Tutorial: Subprocess Strings¶
Strings in xonsh follow two simple rules:
Strings in xonsh are always parsed in the same way, and
Python always wins!
Together these rules mean that even strings in subprocess mode are treated like Python strings! This will (help) preserve your sanity.
Xonsh strings are exactly like Python strings everywhere. Xonsh uses
exactly the same escape characters that Python does; no more and no less.
This is different from other shells, which have a different set of escape
sequences than Python has. Notably, many sh-langs allow you to escape
"\ " (backslash-space).
$ echo A\ Single\ Argument A Single Argument
In the above example, since the spaces are escaped, the
only receves a single argument. Xonsh does not allow this. If you were
to try this in xonsh, you’d see:
$ echo Actually\ Three\ Arguments Actually\ Three\ Arguments
In this example, echo recives three arguments::
"Arguments". Instead, xonsh requires you to use quotes in order to
pass in a single argument:
xonsh or bash
$ echo "A Single Argument" A Single Argument
Using quotes is arguably what should have been done in sh-lang in the first place.
When in doubt in subprocess mode, use quotes!
The reasons for not having additional escape sequences, as in sh-langs, are:
Escape charaters can get overwhemlingly ugly, fast.
We have escape characters, they are called quotes :)
We have literal input in subprocess mode via macros.
On this last point, if you don’t already know about
these allow all input following an
! to be treated as a single argument.
$ echo! A Single Argument A Single Argument
Subprocess macros are the ultimate escape mechanism.
The Quotes Stay¶
In sh-langs, internal quote characters are removed. For instance:
$ echo foo"bar"baz foobarbaz $ echo --key="value" --key=value
Xonsh considers this behavior suboptimal. Instead, xonsh treats these
arguments as if they were surrounded in another, outer level of
'foo"bar"baz'). Xonsh will keep the quotation marks
when leading and trailing quotes are not matched.
$ echo foo"bar"baz foo"bar"baz $ echo --key="value" --key="value"
You can think of these being equivalent to,
$ echo 'foo"bar"baz' foo"bar"baz $ echo '--key="value"' --key="value"
This is yet another major point of departure for xonsh from traditional shells. However, the xonsh subprocess string handling is consistent and predictable.
For deeper details on the great string debate, please feel free to read and comment at: