Bash - Passing an array to remote hosts via ssh

Question

I wrote a Bash script that logs in to multiple remote hosts and runs smartctl (from the smartmontools package) on disks defined in the array DISKS. I'm able to pass the array to the remote hosts, but the script just echos the first element over and over. It seems like it's treating the array as a string, and ignoring all elements after the first.

DISKS=("/dev/sda" "/dev/sdb" "/dev/sdc")
HOSTS=("Ariadne" "Nyx")

for i in "${HOSTS[@]}"; do
    ssh "$i" "bash" << EOF
        for j in "${DISKS[@]}"; do
            echo "$j"
        done
EOF
done

I've tried defining the array on the remote host (before the 2nd for loop): DISKS=("${DISKS[@]}"), but that did not help.

How can I properly pass an array to a remote host via SSH, and then iterate through the array?

Answer 1

You can pass the definition of the array to the remote bash like so (here assuming the login shell of the remote user is bash):

DISKS_definition=$(typeset -p DISKS)
for i in "${HOSTS[@]}"; do
  ssh "$i" "$DISKS_definition"'
    for j in "${DISKS[@]}"; do
      echo "$j"
    done'
done

Or if using the here-document approach:

DISKS_definition=$(typeset -p DISKS)
for i in "${HOSTS[@]}"; do
  ssh "$i" bash <<EOF
    $DISKS_definition
    for j in "\${DISKS[@]}"; do
      echo "\$j"
    done
EOF
done

If your ssh client and server allow passing LC_* variables, you could also do

DISKS_definition=$(typeset -p DISKS)
for i in "${HOSTS[@]}"; do
  LC_DISKS=$DISKS_definition ssh "$i" bash <<'EOF'
    eval "$LC_DISKS"
    for j in "${DISKS[@]}"; do
      echo "$j"
    done
EOF
done

All is in the quoting that determines which of the local or remote shell does the expansions. Remember variables are expanded inside double quotes and here-documents where the delimiter is not quoted (note the <<'EOF' in the last example that prevents expansions inside the here-document while in the earlier examples (with <<EOF) we had to use backslashes for things that we don't want expanded by the local shell).

For arrays that contain non-ASCII data, you'll want to make sure that the locale's character set is the same on the local and remote machine, or at least avoid multi-byte character sets other than UTF-8.

bash-4.3$ locale charmap
ISO-8859-1
bash-4.3$ a=($'foo\xa3$(uname>&2)bar' baz)
bash-4.3$ a_definition=$(typeset -p a)
bash-4.3$ LC_ALL=zh_HK.big5hkscs bash -c "$a_definition"
Linux

By switching to a different character set, the content of $a_definition took a different meaning (here causing the uname>&2 command to be executed).

Answer 2

Your echo "$j" is evaluated when the ssh command line is evaluated, so inside the for j... for loop it's actually a constant (and undefined, at that) rather than a variable.

Try this

for i in "${HOSTS[@]}"; do
    ssh "$i" "bash" << EOF
        for j in "${DISKS[@]}"; do
            echo "\$j"
        done
EOF
done

Or this, if your $DISKS array contains safe words without whitespace.

for i in "${HOSTS[@]}"; do
    ssh "$i" "for j in ${DISKS[@]}; do echo \"\$j\"; done"
done

---

My preferred way of solving this kind of problem is to create a script that performs the necessary work and have that reside on each remote server. Calling the script on each server then becomes a straightforward and simple process:

for h in "${HOSTS[@]}"; do ssh "$h" /usr/local/bin/myscript; done

If you're worried about the script becoming out of date, it's not an impossible task to copy the file across from the local system before running it. Something like this would probably work:

for h in "${HOSTS[@]}"
do
    scp -p /local/path/to/myscript "$h":myscript.$$
    ssh "$h" "myscript.$$; rm -f myscript.$$"
done