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
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.
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 $2}BSD mktemp requires a bit more effort to create a random file in /tmp/, but that probably doesn't even matter: BSD xargs doesn't have an option to read input from a file. The way to go here is to pipe your data straight to xargs.
Either pipe a line-separated list of servers to xargs-ssh, or provide a file with the list with the -f option. You may optionally specifiy a command to run with the -c option and enable easily parsable output with the -s option. If server list is not provided, no -f option neither pipe, the script runs the command interactively as each server is typed (end with Ctrl-D).
| Option | argument | Description |
|---|---|---|
| -f | filename | specifies the input file. Use -f filename to read from a file. If no file is specified, stdin is used. |
| -c | command | lets you specify a command to run on each server. By default this command is "uptime". |
| -s | n/a | enables script mode, which keeps the output for each server on one line, new lines in the output are separated by "\n": |
| -h | n/a | prints help message that explains how to use the script. |
Basic usage of the script using a list of servers in a file:
./xargs-ssh.bash -f /home/kenny/serverlist.txtAn example of piping data to the script:
for i in {1..4}; do echo server$i; done | ./xargs-ssh.bashThese 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" -sThe 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