#!/usr/bin/env python vs. #!/usr/bin/python

[Yves Dorfsman]

On UNIX, some people use
#!/usr/bin/env python

While other use
#!/usr/bin/python

Why is one preferred over the other one ?

Thanks.

 

[Roy Smith]

I've never clearly understood why people want to use "#! /usr/bin/env
python", which is prone to finding a different Python from the one
installed by the operating system. I'd be interested to see what
responses are in favour of it, and what the reasoning is.

One possible reason is that the programmer is attempting to allow for
systems where Python has been installed, but not from an operating
system package.

You've got it exactly.

I'm currently using Python to write unit tests as part of a build system.
Many of our development boxes don't have python installed in /usr/bin (or
perhaps at all). And even if they did, we might want to use a different
version of Python on different branches of the code.

We've got Python built for all our platforms and the binaries stored in our
source control system. When you check out a particular branch, you get the
right version of Python for that branch. By having the Python scripts
start with #!/usr/bin/env python, I can select the version of Python I want
just by changing the environment.

Then, of course, I recently ran across a machine where env was installed in
/opt/gnu/bin instead of /usr/bin. Sigh. Sometimes you just can't win.

 

[D'Arcy J.M. Cain]

I much prefer "#! /usr/bin/python" because I want my Python programs
to, by default, be run with the default Python, and depend on Python
being installed by the operating system's package manager. On systems
that use shebang lines and that actually have standardised filesystem
locations, the default Python is found at '/usr/bin/python'.

You have lived a sheltered life. Not every packaging system puts the
executible in /usr/bin. Many systems use /usr/local/bin. NetBSD
uses /usr/pkg/bin but allows you to define your own pkg root.
Using /usr/bin/env allows your code to run on all these systems.

 

[Jeroen Ruigrok van der Werven]

I've never clearly understood why people want to use "#! /usr/bin/env
python", which is prone to finding a different Python from the one
installed by the operating system. I'd be interested to see what
responses are in favour of it, and what the reasoning is.

Simple, some systems are not as peculiar as a lot of Linux boxes which
chug everything into /usr/bin, which is OS territory (as has been decreed
long ago by hier(7)), but rather use /usr/local/bin (all BSD Unix and
derivatives) or /opt or whatever convention a particular operating system
has.

And prone to find the wrong Python, it all depends upon proper $PATH
administration.

As such, your script with #!/usr/bin/python is as bad as an ash shell script
with #!/bin/bash. #!/usr/bin/env python is more cross-OS friendly, there's
more than just Linux you know.

 

[Tim Roberts]

On UNIX, some people use
#!/usr/bin/env python

While other use
#!/usr/bin/python

Why is one preferred over the other one ?

The /usr/bin/env solution finds the Python interpreter anywhere on the
PATH, whether it be /usr/bin or /usr/local/bin, or whatever. With
/usr/bin/python, it MUST be in /usr/bin.

Way back when, Python wasn't included in Linux distributions by default, so
it was difficult to predict where it would be. /usr/bin/env, on the other
hand, is well-established at that location.

These days, since Python is nearly ubiquitous, I suspect it is not so
important.

 

[Jeroen Ruigrok van der Werven]

To my mind, the Python interpreter installed by a package as
distributed with the OS *is* OS territory and belongs in /usr/bin/.

That's the difference with a distribution, such as Linux, and full OSes ,
such as BSDs or commercial Unix variants. They prefer to keep a pristine
state for the OS vendor files versus what the user can opt to install
himself, hence the /usr/bin - /usr/local/bin separation. Same for sbin, lib,
and so on. It effectively guarantees you can nuke /usr/local without ill
consequences for your OS.

Different philosophies, but after having spent more than 10+ years on too
many Unix and Unix-like systems I know the importance of platform
portability a bit too much and hardcoding a shebang sequence is not the
solution in general. Using env is the, arguably, best solution available.

 

[D'Arcy J.M. Cain]

They use that for the operating-system-installed default Python
interpreter? Colour me incredulous.

OK, let me get out my crayons. However, note that I did not say
"operating-system-installed." I said a packaging system puts it
there. In fact, the NetBSD packaging system works on many systems
including Linux and thus is not an operating system packager.

 

[Thorsten Kampe]

* Ben Finney (Fri, 02 May 2008 23:30:01 +1000)

The OP was asking why people prefer on over the other. My answer is
that I prefer specifying "give me the default OS Python" because
anything not installed by the OS is to non-standardised for me to
worry about.

Others may prefer something different, but then they get to wear
whatever problems occur as a result of that choice. I continue to be
bemused by that preference, and nothing that I've seen so far in this
thread illuminates the issue more.

You're missing the point. Apart from the really dubious terms you use
("OS installable package"), using env in the first line has exactly the
effect to use the default path of Python (which is the first entry in
your path) and not some hard-coded path (which might not even exist).

Thorsten

 

[Roy Smith]

Whereas if Python is *not* installed from an OS package, it's up to
the sys admin to ensure that it works -- not up to my program. So I
don't see the point in making it work by default, when what I want for
my program is that it works *with the default Python*, not with some
non-default installation.

Ben,

Have you ever shipped software to a customer? Imagine the following
conversation:

Customer: "Your product is broken. It says it can't find python, and I
know I have it installed".

Vendor: "Where do you have it installed?"

Customer: "In /opt/bin/python"

Vendor: "Oh, that's your problem, it HAS to be in /usr/bin/python".

Customer: "I can't install it there because <insert whatever silly reason
the customer has>. If you can't make your product work without requiring
me to install python in /usr/bin, I'm afraid I can't buy your product".

Vendor: "No problem sir, I'll be happy to tell our sales folks to stop
bothering you".

If you want to hard-code /usr/bin/python into your application, that's your
decision. If you would like to take on the task of convincing every
sysadmin in the world to do things the way you think they should be done,
have fun.

 

[D'Arcy J.M. Cain]

The OP was asking why people prefer on over the other. My answer is
that I prefer specifying "give me the default OS Python" because
anything not installed by the OS is to non-standardised for me to
worry about.

As someone else pointed out, not all the world is Linux. So your
version of Linux (I'm not sure whether it is true for all versions or
not) delivers Python as part of the OS. That is simply not true of the
whole world. Some OS distributions have an adjunct facility for
installing packages but they are not part of the OS. Some systems
don't even have that and people must download packages such as Python
and install them manually. Even on Linux there are people who won't
install binaries and use NetBSD's pkgsrc instead. Clearly that cannot
install into /usr/bin since it is not part of the OS.

Certainly #! /usr/bin/python is fine if you never expect your software
to run outside of your own little corner of the world but you asked why
people prefer the env version and the answer is that we want to write
software that runs everywhere that Python runs.

 

[D'Arcy J.M. Cain]

Granted, but you must draw the line somewhere anyway. I cannot

No one is talking about if statements here.

 

[D'Arcy J.M. Cain]

It's a good thing I've never implied such to be the case.

You haven't *said* it but you have definitely *implied* it. Installing
Python in /usr/bin is not common. It is very specific to your system.

 

[D'Arcy J.M. Cain]

Yes, and all parties have been quite happy with the results.

When some of us talk about shipping software we aren't talking about a
20 line script delivered to our uncle's construction company office. We
are talking about millions of lines of code in thousands of programs and
modules that has to run out of the box on whatever system the client
happens to run on.

At this point they have the simple option of running the program with
'python /path/to/the/program'. It's certainly not a case of "can't
make the product work".

Simple for your 20 line single script. Not so simple for my million
line, integrated system that has to work everywhere.

It is, however, a case of "can't automatically account for every local
customisation sysadmins choose to make on their systems". Perfectly
willing to work with them to get their specific environment working,
but as a matter of simple economics it's not worth my time to attempt
to make such corner cases work automatically.

If by "corner case" you mean "some system that I don't personally run"
then OK but to some of us your system is the corner case and we would
like our code to run there as well.

Real software has to deal with the fact that support costs money.
You may be able to deal with one or two clients that way but that does
not scale very well.

If they've already chosen to install Python to some unpredictable
location, they know what they're doing enough to invoke the program in
a specific way to get it working.

Unpredictable to you. Perfectly predictable on their system.

I do believe I am done with this thread.

 

[Yves Dorfsman]

Thanks everybody, I didn't mean to start a flamewar...
I do get it now, it's whatever python is in the path, vs. the specific one
you're pointing to.

No, because it's quite common for the PATH variable to have
'/usr/local/bin' appear *before* both of '/bin' and '/usr/bin'.

If the system has a sysadmin-installed '/usr/local/bin/python'
installed as well as the OS-installed '/usr/bin/python', then the two
shebang options the OP raised will behave differently on such a
system. This seems to be quite the point of the discussion.

And I have to admit, I prefer specifying the version (full path) because I
have run into too many problem when users have different PATHs and end up
running different version of an interpreter.

Yves.

 

[Thorsten Kampe]

* Ben Finney (Sat, 03 May 2008 00:37:45 +1000)

No, because it's quite common for the PATH variable to have
'/usr/local/bin' appear *before* both of '/bin' and '/usr/bin'.

If the system has a sysadmin-installed '/usr/local/bin/python'
installed as well as the OS-installed '/usr/bin/python', then the two
shebang options the OP raised will behave differently on such a
system. This seems to be quite the point of the discussion.

Again you're missing the point. If you or whoever installs Python (or
another version of Python) to /usr/local/bin and puts this in the path
to front (as it's often done) then /you/ want that Python to be the
"default" one. It would just be silly to say "no, I the developer want
/usr/bin/python".

So in general "#! env" is better while in certain circumstance
hardcoding the path to /usr/bin/python can be better.


Thorsten

 

[Carl Banks]

When some of us talk about shipping software we aren't talking about a
20 line script delivered to our uncle's construction company office. We
are talking about millions of lines of code in thousands of programs and
modules that has to run out of the box on whatever system the client
happens to run on.

If you're shipping a program that large you out to be packaging the
Python interpreter with it.

Frankly, this whole discussion is silly, as if it's some kind hard
thing to open the script with a text editor and modify the shbang
line.


Carl Banks

 

[Robert Kern]

On UNIX, some people use
#!/usr/bin/env python

While other use
#!/usr/bin/python

Why is one preferred over the other one ?

Caveat: I've only read *most* of this thread, so maybe someone else has already
made the following point.

It depends on the context. Ultimately, when your script is installed, it (almost
certainly) should point to the precise Python executable the installer intends
it to run on. One of the features of distutils is that it will *rewrite*
"#!/usr/bin/env python" to use the exact executable that the installer used to
execute the setup.py.

So *as a developer* I recommend writing your scripts with "#!/usr/bin/env
python". This lets distutils select the correct executable, and it lets your
users play around with your scripts prior to installation without needing to
rewrite the shebang line manually. I hate trying out someone's code just to find
that that they hardcoded /usr/local/bin/python2.3. If you aren't using distutils
to install for some reason, you might want to recommend that the installer
change the shebang line in your installation instructions.

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

 

[Gabriel Genellina]

Yes, and all parties have been quite happy with the results.

When some of us talk about shipping software we aren't talking about a
20 line script delivered to our uncle's construction company office. We
are talking about millions of lines of code in thousands of programs and
modules that has to run out of the box on whatever system the client
happens to run on.
[...]
Simple for your 20 line single script. Not so simple for my million
line, integrated system that has to work everywhere.

In that case you have a setup script, I presume. You use distutils or a
better alternative, I presume. You use the scripts= argument to setup, or
the install_scripts distutils command, I presume. The first line on your
scripts starts with #! and contains the word python somewhere, I presume.
Then, distutils will adjust that shebang line using the same python
executable that was used to run the installation.
It doesn't matter whether the line read #!/usr/bin/python, #!/usr/bin/env
python, #~/bin/python2.3 or just #!python: whatever Python was used to
install your program, that will be written as the first line on the
script, and consequentely that will be used to execute the script in the
future. So the admin (or whoever installs the system) only has to make
sure to use the right Python version from the right directory. That's all.
Plain easy, isn't it?

I can't believe some angry responses in this thread - it's just a
technical question, not about which is the best team in the [preferred
sports here] National Championship...

 

[Lou Pecora]

It is common. That's where it's installed by almost all Linux
distributions.

MacOS X system python (or links to them) is in the same place.

 

[Aahz]

I can't believe some angry responses in this thread - it's just a
technical question, not about which is the best team in the [preferred
sports here] National Championship...

http://www.netfunny.com/rhf/jokes/91q3/usolym.html