xargs-ssh

This simple Linux bash script runs multiple ssh commands in parallel by leveraging the parallel processing power of xargs. I first learned of this trick on Jordan Sissel's blog.

There are already a number of tools available to run ssh in parallel, such as

• pssh
• sshpt
• ClusterSSH
• Capistrano
• Fabric
• Func
• MCollective

These tools all have numerous advantages and features that this simple script doesn't. The single advantage that xarg-ssh has over these alternatives is that it's just a bash script that you can run without having to install anything new on either server or client.

At the heart of this script is this command

xargs -a $serverlist -I"SERVER" -P${threads} -n1 sh -c "ssh SERVER $cmd"

• $serverlist is a file containing a newline-separated list of hostnames.
• ${threads} is the maximum number of threads you want it to run simultaneously at any given time.
• $cmd is the command to be run on each server.

If this script is a bit much for you, you could distill it down to just that one line. I recommend wrapping the ssh part in a command substitution in some sort of printf/echo statement if the output matters.

Dependencies:

This script only works on Linux. The following tools are needed to run xargs-ssh.bash:

• bash
• openssh-client (also known as "ssh")
• xargs
• sed
• awk
• tput
• find
• POSIX shell (also known as "sh")
• printf

These are all tools that are commonly found on any GNU/Linux system.

The versions of xargs and mktemp on OSX (the BSD versions) behave very differently to their Linux counterparts. I could adapt this script to work on OSX, but since I personally intend to use this only on Linux (I don't even own a Mac), I probably won't.

In case you want to do it, here are some tips to help you on your way:
Counting the cores on OSX is done like this:

sysctl hw.ncpu | awk '{print $2}

BSD mktemp requires a bit more effort to create a random file in /tmp/, but that probably doesn't even matter: BSD xargs doesn't have an option to read input from a file. The way to go here is to pipe your data straight to xargs.

Usage:

Either pipe a line-separated list of servers to xargs-ssh, or provide a file with the list with the -f option. You may optionally specifiy a command to run with the -c option and enable easily parsable output with the -s option. If server list is not provided, no -f option neither pipe, the script runs the command interactively as each server is typed (end with Ctrl-D).

Option	argument	Description
-f	filename	specifies the input file. Use -f filename to read from a file. If no file is specified, stdin is used.
-c	command	lets you specify a command to run on each server. By default this command is "uptime".
-s	n/a	enables script mode, which keeps the output for each server on one line, new lines in the output are separated by "\n":
-h	n/a	prints help message that explains how to use the script.

Examples:

Basic usage of the script using a list of servers in a file:

./xargs-ssh.bash -f /home/kenny/serverlist.txt

An example of piping data to the script:

for i in {1..4}; do echo server$i; done | ./xargs-ssh.bash

These two commands would have the same output:

###### server3 ######
 11:31:22 up 34 days, 17:09,  2 users,  load average: 0.00, 0.00, 0.00

###### server4 ######
 11:31:22 up 153 days, 22:44,  1 user,  load average: 0.00, 0.00, 0.00

###### server2 ######
 11:31:22 up 34 days, 17:13,  8 users,  load average: 0.12, 0.11, 0.05

###### server1 ######
 05:28:35 up 76 days, 12:29,  0 users,  load average: 0.00, 0.00, 0.00

Specify a custom command and enable script mode:

./xargs-ssh.bash -f /home/kenny/serverlist.txt -c "hostname --fqdn && uptime" -s

The output of each individual ssh reduced to one line of output, where new lines in the output are separated by "\n":

server4: server4.customer.local\n 11:31:22 up 153 days, 22:44,  1 user,  load average: 0.00, 0.00, 0.00
server1: server1.customer.local\n 05:28:35 up 76 days, 12:29,  0 users,  load average: 0.00, 0.00, 0.00
server2: server2.customer.local\n 11:31:22 up 34 days, 17:13,  8 users,  load average: 0.12, 0.11, 0.05
server3: server3.customer.local\n 11:31:22 up 34 days, 17:09,  2 users,  load average: 0.00, 0.00, 0.00