How to write a dynamically generated heredoc in bash?

Question

I have a shell script that connects to a remote machine to perform actions. One of those actions, is to set one or more DNS servers. While largely static data is easy to capture and pipe to the remote machine via SSH:

config_ntp()
{
    ssh -T admin@server_ip <<-NTPSERVER
    sysconf ntp addserver $NTPSERVER
    NTPSERVER
}

Creating a dynamically sized list of commands, is trickier than I thought. What I have:

DNSSERVERS=(8.8.8.8 8.8.8.7)

config_dns()
{
    cmd=""
    for server in ${DNSSERVERS[@]}; do
        cmd+="network dns add nameserver $server$'\n' "
    done
    cmd+="service dns restart$'\n'"
    echo -e "cmd: $cmd"
    ssh -T admin@server_ip $cmd
}

The result of calling this:

$ sh setup.sh
cmd: network dns add nameserver 8.8.8.8$'
' network dns add nameserver 8.8.8.7$'
' service dns restart$'
'
Syntax Error: Invalid character detected: '\'.
Command Result : 22 (Invalid argument)
Exiting...

That was my latest incarnation. I was playing with $'\n' as suggested elsewhere... previously, I just had \n, which resulted in the same error.

How do I create a variable containing a list (variable length, dynamically generated) of commands to pipe, via ssh, to a remote machine?

Answer 1

Why do you want or need a here document? Providing standard input to the process by some other means is almost definitely easier if it's not just basically a static template with a few placeholders you fill with variables.

ssh <<____HERE
commands
more commands "$variable"
____HERE

is exactly equivalent to

printf "commands\nmore commands \"%s\"\n" "$variable" | ssh

If some parts are more complex than just printing something, you can make it arbitarily complex:

{ complex_function --with --options and arguments  -o stdout
  printf "echo 'anything we send to stdout goes into the pipe'\n"
  if command; then
      while another command; do
          for arguments in $(something to drive a loop); do
               complex stuff
          done
      done
  fi
  : and etc
} | ssh

Use curly parentheses ( commands ) instead of braces { commands } before the pipe to run commands in a subshell.

Your concrete problem is that you can't put $'\n' inside a regular double-quoted string. echo already produces a newline so you seem to be taking a very roundabout way of constructing things. But try this:

# Does this really need to be in a separate variable?
DNSSERVERS=(8.8.8.8 8.8.8.7)
config_dns()
{
    {
      # To be completely correct, notice quoting around array
      for server in "${DNSSERVERS[@]}"; do
        echo "network dns add nameserver $server"
      done
      echo "service dns restart"
    } |
    # maybe add a tee /dev/stderr or something here to see what's going on
    ssh -T admin@server_ip
}

For aesthetics, I would perhaps refactor to

config_dns()
{
    {
        printf 'network dns add nameserver %s\n' "${DNSSERVERS[@]}"
        printf 'service dns restart\n'
    } |
    ssh -T admin@server_ip
}

Answer 2

First of all, you're mixing up passing text to stdin of a command with passing it as an argument. You should change your ssh invocation to:

ssh -T admin@server_ip <<< "$cmd"

It uses herestrings to pass text to stdin of a command (similar to what heredocs do).

Second, for $'\n' notation to work, you need to place it outside of double quotes:

cmd+="network dns add nameserver $server"$'\n'" "

Answer 3

Quoting through ssh is very complicated. Don't do it if you don't have to.

Instead of trying to separate your commands with \n, just use ;. It works just as well and doesn't require so much quoting.

DNSSERVERS=(8.8.8.8 8.8.8.7)

config_dns()
{
    cmd=""
    for server in ${DNSSERVERS[@]}; do
        cmd+="network dns add nameserver $server;"
    done
    cmd+="service dns restart"
    echo -e "cmd: $cmd"
    ssh -T admin@server_ip "$cmd"
}