Why doesn't a command with quoted whitespace w

问题:

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?

回答1:

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.

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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.

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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.

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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".

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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

回答2:

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.

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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"