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I am building this project in which I have to execute UNIX commands using ssh. Here is my question : When I execute the following command in a UNIX shell,

echo "Hello           World"

It produces the output : 

Hello           World

With 5 spaces in between as the arguments were given within double quotes. I want to achieve the same effects with ssh as well. I am trying to escape the double quotes using backslash as follows : 

ssh localhost echo \"Hello           World\"

as I do not want bash to process the quotes. But it just produces 

Hello           World

There is only one space after "Hello" Why is that?

Charles Duffy
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Sudha Sompura
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2 Answers2

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ssh executes a command it's given by invoking a remote shell with a script composed of the full set of local arguments concatenated together with whitespace between them.

If you run:

# This has only syntactic quotes -- none of the quotes are literal.
ssh somehost echo "Hello   World"

..then this list of arguments passed to ssh (in C syntax, excluding the hostname and the ssh command itself) is { "echo", "Hello World", NULL }.

Concatenated together into a string, this then becomes "echo Hello World".

Thus, the remote shell runs echo Hello World, which has no syntactic quotes.

If you run:

ssh somehost echo \"Hello   World\"

...then the list of commands (again, in C syntax) is { "echo", "\"Hello", "World\"", NULL }: The quotes are literal, but the spaces were removed by the local shell (before SSH was run) during the word-splitting process.

Concatenated into a string, this becomes echo "Hello World". Thus, you get quotes, but lose your spaces.

By contrast, as a working (but not particularly best-practice) example, consider:

ssh somehost echo \"Hello"   "World\"

Here, you're passing ssh two arguments: The \"s at the beginning and end are literal to the first shell (having no syntactical meaning, so the word Hello is to that first shell unquoted); but the "s just surrounding the whitespace is syntactical, and tells the shell to preserve that whitespace (so the spaces are quoted) when invoking ssh. Thus, you get { "echo", "\"Hello World\"", NULL }, which concatenates to the string echo "Hello World".

The best practice is to avoid this behavior by passing only a single string to ssh containing the entire script you want to run.

# This works
ssh somehost 'echo "Hello   World"'

...if you want to generate that string programatically, you can do so as such (if you know that your remote shell is bash -- even bash-in-/bin/sh mode is safe):

remote_command=( echo "Hello   World" )
printf -v remote_command_q '%q ' "${remote_command[@]}"
ssh somehost "$remote_command_q"

To do that safely with any POSIX-compliant remote shell, see the Python-assisted answer in How to have simple and double quotes in a scripted ssh command

Charles Duffy
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  • What happens when I escape double quotes in the following manner ? ssh somehost echo \"Hello World\" Then I don't just have 2 arguments. I have 3. The 3 will be concatenated as "echo \"Hello World\"". If ssh runs that command, shouldn't it print "Hello world"? – Sudha Sompura Sep 07 '17 at 12:33
  • ...edited to amend my comments into the answer, so I'm clearing them out of the history here. – Charles Duffy Sep 07 '17 at 12:43
  • @SudhaSompura, ...did I miss a question there? Passing `echo "Hello World"` to the remote shell (after using escaping to preserve the quotes on the local side), the quotes are (on the remote end) treated as syntactical; they're used to guide string-splitting by the remote shell before the argument vector is passed to `echo`, so the string `echo` gets is `Hello World` (as opposed to two separate arguments, `Hello` and `World`, as would be the case if there had been no syntactic quotes). – Charles Duffy Sep 07 '17 at 12:54
  • Regarding the "programmatic" approach, can the array idea be applied to a more complex example, such as one with multiple statements? – Tom Fenech Sep 15 '17 at 10:58
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    @TomFenech, the easiest way to pass multiple statements over ssh reliably is to encapsulate them in a function. `myfunc() { echo "Hello World"; echo " * Goodbye World * "; }`, and then you can `ssh somehost "$(declare -f myfunc); myfunc"`, and all the quoting/escaping/etc in your function will be represented correctly. – Charles Duffy Nov 11 '21 at 15:44
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The quotes are present only on the client, and mean that the server sees this:

echo Hello     World

But the quotes are already lost at this point, so that whitespace is collapsed.

You can use single quotes around the whole command:

ssh localhost 'echo "Hello     World"'

Now the local shell does not process the string, so the inner quotes remain intact.

The case with the escaped quotes means that the quotes are ineffective on the local shell, so the whitespace is collapsed locally, then the server sees:

echo "Hello World"
Tom Fenech
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