Run command on remote server over SSH - without exiting

Question

I connect to my school's Linux Lab frequently to work on my programming assignments remotely. Sometimes, when there are lots of other students logged in to the server, the connection is slow, and I lose work during connection timeouts. There are several servers to choose from in the lab, and I want to be able to automatically run who as soon as the connection is made, so I can see how crowded the server is, and use another one if it's pretty full. Right now, I use a function in my .bash_aliases file to streamline the connection and password entry:

In file ~/.bash_aliases:

#!/bin/bash

# ssh to the Linux lab. Takes hostnames defined in ~/.ssh/config as 
# parameters
function sshll()
{
    if [ "$@" ]
      echo "Connecting to hostname $@";
      sshpass -f <password_file> ssh $@;
    else
      echo "Connecting to default host";
      sshpass -f <password_file> ssh <user@ipaddress>;
    fi
}

This works, so I added who to the end of the ssh commands:

In file ~/.bash_aliases:

#!/bin/bash

# ssh to the linux lab. Takes hostnames defined in ~/.ssh/config as 
# parameters
function sshll()
{
    if [ "$@" ]
      echo "Connecting to hostname $@";
      sshpass -f <password_file> ssh $@ 'who';
    else
      echo "Connecting to default host";
      sshpass -f <password_file> ssh <user@ipaddress> 'who';
    fi
}

This connects, enters my password automatically, and runs who, but then closes the connection. Is there a way I can run who automatically, without closing the connection afterward?

score 6 · Accepted Answer · edited May 23 '17 at 12:39

TL;DR: int_ua's answer is the way to go, simplest and effortless.

Why this works the way it does

This goes back to the basic behavior of any shell. When you login simply as ssh user@server.domain.whatever you get interactive version of the shell. When you run ssh user@some.server.net -t "command1;command2" you basically get /bin/sh -c 'command1;command2'. The first one lets you run whatever you put into the stdin while sourcing the dot files, while second just executes those commands and exits. Essentially, there is no way to run a command before interactive use ( unless it is in one of the dot files ) or get an interactive shell from the single-shot command sh -c (or whichever shell it may be, not necessarily sh ).

Failed or incomplete ideas

My first idea I had is this : if the shell uses server's local .bashrc , could one make it use client's local file on the server ? Well,no. Not unless you copy the dot file over to remote server with scp or rsync.

Second idea that i had is the PROMPT_COMMAND. This very nice bash variable runs command each time before showing your PS1 prompt, so why not set this variable before spawining bash to PROMPT_COMMAND="who; unset PROMPT_COMMAND" so that we run it as single-shot?

Problem is this variable has to be exported to the remote server, and Gilles answer helped me to do something like this:

ssh -o SendEnv=PS1 ssh localhost -t bash

or

ssh localhost -t " PROMPT_COMMAND='who; unset PROMPT_COMMAND' bash  "

Problem ? -o option SendEnv must allow that specific variable that you are trying to send in /etc/ssh/sshd_config , and besides in either case you still get to run all that stuff with /bin/sh -c

Slight improvement to int_ua's answer

So by now we can see that ssh user@server -t 'command1;shell' pattern works the best. Except there is one minor nitpick: /bin/sh -c process is still there , so why not run bash with exec to replace that process out?

 ssh user@server.net -t 'who;exec bash'

score 5 · Answer 2 · answered Feb 21 '16 at 03:43

5

Just executing bash should be enough:

 sshpass -f <password_file> ssh <user@ipaddress> 'who; bash';

answered Feb 21 '16 at 03:43

int_ua

8,892

Michael Hoffmann · Answer 3 · 2016-02-21T02:53:47.120

This is an answer, but definitely not the answer.

I discovered that sshd (the daemon for ssh services on the remote host) will run commands in ~/.ssh/rc upon connection. I just created this file, and added who there, and now a list of users is displayed every time I make a remote connection, and I still get a login shell.

On remote host:

File ~/.ssh/rc:

#!/bin/bash

# This file is executed by sshd when a remote connection is started over ssh
who

On client:

In file ~/.bash_aliases:

#!/bin/bash

# ssh to the linux lab. Takes hostnames defined in ~/.ssh/config as 
# parameters
function sshll()
{
    if [ "$@" ]
      echo "Connecting to hostname $@";
      sshpass -f <password_file> ssh $@;
    else
      echo "Connecting to default host";
      sshpass -f <password_file> ssh <user@ipaddress>;
    fi
}

I am still interested in knowing how to do this from the client side, however, for other users with similar problems who can't use ~/.ssh/rc for lack of permissions, or whatever other reason.

score 3 · Answer 4 · edited Apr 13 '17 at 12:24

int_ua's answer is what I'd use. Just for completeness, if you can edit your own .profile or .bashrc on the remote server, you could append this to either one:

if [[ -n $SSH_TTY ]]
then
    who
fi

SSH sets various variables, one of which is SSH_TTY - you can test for these variables to determine if you're connected via SSH. Restrictions on ~/.ssh/rc should not prevent you from using this.

I use this to set my prompt (note how I skip the hostname for local shells):

if [[ -n $SSH_TTY ]]
then
    PS1="\u@\h:\w \$ "
else
    PS1="\u:\w \$ "
fi

score 1 · Answer 5 · answered Jun 18 '21 at 14:42

I know this is an old question, but I wonder why nobody mentioned expect, that seems to be just designed for such cases. (Of course, you need to install the expect package first).

!#/usr/bin/expect
spawn ssh user@host
expect "assword:"
send "mypassword\r"
expect "$ "
send "who\r"
interact

Jafar Amini · Answer 6 · 2021-10-24T13:25:58.357

If you want ssh and execute commands without exiting:

Method 1

ssh  -t USER@SERVER "command1; command2;bash --rcfile ~user/.bashrc"

OR

Method 2

ssh -t USER@SERVER "$( cat <<'EOT'
COMMANDS
bash --rcfile ~user/.bashrc
EOT
)"

Sending variables to commands in Method One:

var1="Hello World"
var2="Salam World"
ssh  -t USER@SERVER "echo $var1; echo $var2; command3;bash --rcfile ~user/.bashrc"

Works fine

I recommend use Method One and call stored shell in server and send needed variables to that:

ssh  -t USER@SERVER "bash /path/to/server/stored/script.sh $var1 $var2 $other_var;bash --rcfile ~user/.bashrc"

Run command on remote server over SSH - without exiting

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