通过SSH执行存储在关联数组中的bash命令，存储结果

Question

For a larger project that's not relevant, I need to collect system stats from the local system or a remote system. Since I'm collecting the same stats either way, I'm preventing code duplication by storing the stats-collecting commands in a Bash associative array.

declare -A stats_cmds
# Actually contains many more key:value pairs, similar style
stats_cmds=([total_ram]="$(free -m | awk '/^Mem:/{print $2}')")

I can collect local system stats like this:

get_local_system_stats()
{
    # Collect stats about local system
    complex_data_structure_that_doesnt_matter=${stats_cmds[total_ram]}
    # Many more similar calls here
}

A precondition of my script is that ~/.ssh/config is setup such that ssh $SSH_HOSTNAME works without any user input. I would like something like this:

get_remote_system_stats()
{
    # Collect stats about remote system
    complex_data_structure_that_doesnt_matter=`ssh $SSH_HOSTNAME ${stats_cmds[total_ram]}`
}

I've tried every combination of single quotes, double quotes, backticks and such that I can imagine. Some combinations result in the stats command getting executed too early ( bash: 7986: command not found ), others cause syntax errors, others return null (single quotes around the stats command) but none store the proper result in my data structure.

How can I evaluate a command, stored in an associative array, on a remote system via SSH and store the result in a data structure in my local script?

Answer 1

Make sure that the commands you store in your array don't get expanded when you assign your array!

Also note that the complex-looking quoting style is necessary when nesting single quotes. See this SO post for an explanation.

stats_cmds=([total_ram]='free -m | awk '"'"'/^Mem:/{print $2}'"'"'')

And then just launch your ssh as:

sh "$ssh_hostname" "${stats_cmds[total_ram]}"

(yeah, I lowercased your variable name because uppercase variable names in Bash are really sick). Then:

get_local_system_stats() {
    # Collect stats about local system
    complex_data_structure_that_doesnt_matter=$( ${stats_cmds[total_ram]} )
    # Many more similar calls here
}

and

get_remote_system_stats() {
    # Collect stats about remote system
    complex_data_structure_that_doesnt_matter=$(ssh "$ssh_hostname" "${stats_cmds[total_ram]}")
}

Answer 2

First, I'm going to suggest an approach that makes minimal changes to your existing implementation. Then, I'm going to demonstrate something closer to best practices.

---

Smallest Modification

Given your existing code:

declare -A remote_stats_cmds
remote_stats_cmds=([total_ram]='free -m | awk '"'"'/^Mem:/{print $2}'"'"''
            [used_ram]='free -m | awk '"'"'/^Mem:/{print $3}'"'"''
            [free_ram]='free -m | awk '"'"'/^Mem:/{print $4}'"'"''
            [cpus]='nproc'
            [one_min_load]='uptime | awk -F'"'"'[a-z]:'"'"' '"'"'{print $2}'"'"' | awk -F "," '"'"'{print $1}'"'"' | tr -d " "'
            [five_min_load]='uptime | awk -F'"'"'[a-z]:'"'"' '"'"'{print $2}'"'"' | awk -F "," '"'"'{print $2}'"'"' | tr -d " "'
            [fifteen_min_load]='uptime | awk -F'"'"'[a-z]:'"'"' '"'"'{print $2}'"'"' | awk -F "," '"'"'{print $3}'"'"' | tr -d " "'
            [iowait]='cat /proc/stat | awk '"'"'NR==1 {print $6}'"'"''
            [steal_time]='cat /proc/stat | awk '"'"'NR==1 {print $9}'"'"'')

...one can evaluate these locally as follows:

result=$(eval "${remote_stat_cmds[iowait]}")
echo "$result" # demonstrate value retrieved

...or remotely as follows:

result=$(ssh "$hostname" bash <<<"${remote_stat_cmds[iowait]}")
echo "$result" # demonstrate value retrieved

No separate form is required.

---

The Right Thing

Now, let's talk about an entirely different way to do this:

# no awful nested quoting by hand!
collect_total_ram() { free -m | awk '/^Mem:/ {print $2}'; }
collect_used_ram()  { free -m | awk '/^Mem:/ {print $3}'; }
collect_cpus()      { nproc; }

...and then, to evaluate locally:

result=$(collect_cpus)

...or, to evaluate remotely:

result=$(ssh "$hostname" bash <<<"$(declare -f collect_cpus); collect_cpus")

...or, to iterate through defined functions with the collect_ prefix and do both of these things:

declare -A local_results
declare -A remote_results
while IFS= read -r funcname; do
  local_results["${funcname#collect_}"]=$("$funcname")
  remote_results["${funcname#collect_}"]=$(ssh "$hostname" bash <<<"$(declare -f "$funcname"); $funcname")
done < <(compgen -A function collect_)

...or, to collect all the items into a single remote array in one pass, avoiding extra SSH round-trips and not eval 'ing or otherwise taking security risks with results received from the remote system:

remote_cmd=""
while IFS= read -r funcname; do
  remote_cmd+="$(declare -f "$funcname"); printf '%s\0' \"$funcname\" \"\$(\"$funcname\")\";"
done < <(compgen -A function collect_)

declare -A remote_results=( )
while IFS= read -r -d '' funcname && IFS= read -r -d '' result; do
  remote_results["${funcname#collect_}"]=$result
done < <(ssh "$hostname" bash <<<"$remote_cmd")