ayrton - a shell like language with the power of python.
ayrton is an extension of the Python language that tries to make it look more
like a shell programming language. It takes ideas already present in sh, adds
a few functions for better emulating envvars, and provides a mechanism for (semi)
transparent remote execution via ssh.
This code is released under the GPLv3. If you're unsure on how this apply to your interpreted programs, check this entry in their FAQ.
Currently ayrton is under heavy development, so if you're following it and
clone it (there are no releases yet), use the branch develop.
ayrton depends on three pieces of code. Python is the most obvious; it has been
developed in its version 3.3. Python 3.2 is not enough, sorry. Next is sh, version
1.08. The last item is more complicated. It uses
paramiko, but as this project tries to
be compatible with lower versions of Python2, there's no official port for Python3.
We used an unofficial port that works pretty
well so far. As Python3 has not completely caught yet, most probably even less
in stable server environments, we plan to support at least Python2.7.
So, in short:
# apt-get install python3
# git clone https://github.com/amoffat/sh.git
# cd sh
# python3 setup.py install
# cd ..
# apt-get install python3-crypto
# git clone https://github.com/nischu7/paramiko.git
# cd paramiko
# python3 setup.py install
# cd ..
# git clone https://github.com/StyXman/ayrton.git
# cd ayrton
# make tests
# python3 setup.py install
or edit Makefile and
# make install
To generate the docs:
# make docs
ayrton syntax is Python3's with some things changed. Here's the unavoidable
'Hello world' example:
print ('Hello, World!')
Nothing fancy, right? Let's try something slightly different:
echo ('Hello, World!')
Not interested yet? What if I tell you that that echo function just
executed /bin/echo?:
mdione@diablo:~/src/projects/ayrton$ strace -e process -ff ayrton doc/examples/hw.ay
execve("/home/mdione/local/bin/ayrton", ["ayrton", "doc/examples/hw.ay"], [/* 40 vars */]) = 0
[...]
[pid 404] execve("/bin/echo", ["/bin/echo", "Hello, World!"], [/* 40 vars */]) = 0
With sh you could from sh import echo and it will create a callable that will
transparently run /bin/echo for you; ayrton takes a step further and creates
the callable on the fly, so you don't have to pre-declare it. Another difference
is that under sh, echo's output gets captured by default, which means that
you don't see it unless you later print it. ayrton tries to be more shell-like,
sending the output where it should. If you want to capture the output, just tell
it so:
hw= echo ('Hello, World!', _out=Capture)
While we're discussing output, check out this:
echo ('Hello, World!', _out=None)
Just guess were the output went :) ... (ok, ok, it went to /dev/null).
Just like sh, you can nest callables, but you must explicitly tell it that you
want to capture the output so the nesting callable gets its input:
root= grep (cat ('/etc/passwd', _out=Capture), 'root', _out=Capture)
This seems more cumbersome than sh, but if you think that in any shell language
you do something similar (either using $(), | or even redirection), it's not
a high price to pay.
Another improvement over sh is that you can use commands as conditions:
if grep (cat ('/etc/passwd', _out=Capture), 'mdione', _out=None):
print ('user «mdione» is present on your system; that's a security vulnerability right there!')
As a consequence, you can also use and, or and not.
Of course, no shell scripting language can call itself so without piping, so we had to implement it:
if cat ('/etc/passwd') | grep ('mdione', _out=None):
print ('I'm here, baby!')
Notice that this time you don't have to be explicit about the cat's output;
we know it's going to a pipe, so we automatically Capture it. Of course, we
also have redirection:
grep ('mdione') < '/etc/passwd' > '/tmp/foo'
grep ('root') < '/etc/passwd' >> '/tmp/foo'
Do I have you attention? Let's go for your interest. Something also useful is a
behavior similar to pushd/popd:
with cd ('bin'):
print (pwd ())
print (pwd ())
If you were in ayrton's source directory, you would get something in the lines
of:
/home/mdione/src/projects/ayrton/bin
/home/mdione/src/projects/ayrton
bash() applies brace, tilde and glob (pathname) expansions:
>>> from ayrton.expansion import bash
>>> import os
>>> os.chdir (bash ('~/src/pro*/osm/mapn*')[0])
>>> os.getcwd ()
'/home/mdione/src/projects/osm/mapnik-stylesheets'
>>> bash ("Elevation/{legend*,Elevation.dgml,preview.png,Makefile}")
['Elevation/legend.html', 'Elevation/legend', 'Elevation/Elevation.dgml', 'Elevation/preview.png', 'Elevation/Makefile']
Notice that bash() always returns a list.
Parameter expansion can be achieved with the str operator % or the format()
method. Arithmetic expansion can be achieved with normal arithmetic operators.
Process substitution is planned but not yet implemented.
There is no need for a test/[/[[ equivalent, but there are for the
operators. As -cannot be part of the name of a function, we replaced it with _.
So, -f became _f() and so on. Some of the operators are not implemented yet.
Of course, string and integer operators are better implemented in Python's str,
int and, why not, float types.
One main difference between Python and shell languages is that in the latter, you only have
environment variables, which after being exported, can be seen by any subprocess.
In Python there are two worlds: Python variables and environment variables.
ayrton again reaches to shell languages, mixing the environment into the globals,
so envvars can be reached from any place, just like in shell scripts. Notice that
new variables in ayrton (f.i., foo=42) are Python variables; therefore they
can hold any Python object, but won't be exported. The export() function
gives the same behavior as bash's export command, with the caveat that values
will be automatically converted to str.
The cherry on top of the cake, or more like the melon of top of the cupcake, is (semi) transparent remote execution. This is achieved with the following construct:
a= 42
with remote ('localhost') as streams:
foo= input ()
print (foo)
# we can also access variables already in the scope
# even when we're actually running in another machine
print (a)
# streams returns 3 streams: stdin, stdout, stderr
(i, o, e)= streams
# notice that we must include the \n at the end so input() finishes
# and that you must transmit bytes only, no strings
i.write (b'bar\n')
print (o.readlines ())
The body of the with remote(): ... statement is actually executed in a remote
machine after connecting via ssh. The remote() context manager accepts the
same parameters as paramiko's
SSHClient.connect()
method.
The implementation of this construct limits a bit what can be done in its body. The code is converted into a AST subtree and the local environment is pickled. If the latter fails the construct fails and your script will finish. We're checking its limitations to see where we can draw the line of what will be possible or not.
Here you'll find the docs.
Q: Why bother? Isn't bash great?
A: Yes and no. bash is very powerful, both from the CLI and as a language. But
it's clumsy, mainly due to two reasons: parsing lines into commands and their
arguments, and the methods for preventing overzealous word splitting, which leads
to several pitfalls, some of them listed here);
and poor data manipulation syntax. It also lacks of good remote
execution support. Most scripts start small, but once they reach
a certain size/complexity, either they become monsters (resembling a Frankenstein
built using a Kafkian method) or they are rewritten in Perl (which makes them a
different kind of monster, closer to the Thing in «The Thing»).
Q: Why not contribute all this to sh?
A: sh has a very specific objective, which is to make easy to capture the
output of commands into a Python script, and even pipe output to other commands
in a functional/pythonic way. ayrton aims to make python+sh behave more like
bash so it's easier for sysadmins to learn and use. Anything that still holds
sh's objective will be sent as a patch over time, but for the moment being,
we're still playing with the shape of ayrton.
Q: ayrton is too verbose! I don't want to put extra ()'s or ''s everywhere.
A: Shell languages have evolved from shell interpreters. Command execution are their main objective, and its syntax is designed around it. That leads to shortcuts that later are more difficult to read and creates problems when handling filenames that have special characters.
rbilstolfi, marianoguerra, facundobatista, ralsina, nessita for unit
testing support, Darni for pointing me to
nvie's workflow for git,
Andrew Moffat for sh and Richard Jones for
this talk (thanks again, ralsina), even when I ended up doing something
different:
See TODO.rst