Tutorial: Subprocess Strings

Strings in xonsh follow two simple rules:

  1. Strings in xonsh are always parsed in the same way, and
  2. Python always wins!

Together these rules mean that even strings in subprocess mode are treated like Python strings! This will (help) preserve your sanity.

No Escape

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 spaces with "\ " (backslash-space).

bash

$ echo A\ Single\ Argument
A Single Argument

In the above example, since the spaces are escaped, the echo command only receves a single argument. Xonsh does not allow this. If you were to try this in xonsh, you’d see:

xonsh

$ echo Actually\ Three\ Arguments
Actually\ Three\ Arguments

In this example, echo recives three arguments:: "Actually\\", "Three\\", and "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.

Note

When in doubt in subprocess mode, use quotes!

Justification

The reasons for not having additional escape sequences, as in sh-langs, are:

  1. Escape charaters can get overwhemlingly ugly, fast.
  2. We have escape characters, they are called quotes :)
  3. We have literal input in subprocess mode via macros.

On this last point, if you don’t already know about Subprocess Macros, these allow all input following an ! to be treated as a single argument. For example,

xonsh

$ 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 insane. Instead, xonsh treats these arguments as if they were surrounded in another, outer level of quotation ('foo"bar"baz'). Xonsh will keep the quotation marks when leading and trailing quotes are not matched.

xonsh

$ echo foo"bar"baz
foo"bar"baz

$ echo --key="value"
--key="value"

You can think of these being equivalent to,

xonsh

$ 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.

Further Reading

For deeper details on the great string debate, please feel free to read and comment at: