Replace your SSH loop script

From ControlTier

This page introduces the idea of command dispatching using ctl-exec as an alternative to SSH looping scripts.

Scenario

You have a shell script that basically is an SSH loop around a set of remote hosts, where at each pass, you execute one or more commands. It has a structure something like this:

FORALL host in HOST-LIST
  SSH host command statement
  .
  .
  .
END FORALL

Here's an example Bash script that executes the uname command across two hosts:

#!/bin/bash
NODES="centos demo@development"
for n in $NODES
do
  ssh $n uname -a
done

Issues

• Can't debug it on one of the remote nodes
• It mixes looping and ssh calls
• You have to edit your script anytime you need to change host targets. This is undesirable because it leads to a lot of one-off copies which end up being a burden to maintain.
• Often these are just a temporary solution so functionality, like error handling, are not implemented

Alternatives

Here are some alternatives that use ctl-exec.

Benefits:

• Less code complexity: Looping, node addressing, and individual steps separated.
• Flexibility: Use the node filtering options
• Error recovery: Use "-K" to skip over any node where the command fails

Single command

If you have just a single command statement, you can use ctl-exec like so:

ctl-exec -- uname -a

Scripts

You can have ctl-exec execute multiple commands in the form of a shell script.

Script file

You might prefer to save a series of command statements to a script file. The ctl-exec command will copy the script and execute it on each remote host:

File listing: info.sh

#!/bin/bash
uname -a;
who;

You can let ctl-exec copy that script to the target host and then run it:

ctl-exec -s info.sh

Stdin

The ctl-exec command can read the script statements from standard input. You can specify the "-S" or "--stdin" option to have ctl-exec read from standard input.

Here's an example of the uname command being read from stdin:

$ echo "uname -a"|ctl-exec -S

You can also use the "here document" syntax:

ctl-exec -S <<EOS
command statement 1;
command statement 2;
...
command statement N;
EOS

Here's an example using the here document syntax:

ctl-exec -S <<EOS
uname -a;
who;
EOS