Argument values containing quotes are unexpectedly parsed as string literals in conditionals

[tomqext]

This came up when calling xacro from python. Not sure if this is an actual problem or expected behavior. I've lost a couple of days on this so at the very least maybe this can serve as a warning to the next person

Problem (?)

When passing argument values that contain single quote characters to xacro, the single quotes are interpreted as string literal delimiters within conditional expressions rather than as part of the value itself. Double quotes are not. This leads to (in my opinion) surprising and inconsistent behavior.

Steps to reproduce

example_problem.xacro

<?xml version="1.0" ?>
<robot name="example_problem" xmlns:xacro="http://www.ros.org/wiki/xacro">

  <xacro:arg name="example_arg" default="" />
  <xacro:property name="example_arg" value="$(arg example_arg)" />

  <xacro:if value="${example_arg == ''}">
    <link name="example_link"/>
  </xacro:if>

</robot>

Example calls to xacro

# Empty string: if conditional evaluates to true - resulting URDF contains example_link
xacro example_problem.xacro example_arg:=

# Literal single quotes: also matches empty string - resulting URDF contains example_link
xacro example_problem.xacro example_arg:="''"

# Literal double quotes: does not match empty string - resulting URDF does not contain example_link
xacro example_problem.xacro example_arg:='""'

This is particularly confusing when calling xacro from Python, where quote characters naturally end up in argument values:

# User intends to pass an empty string, but actually passes literal ""
subprocess.run(['xacro', 'example_problem.xacro', 'example_arg:=""'])

# User intends to pass an empty string, but '' gets parsed as empty string
# (happens to work, but maybe for the wrong reason)
subprocess.run(["xacro", "example_problem.xacro", "example_arg:=''"])

Is this behavior expected? Personally, I'd expect example_arg:="''" to be evaluated in the same way as example_arg:='""', but it feels like there might be some complexities here that I'm not fully aware of

EDIT: Perhaps a better example

example_problem_2.xacro

What if we're trying to match a single space in the conditional?

<?xml version="1.0" ?>
<robot name="example_problem" xmlns:xacro="http://www.ros.org/wiki/xacro">

  <xacro:arg name="example_arg" default="" />
  <xacro:property name="example_arg" value="$(arg example_arg)" />

  <xacro:if value="${example_arg == ' '}">
    <link name="example_link_space"/>
  </xacro:if>

</robot>

xacro example_problem_2.xacro example_arg:='" "'

Results in:

<?xml version="1.0" ?>
...
<robot name="example_problem">
</robot>

Whereas:

xacro example_problem_2.xacro example_arg:="' '"

Results in:

<?xml version="1.0" ?>
...
<robot name="example_problem">
  <link name="example_link_space"/>
</robot>

Environment

xacro version: 1.14.20
ROS: Noetic
OS: Ubuntu 20.04

[rhaschke]

This is expected behavior due to shell's processing of single and double quotes:

The Bash manual states:

3.1.2.2 Single Quotes

Enclosing characters in single quotes (') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

3.1.2.3 Double Quotes

Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their special meaning within double quotes (see Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or newline. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed.

[tomqext]

Hi @rhaschke, thanks for getting back to me.

It is highly possible I have misunderstood the shell's processing of single and double quotes.
I've read your response, and I'm sure I'm missing something, but I can't see how that fits into this.

Enclosing characters in single quotes (') preserves the literal value of each character within the quotes.

Enclosing characters in double quotes (") preserves the literal value of all characters within the quotes, ... - exceptions listed, our case is not one of these exceptions.

Is there a subtle difference in these two statements from the bash manual that I'm missing?

These make sense to me:

echo '"foo"'
"foo"

and

echo "'foo'"
'foo'

But this:
example.xacro

<?xml version="1.0" ?>
<robot name="example_problem" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <xacro:arg name="example_arg" default="" />
  <xacro:property name="example_arg" value="$(arg example_arg)" />
  <xacro:if value="${example_arg == 'foo'}">
    <link name="example_link_foo"/>
  </xacro:if>
</robot>

When called via xacro --verbosity=4 example.xacro example_arg:="'foo'"
giving:

set example_arg: $(arg example_arg) (example.xacro)
use example_arg: foo (example.xacro)
use example_arg: foo (example.xacro)
<?xml version="1.0" ?>
...
<robot name="example_problem">
  <link name="example_link_foo"/>
</robot>

but when called via xacro --verbosity=4 example.xacro example_arg:='"foo"'
giving:

set example_arg: $(arg example_arg) (example.xacro)
use example_arg: "foo" (example.xacro)
<?xml version="1.0" ?>
...
<robot name="example_problem">
</robot>

Doesn't make sense to me

[rhaschke]

In both cases, either '' or "" is passed to python. Maybe, python interprets these inputs differently, maybe xacro does.