From: Brian Candler <b.candler@...>
Date: 2011-02-10T22:18:08+09:00
Subject: Re: System calls with ` in parameters

Gerad S. wrote in post #980690:
> Hi All,
>
> I am trying to add functionality to an interesting script.  The part
> that is tripping me up is that I have to make a system call to echo, and
> feed several parameters.  One of the parameters occasionally includes
> the ` character.
>
> Because of this, the syntax of:
>
> `echo "#{someVar}" | ./some-script.py > "#{outputFile}"`
>
> is breaking when #{someVar} contains the tick `.

Does this test case represent your problem?

>> someVar="foo`bar"
=> "foo`bar"
>> `echo "#{someVar}"`
sh: Syntax error: EOF in backquote substitution
=> ""

Quick workaround: use single quotes instead (but see below).

>> `echo '#{someVar}'`
=> "foo`bar\n"

The same occurs with system():

>> system("echo \"#{someVar}\"")
sh: Syntax error: EOF in backquote substitution
=> false
>> system("echo '#{someVar}'")
foo`bar
=> true

It's an issue with the shell, not with ruby; the same happens at the 
shell prompt too.

$ echo "foo`bar"
> bash: unexpected EOF while looking for matching ``'
bash: syntax error: unexpected end of file

The issue is that in the shell, double-quoted strings can *contain* 
backtick expansions, for example

$ echo "foo`hostname`bar"
foox100bar

Backticks inside single quotes are not treated specially. However, 
single quotes inside single quotes are still a problem:

>> someVar="foo'bar"
=> "foo'bar"
>> `echo '#{someVar}'`
sh: Syntax error: Unterminated quoted string
=> ""

So using the Shellwords escaping mechanism, as already shown, is 
probably what you want.

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