Run a command on remote machine and store its output in variable on remote machine - linux

Question

I want to capture number of rules of iptables that start with specific pattern in comment and then delete them. This is what I want to achieve. Here is my bash script
ssh -o "StrictHostKeyChecking no" root#$ip_address << EOF
echo "Now Removing your IPTables";
#storing output in input variable
input=$(iptables -nL INPUT --line-number | grep ip.* | cut -d " " -f1 | xargs)
#converting variable into an array
arr1=($input);
#loop through each element of array
echo "length:${#arr1[#]}";
for (( i="${#arr1[#]}"-1;i >=0; i-- ));
do
echo "$i:${arr1[$i]}"
iptables -D INPUT $i;
done;
EOF
Problem is the iptables command is not being executed on the remote machine and the output shows the length of arr1 is 0. But I am sure iptables has rules with my desired pattern.
Error being shown in terminal:
-bash: line 9: 3: command not found
Adding 2>&1 in the end of command also not working:
input=$(iptables -nL INPUT --line-number | grep ip.* | cut -d " " -f1 | xargs 2>&1)

Answer 1

TL;DR: Use <<"EOF" instead of <<EOF.
Your Here-Document will expand all variables and evaluate all subshells before the script is even sent to your ssh server.
Consider the following script:
ssh user#servername <<EOF
echo "$(hostname)"
EOF
This will not print servername (the name of the computer you are connecting to) but the name of your localhost instead (the name of the computer you working on).
Before ssh is executed, the subshell $(hostname) is executed. The resulting string "echo localhostname" is then passed to ssh and executed on the remote server.
To fix the problem you have to escape the $ inside the Here-Document or use a literal Here-Document:
ssh user#servername <<"EOF"
echo "$(hostname)"
EOF