Wani, command scp -v -t -- .
OpenSSH_5.6p1, OpenSSL 1.0.0j-fips 10 May 2012
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to localhost [127.0.0.1] port 22.
debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: identity file /root/.ssh/id_rsa type -1
debug1: identity file /root/.ssh/id_rsa-cert type -1
debug1: identity file /root/.ssh/id_dsa type -1
debug1: identity file /root/.ssh/id_dsa-cert type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.6
debug1: match: OpenSSH_5.6 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_5.6
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 'localhost' is known and matches the RSA host key.
debug1: Found key in /root/.ssh/known_hosts:2
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-     with-mic,password
debug1: Next authentication method: gssapi-keyex
debug1: No valid Key exchange context
debug1: Next authentication method: gssapi-with-mic
debug1: Unspecified GSS failure.  Minor code may provide more information
Credentials cache file '/tmp/krb5cc_0' not found

debug1: Unspecified GSS failure.  Minor code may provide more information
Credentials cache file '/tmp/krb5cc_0' not found

debug1: Unspecified GSS failure.  Minor code may provide more information


debug1: Unspecified GSS failure.  Minor code may provide more information


debug1: Next authentication method: publickey
debug1: Trying private key: /root/.ssh/id_rsa
debug1: Trying private key: /root/.ssh/id_dsa
debug1: Next authentication method: password
Wani@localhost's password: 
debug1: Authentication succeeded (password).
Authenticated to localhost ([127.0.0.1]:22).
debug1: channel 0: new [client-session]
debug1: Requesting [e-mail]
debug1: Entering interactive session.
debug1: Sending environment.
debug1: Sending env XMODIFIERS = @im=none
debug1: Sending env LANG = en_US.UTF-8
debug1: Sending command: scp -v -t -- .
Hello
[root@Dell Wani]# debug1: client_input_channel_req: channel 0 rtype exit-status      reply      0
debug1: channel 0: free: client-session, nchannels 1
debug1: fd 0 clearing O_NONBLOCK
debug1: fd 1 clearing O_NONBLOCK
Transferred: sent 1664, received 1976 bytes, in 0.1 seconds
Bytes per second: sent 22961.5, received 27266.8
debug1: Exit status 0

(And after I press Enter)

[root@Dell Wani]# 

Can anyone please shed some light as to what exactly happened here? Why did the file not get copied to Wani from root?

回答1:

Using echo in a .bashrc will break scp, as scp expects to see its protocol data over the stdin/stdout channels. See https://bugzilla.redhat.com/show_bug.cgi?id=20527 for more discussion on this issue.

There's a few workarounds available:

• Condition on the 'interactive' flag (e.g. case $- in *i* as suggested by tripleee)
• Use the tty utility to detect an interactive shell (e.g. if tty > /dev/null or if [ -t 0 ])
• Check the value of $SSH_TTY

I suppose you should use whichever one works for you. I don't know what the best (most portable/most reliable) option is, unfortunately.

回答2:

To add to nneonneo's options, you can also condition with the interactive flag with

if [[ $- =~ "i" ]]

which I think is possibly the clearest way in bash.

回答3:

This works for me,
In .bashrc add first line as:

if [ -z "$PS1" ]; then
    return
fi

https://superuser.com/questions/690735/can-i-tell-if-im-in-an-scp-session-in-my-bashrc

回答4:

The default Ubuntu .bashrc contains the following snippet which already takes care of the problem:

# If not running interactively, don't do anything
case $- in
    *i*) ;;
    *) return;;
esac

回答5:

In .bashrc, use STDERR as output instead:

echo "# Important Notice" >&2

Update: do not use it! We had an issue recently that a (closed source) tool failed due to an echo to STDERR in .bashrc. The tool (using rcp) expected no output at all, neither on STDOUT nor STDERR. And it stuck when it got the echo. Lesson learned: make separate accounts for humans and for machines (scripts), or just stop tattling via .bashrc.

回答6:

The most portable way of testing for an interactive shell seems to be:

test -t 0
if [ $? -eq 0 ]
then
    # interactive
    ;
else
    # non-interactive
    ;
fi

回答7:

nneonneo's solution worked for me as well. But since my default shell is TCSH, I had to slightly edit the fix as follows (in .tcshrc):

if ( $?SSH_TTY ) then
    exec /bin/bash
endif

Just thought I would share for everyone's benefit.

回答8:

if [ 0 -eq $(shopt -q login_shell; echo $?) ]; then
  echo "do something?"
fi

Source

回答9:

If you're on Red Hat Enterprise Linux (RHEL) or variant, drop a script that does the echo, or whatever you want, into /etc/profile.d/