Multiple ssh sessions with different users

Question

I am writing a script that connects to multiple servers which uses 3 different users.

#!/bin/ksh
server1=("abc1" "abc2")
server2=("abc3" "abc4")
server3=("abc5" "abc6")
for i in "${server1[@]}"
do ssh sam@$i "some command"
done
for i in "${server2[@]}"
do ssh danny@$i "some command"
done
for i in "${server3[@]}"
do ssh robert@$i "some command"
done

Is there a way to combine all three for loops` into just one. Actually I want just want to write just one ssh for multiple connections.

I have tried putting all servers in 1 array and users in anotherlike

servers=("abc1" "abc2" "abc3" "abc4" "abc5" "abc6")
users=("sam" "Danny" "Robert")
for i in ${servers[@]}
do
 ssh ${users[@]}@$i "some command"
done

But this way the script puts each user with each server, which ends up locking the service accounts…

What I want is some script that loops all the servers and users and only match user Sam with array server1 user danny with array server2 and user robert with array server3 only. is there a better way to match just the right user with right server in a clean and better way…

Answer 1

Assuming Bash, as the tags say (not ksh as the shebang says), we can use a nameref to use the corresponding array variable given the integer identifier:

users=(. userxyz user123 user221)
for n in 1 2 3
do
    declare -n server=server$n
    for i in "${server[@]}"
    do ssh "${users[$n]}@$i" "some command"
    done
done

With a suitable echo, we get the same output as original script:

ssh userxyz@abc1 some command
ssh userxyz@abc2 some command
ssh user123@abc3 some command
ssh user123@abc4 some command
ssh user221@abc5 some command
ssh user221@abc6 some command

To adapt this for Korn shell, you would need to use the nameref built-in command instead of declare -n.

---

A simpler approach may be just to consider the username/hostname together:

servers=("userxyz@abc1" "userxyz@abc2"
         "user123@abc3" "user123@abc4"
         "user221@abc5" "user221@abc6")
for i in "${servers[@]}"
do ssh "$i" "some command"
done

Answer 2

If user1 is only associated with server1a and server1b, user2 is associated with server2a, server2b and server2c etc., then

#!/bin/sh
set -- 

    user1 server1a server1b -- 

    user2 server2a server2b server2c -- 

    user3 server3a --
while [ "$#" -ne 0 ]; do
    user=$1; shift
until [ "$1" = -- ]; do
    ssh "$user@$1" 'some command'
    shift
done

# Shift off that "--" delimiter
shift

done

This uses the positional parameters to store the user names and the corresponding list of servers. Since it's a single list of strings, we delimit the sections of the list containing user names and server names with -- so that we easily can see when to expect either a new user name or the end of the list.

The outer loop loops over the sections of the list and assigns the user name to the variable user, while the inner loop loops over the server names and calls ssh until the string -- is found.

The script is written for /bin/sh and would therefore also work for both ksh and bash.

Given the code above, the following ssh commands would get executed:

ssh user1@server1a 'some command'
ssh user1@server1b 'some command'
ssh user2@server2a 'some command'
ssh user2@server2b 'some command'
ssh user2@server2c 'some command'
ssh user3@server3a 'some command'

If you remove the initial set command, you would be able to call the script from the command line like this, for the same effect:

$ ./script user1 server1a server1b -- user2 server2a server2b server2c -- user3 server3a --

With an extra check (|| [ "$#" -eq 0 ]) in the inner loop, you could leave off that last -- on the command line.

---

If what you want to do is to apply the DRY principle and only call ssh from a single point in your code, then lift that bit out into a function. Here's your code, modified to do that:

#!/bin/ksh
server1=("abc1" "abc2")
server2=("abc3" "abc4")
server3=("abc5" "abc6")
process () {
    ssh "$1@$2" 'some command'
}
for server in "${server1[@]}"; do
    process sam "$server"
done
for server in "${server2[@]}"; do
    process danny "$server"
done
for server in "${server3[@]}"; do
    process robert "$server"
done