Bash: shell quoting

Another issue I had during the weekend that took me hours. Thanks that I have been reading a bit this book (1.6) and had some clues.

I was trying to test a repo to start an Arista lab using docker and I assumed that everything should work if I followed the instructions. My problem was the script trying to push some basic config to the switches.

This is was the initial function:

#!/usr/bin/env bash
...
function fast_cli() {
  params="${*:2}"
  commands="${params//;/\\\n}"
  docker exec "${1}" bash -c "echo -e ${commands} | FastCli -p15 -e
}
...

If you type that command in a bash shell directly is something like this:

$ docker exec DOCKER_ID bash -c 'echo -e "configure\n hostname sp01\n end\n write\n" | FastCli -p15 -e'

As you can see that differs with what we have inside the bash script. So from the bash script we need to put between ‘ the parameter for -c but inside the parameter we need to use “. So I had to make the change below:

-  docker exec "${1}" bash -c "echo -e ${commands} | FastCli -p15 -e"
+  # need to update this command as the quoting doesnt work in my bash
+  docker exec "${1}" bash -c 'echo -e '"'${commands}'"' | FastCli -p15 -e'

The books says Enclose a string in single quotes ‘ unless it contains elements that you want the shell to interpolate. So let’s divide the solution in parts so can be easier to digest (and remember for me in the future because this will bit me again for sure)

• 1st part: ‘echo -e ‘
• 2nd part: “
• 3rd part: ‘${commands}’
• 4th part: “
• 5th part: ‘ | FastCli -p15 -e’

The ” need to be outside the ‘ region because the commands need to be between ” for the docker command. The 3rd part will expand the variable commands.

I guess the author is using a different version of bash? This is mine

$ bash --version
GNU bash, version 5.0.16(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 

This is free software; you are free to change and redistribute it.