Quick evaluation of Python expressions in Bash

A Bash function and alias combo that lets you evaluate simple Python expressions from the Bash command-line:

The function and alias combo is necessary to prevent Bash from expanding wildcard expressions like * and ? and such. I borrowed the so-called “magic aliases” technique from Simon Tatham’s “Magic Aliases: A Layering Loophole in the Bourne Shell”. Even so, I have not found a way to get Bash to not parse out quotes and redirection characters and what not so some stuff works as expected and some stuff doesn’t.

$ pyval 23 * 2
46
$ pyval 0x20 * 2
64
$ pyval [2 * x for x in [1, 2, 3]]
[2, 4, 6]

Characters like < and > get parsed by Bash as redirections. You can stop this by quoting:

$ pyval 1 < 2
-bash: 2: No such file or directory
$ pyval '1 < 2'
True
$ pyval '1 << 5'
32

I have not found a way to stop Bash from parsing out quotes so that unfortunately you have to do this ugly double quoting of string literals:

$ pyval 'hello'
Traceback (most recent call last):
  File "", line 1, in 
NameError: name 'hello' is not defined
$ pyval "'hello'.upper()"
HELLO
$ pyval 72 * "'-'"
------------------------------------------------------------------------

Just for fun:

$ pyval "' '.join([word.capitalize() for word in 'the green mile'.capitalize().split()])"
The Green Mile

Unfortunately, this simple implementation doesn’t let you import Python modules. I could imagine a way to rewrite this so that you could specify modules to import as arguments, but I don’t know if it’s worth it. After all, it’s not that hard to type “python” and if you really want a pythonic shell, you could use ipython with the so-called “Sh profile” (which I’ve never got into the habit of).

Leave a Reply

Your email address will not be published.