Which of these is better or faster to use as the shebang line for a Perl script?
#! perl
#! perl.exe
#! fullpath/perl(/perl.exe)
#! partialpath/perl(/perl.exe)
And, when using #!perl, when it works on a particular system, how do I find out in the script which perl interpreter I'm using so I can put that one into the shebang line?
And, if using a /path/path/perl, are * or ... allowed to be used for the folders?
If you have to hard code #!, use #!/usr/bin/env perl. Why? What you want is for the Perl program to run with the user's preferred Perl. That's going to be the first on in their PATH. #!perl doesn't do what I mean, it doesn't search the user's PATH, #!/usr/bin/env perl is how you pull that off. /usr/bin/env will always be there on Unix systems.
If the user is using Windows, as others have pointed out, it doesn't matter. Windows doesn't use #! it uses file extension associations. Make sure your program is called foo.pl or something and it'll work. But include the #! line anyway as some utilities and editors make use of it.
If you're shipping code, let the installer take care of it. Both MakeMaker/Makefile.PL and Module::Build/Build.PL will change your #! line to match the perl the user used to install with. They will take care of this problem for you.
If you are installing code for your own production use, you should use the full path to a particular copy of perl. Which copy of perl? One specific to your project. Does this mean you need to compile perl for every project? No, you can make a symlink. Project foo might have /usr/local/bin/fooperl point at /usr/bin/perl5.18. Use #!/usr/local/bin/fooperl. Now if you decide to upgrade perl you can do it per project by changing the symlink.
If you are running CGI via Apache on Windows, the SHEBANG IS USED. You will need the fullpath to perl.
A Windows she-bang (deduced from the perl.exe bit) seems irrelevant since your (ahem) "shell" probably does not even parse it (correct me if I am wrong, could have been changed lately).
Some command line flags may still be picked up by Perl itself though (according to this thread).
As ChristopheD noted, I can confirm from practice (ActivePerl on XP) that the shebang line is not really necessary on Windows.
A shebang line tells a Unix shell which interpreter to pass the script to.
On Windows, the program to pass the script to will be determined by associations based on the extension.
On Unix, the third option (full path to perl executable) is best.
And yes, you can use ".." in theory (shell doesn't care) but you should not really use relative path - you never know what your current working directory when executing a script will be.
If you're developing in Unix using Perl and you use "perlbrew" to easily switch between different versions of Perl, then the "#!/usr/bin/env perl" shebang line works well.
I originally had the first 2 characters in the shebang line reversed. Just fixed/edited that.
The first line stands for shebang. It basically tells the program where Perl interpreter is located since Perl is interpreted language. On Linux you can type in terminal:
whereis perl
which will give you exact location of it. Usually it's inside /usr/bin/perl. This means that you want to make shebang regarding to /usr/bin/perl
#! /usr/bin/perl
use strict;
use warnings;
use v5.10.1;
.
.
.
This is just some good practice, hence it's obviously fastest solution.
I hope you find this useful,
Thanks.
And, when using "#! perl", when it works on a particular system, what is the print() for showing the full path to perl.exe, that could be included into the Shebang Line ?
Well, if you're using the print statement you are already executing perl code, so...
This is one of the things which I dislike about Perl.
On Windows, if you are using ActiveState Perl at least, if the file ends with .pl then the Windows registry will run the Perl interpreter for you, regardless of the shebang line. On Cygwin, I am not sure why but #! perl works too. On Unix you should put the full path to your Perl executable in the shebang line. Schwern's idea of using env is convenient, but has some danger, as I pointed out in a comment.
This is why I suggest to you that the best solution is to package your Perl scripts as CPAN modules. CPAN installers like Module::Build will then change the shebang line to the full path to your Perl interpreter. (I am not sure whether Schwern's installer, ExtUtils::MakeMaker, does this or uses env, since I don't use it.)
Related
Should I put the shebang in my Python scripts? In what form?
#!/usr/bin/env python
or
#!/usr/local/bin/python
Are these equally portable? Which form is used most?
Note: the tornado project uses the shebang. On the other hand the Django project doesn't.
The shebang line in any script determines the script's ability to be executed like a standalone executable without typing python beforehand in the terminal or when double clicking it in a file manager (when configured properly). It isn't necessary but generally put there so when someone sees the file opened in an editor, they immediately know what they're looking at. However, which shebang line you use is important.
Correct usage for (defaults to version 3.latest) Python 3 scripts is:
#!/usr/bin/env python3
Correct usage for (defaults to version 2.latest) Python 2 scripts is:
#!/usr/bin/env python2
The following should not be used (except for the rare case that you are writing code which is compatible with both Python 2.x and 3.x):
#!/usr/bin/env python
The reason for these recommendations, given in PEP 394, is that python can refer either to python2 or python3 on different systems.
Also, do not use:
#!/usr/local/bin/python
"python may be installed at /usr/bin/python or /bin/python in those
cases, the above #! will fail."
―"#!/usr/bin/env python" vs "#!/usr/local/bin/python"
It's really just a matter of taste. Adding the shebang means people can invoke the script directly if they want (assuming it's marked as executable); omitting it just means python has to be invoked manually.
The end result of running the program isn't affected either way; it's just options of the means.
Should I put the shebang in my Python scripts?
Put a shebang into a Python script to indicate:
this module can be run as a script
whether it can be run only on python2, python3 or is it Python 2/3 compatible
on POSIX, it is necessary if you want to run the script directly without invoking python executable explicitly
Are these equally portable? Which form is used most?
If you write a shebang manually then always use #!/usr/bin/env python unless you have a specific reason not to use it. This form is understood even on Windows (Python launcher).
Note: installed scripts should use a specific python executable e.g., /usr/bin/python or /home/me/.virtualenvs/project/bin/python. It is bad if some tool breaks if you activate a virtualenv in your shell. Luckily, the correct shebang is created automatically in most cases by setuptools or your distribution package tools (on Windows, setuptools can generate wrapper .exe scripts automatically).
In other words, if the script is in a source checkout then you will probably see #!/usr/bin/env python. If it is installed then the shebang is a path to a specific python executable such as #!/usr/local/bin/python (NOTE: you should not write the paths from the latter category manually).
To choose whether you should use python, python2, or python3 in the shebang, see PEP 394 - The "python" Command on Unix-Like Systems:
... python should be used in the shebang line only for scripts that are
source compatible with both Python 2 and 3.
in preparation for an eventual change in the default version of
Python, Python 2 only scripts should either be updated to be source
compatible with Python 3 or else to use python2 in the shebang line.
If you have more than one version of Python and the script needs to run under a specific version, the she-bang can ensure the right one is used when the script is executed directly, for example:
#!/usr/bin/python2.7
Note the script could still be run via a complete Python command line, or via import, in which case the she-bang is ignored. But for scripts run directly, this is a decent reason to use the she-bang.
#!/usr/bin/env python is generally the better approach, but this helps with special cases.
Usually it would be better to establish a Python virtual environment, in which case the generic #!/usr/bin/env python would identify the correct instance of Python for the virtualenv.
The purpose of shebang is for the script to recognize the interpreter type when you want to execute the script from the shell.
Mostly, and not always, you execute scripts by supplying the interpreter externally.
Example usage: python-x.x script.py
This will work even if you don't have a shebang declarator.
Why first one is more "portable" is because, /usr/bin/env contains your PATH declaration which accounts for all the destinations where your system executables reside.
NOTE: Tornado doesn't strictly use shebangs, and Django strictly doesn't. It varies with how you are executing your application's main function.
ALSO: It doesn't vary with Python.
You should add a shebang if the script is intended to be executable. You should also install the script with an installing software that modifies the shebang to something correct so it will work on the target platform. Examples of this is distutils and Distribute.
Sometimes, if the answer is not very clear (I mean you cannot decide if yes or no), then it does not matter too much, and you can ignore the problem until the answer is clear.
The #! only purpose is for launching the script. Django loads the sources on its own and uses them. It never needs to decide what interpreter should be used. This way, the #! actually makes no sense here.
Generally, if it is a module and cannot be used as a script, there is no need for using the #!. On the other hand, a module source often contains if __name__ == '__main__': ... with at least some trivial testing of the functionality. Then the #! makes sense again.
One good reason for using #! is when you use both Python 2 and Python 3 scripts -- they must be interpreted by different versions of Python. This way, you have to remember what python must be used when launching the script manually (without the #! inside). If you have a mixture of such scripts, it is a good idea to use the #! inside, make them executable, and launch them as executables (chmod ...).
When using MS-Windows, the #! had no sense -- until recently. Python 3.3 introduces a Windows Python Launcher (py.exe and pyw.exe) that reads the #! line, detects the installed versions of Python, and uses the correct or explicitly wanted version of Python. As the extension can be associated with a program, you can get similar behaviour in Windows as with execute flag in Unix-based systems.
When I installed Python 3.6.1 on Windows 7 recently, it also installed the Python Launcher for Windows, which is supposed to handle the shebang line. However, I found that the Python Launcher did not do this: the shebang line was ignored and Python 2.7.13 was always used (unless I executed the script using py -3).
To fix this, I had to edit the Windows registry key HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Python.File\shell\open\command. This still had the value
"C:\Python27\python.exe" "%1" %*
from my earlier Python 2.7 installation. I modified this registry key value to
"C:\Windows\py.exe" "%1" %*
and the Python Launcher shebang line processing worked as described above.
Answer: Only if you plan to make it a command-line executable script.
Here is the procedure:
Start off by verifying the proper shebang string to use:
which python
Take the output from that and add it (with the shebang #!) in the first line.
On my system it responds like so:
$which python
/usr/bin/python
So your shebang will look like:
#!/usr/bin/python
After saving, it will still run as before since python will see that first line as a comment.
python filename.py
To make it a command, copy it to drop the .py extension.
cp filename.py filename
Tell the file system that this will be executable:
chmod +x filename
To test it, use:
./filename
Best practice is to move it somewhere in your $PATH so all you need to type is the filename itself.
sudo cp filename /usr/sbin
That way it will work everywhere (without the ./ before the filename)
If you have different modules installed and need to use a specific
python install, then shebang appears to be limited at first. However,
you can do tricks like the below to allow the shebang to be invoked
first as a shell script and then choose python. This is very flexible
imo:
#!/bin/sh
#
# Choose the python we need. Explanation:
# a) '''\' translates to \ in shell, and starts a python multi-line string
# b) "" strings are treated as string concat by python, shell ignores them
# c) "true" command ignores its arguments
# c) exit before the ending ''' so the shell reads no further
# d) reset set docstrings to ignore the multiline comment code
#
"true" '''\'
PREFERRED_PYTHON=/Library/Frameworks/Python.framework/Versions/2.7/bin/python
ALTERNATIVE_PYTHON=/Library/Frameworks/Python.framework/Versions/3.6/bin/python3
FALLBACK_PYTHON=python3
if [ -x $PREFERRED_PYTHON ]; then
echo Using preferred python $PREFERRED_PYTHON
exec $PREFERRED_PYTHON "$0" "$#"
elif [ -x $ALTERNATIVE_PYTHON ]; then
echo Using alternative python $ALTERNATIVE_PYTHON
exec $ALTERNATIVE_PYTHON "$0" "$#"
else
echo Using fallback python $FALLBACK_PYTHON
exec python3 "$0" "$#"
fi
exit 127
'''
__doc__ = """What this file does"""
print(__doc__)
import platform
print(platform.python_version())
Or better yet, perhaps, to facilitate code reuse across multiple python scripts:
#!/bin/bash
"true" '''\'; source $(cd $(dirname ${BASH_SOURCE[#]}) &>/dev/null && pwd)/select.sh; exec $CHOSEN_PYTHON "$0" "$#"; exit 127; '''
and then select.sh has:
PREFERRED_PYTHON=/Library/Frameworks/Python.framework/Versions/2.7/bin/python
ALTERNATIVE_PYTHON=/Library/Frameworks/Python.framework/Versions/3.6/bin/python3
FALLBACK_PYTHON=python3
if [ -x $PREFERRED_PYTHON ]; then
CHOSEN_PYTHON=$PREFERRED_PYTHON
elif [ -x $ALTERNATIVE_PYTHON ]; then
CHOSEN_PYTHON=$ALTERNATIVE_PYTHON
else
CHOSEN_PYTHON=$FALLBACK_PYTHON
fi
This is really a question about whether the path to the Python interpreter should be absolute or logical (/usr/bin/env) with respect to portability.
My view after thoroughly testing the behavior is that the logical path in the she-bang is the better of the two options.
Being a Linux Engineer, my goal is always to provide the most suitable, optimized hosts for my developer clients, so the issue of Python environments is something I really need a solid answer to. Encountering other answers on this and other Stack Overflow sites which talked about the issue in a general way without supporting proofs, I've performed some really granular testing & analysis on this very question on Unix.SE.
For files that are intended to be executable from the command-line, I would recommend
#! /usr/bin/env python3
Otherwise you don't need the shebang (though of course it doesn't harm).
If you use virtual environments like with pyenv it is better to write #!/usr/bin/env python
The pyenv setting will control which version of python and from which file location is started to run your script.
If your code is known to be version specific, it will help others to find why your script does not behave in their environment if you specify the expected version in the shebang.
If you want to make your file executable you must add shebang line to your scripts.
#!/usr/bin/env python3
is better option in the sense that this will not be dependent on specific distro of linux but could be used on almost all linux distro since it hunts for the python3 path from environment variables, which is different for different distros of linux.
whereas
#!/usr/local/bin/python3
would be a distro specific path for python3 and would not work if python3 is not found on this path, and could result in confusion and ambiguity for developer when migrating from one distro to another of linux.
Use first
which python
This will give the output as the location where my python interpreter (binary) is present.
This output could be any such as
/usr/bin/python
or
/bin/python
Now appropriately select the shebang line and use it.
To generalize we can use:
#!/usr/bin/env
or
#!/bin/env
In the pip program, the She-bang is
#!/usr/local/bin/python
if __name__ == "__main__":
# Python program body
while in the Install Certificates.command that Python Launcher offers:
#!/bin/sh
/Library/Frameworks/Python.framework/Versions/3.6/bin/python3.6 << "EOF"
# python program body
EOF
Are there any differences between those two approaches? And is there any reason to prefer one to another?
It seems to me they are all the same, except for the second one has one more bash subroutine. Is this right?
In the general case, you simply want to specify the interpreter you actually want.
Outside of this, you sometimes see workarounds like this as portability hacks. On POSIX systems, /usr/bin/env covers the majority of scenarios quite nicely; but if you need portability to older or otherwise peculiar systems, falling back to the lowest common denominator and then working your way back up to a place where you can reliably run e.g. Python on a variety of systems may require all kinds of unobvious constructs. (The previous - upvoted! - answer by Dan D. is a good example.)
There are also cases where you want sh to set something up (fetch some environment variables which are specified in a file which uses sh syntax, for example) and then hand over execution to Python;
#!/bin/sh
# source some variables
. /etc/defaults/myenv.sh
# Then run Python
exec env python -c '
# ... Your Python script here
' "$#"
There is a line length limit on the #! line. Perhaps they did that to get around that.
The options are the path to the program but only if it is short enough. Use of env python which uses the path. Or chain loading like this.
This specific code for the Install Certificates.command script was introduced in Python Issue #17128. As far as I can tell, the author hasn't explained why he wrote the code this way.
Note that .command files are Shell scripts on Mac OS X that can be executed by double-clicking on them in Finder.
I believe the likely explanation is that the author simply wanted to honour Mac OS X's expectation that .command files should be Shell scripts.
You could test this by placing the following content in a file ~/Desktop/test.command:
#!/usr/bin/env python
print "Hello world"
Then view the Desktop folder in Finder, and note that it is reported as a "shell" file:
(Although it is reported incorrectly as a Shell file, this Python script can still be executed by double-clicking on it. It doesn't break Finder or anything.)
To answer the specific question, one reason for preferring this pattern might be, as Dan D. said, to avoid a Shebang line limit.
In general, you would prefer to use #!/usr/bin/env python as your Shebang line. Creating a Bash Heredoc (i.e. the python3.6 << EOF pattern) would create all sorts of problems, such as your syntax highlighting won't work, you have to watch out for Bash variable interpolation inside the Heredoc, etc.
I have several python scripts which work just fine but one script has (as of this morning) started giving me this error if I try to run it from the bash:
: No such file or directory
I am able to run the 'broken' script by doing python script_name.py and after looking around a bit the general idea that I picked up was that maybe my line ending of the hashbang got changed (silently) so I looked at the line ending of a working script and a broken script via the :set list option in VI as indicated in this question -> View line-endings in a text file
Both files appear to end using the same character (a $) so I am kind of at a loss on how to proceed from here. Specifically, how to actually 'see' the line ending in case the set list was not the right method.
PS: The script is executable and the shebang is in there, I stated that it's just this 1 script that was working fine before the weekend but it started giving me this error as of this morning.
-- edit: --
Running the script through dos2unix does get it working again but I would like to know of any way to visualize the line ending somehow in VI(M) or why Geany somehow converted the line endings in the first place (as I never work on a dos/windows system anyhow).
From the comments above it looks like you have dos line endings, and so the hashbang line is not properly processed.
Line ending style are not shown with :set list in Vim because that option is only used when reading/writing the file. In memory line endings are always that, line-endings. The line ending style used for a file is kept in a Vim per-file option, weirdly called fileformat.
To see/change the line ending style from Vim, you can use the following commands:
:set fileformat
:set ff
It will show dos or unix. You want unix, of course ;-).
To change it quickly you can save the file with:
:w ++ff=unix
Or if you prefer:
:set ff=unix
And then save the file normally.
So see all the gory details just do :help fileformat, :help file-formats and :help fileformats
You can also use the dos2unix command to convert the file format
dos2unix
This helped me to run the python scripts
This normally happens when we open files in windows do changes and save it.
if you open the file locate the ^M characters at the end of every line
Thanks
Personally, I find it kinda wrong using direct path to python interpreter. As you dont use windows platform, you should have program env, usually in /usr/bin (/usr/bin/env). Try using following shebang:
#!/usr/bin/env python
Different distros store python binary in /bin or /usr/bin (or some weird locations), and this one makes your script config-independent (as far as possible, here we have possibility that env is stored elsewhere; still - it is less possible that env is not in /usr/bin than that python is mislocated).
I had similiar problem (if not exactly the same) and that worked for me.
Also, I have both python interpreters (2.7.x and 3.x) installed, so I need to use "python3" argument for env. AFAIR usually distros link different names to different binaries, so "env python" will run python2.7 on my system, "env python3" (also python33, or smth like that) will run p3k, and "env python2" (also python27, etc) will run python 2.7.x. Declaring which version of interpreter should be used seems like a good idea too.
I came across this problem editing my code on Windows, checking it in with git, and checking out and running it on Linux.
My solution was: tell git to Do The Right Thing. I issued this command on the Windows box:
git config --global core.autocrlf true
Modified the files and checked them in; voila, no such problem any more.
As discussed on the Git documentation.
I know that in the begining of .sh bash scripts is
#!/bin/bash
which points to the command interpeter executable.
But during watching Google Python Class http://www.youtube.com/watch?v=tKTZoB2Vjuk I noticed that for python they use
#!/usr/bin/python -tt
. Surfing the Internet I also have found such styles of this notation:
#!/usr/local/bin/python
and even
#!/usr/bin/env python
.
So, I'm new with Python and I'm ordinary Linux user and I have a few questions about this "magic" line:
First of all, what is the right form of this line? and why?
What does -tt key means in #!/usr/bin/python -tt ?
What program is parsing this line in Linux?
What syntax of this line for any script?
Why this line is so necessary if each file have it's extension?
And what about that in each computer interpreter for some kind of scripts will be stored in different place than in another? And script couldn't be run.
It's really interesting to me.
What's this line? Why this line? How to write this line? Why in such a way?...
Question #1) The line is called a shebang, and there's no right form that works universally. e.g.
#!python
#!/usr/bin/python
#!/usr/local/bin/python
#!/usr/bin/python -t
are all valid/acceptable forms, but may not work on all systems:
#!python will work only if the python executable is somewhere in your shell's PATH
#!/usr/bin/python only works if the python binary is actually in /usr/bin
#!/usr/local/bin/python also only works if python is in /usr/local/bin
Question #2)
#!/usr/bin/python -tt is passing the -tt option to python, as if you'd done:
$ python -t somescript.py
at the shell prompt. You can pass arbitary command line arguments to the interpreter on the shebang line.
Question #3)
The line is interpreted by the OS kernel and the shell you're currently using. The stuff after the #! simply tells the OS which program should be fired up to "execute" the rest of the script.
Question #4)
The script syntax depends on the language you're using. E.g. a PHP shell script must take the form of
#!/usr/bin/php
<?php
... php code here ...
A #!/usr/bin/perl perl script must use Perl syntax, etc... If you put PHP code with a Perl shebang, you'll just have Perl barf up the script with syntax errors, as PHP code is NOT perl code
Question #5)
Shebangs are for Unix systems, where file extensions were never really used to identify file types to the OS. A .c file was understood to be a C language source code file, but that's merely a convention. You could put a Bash shell script into a .c file, make it executable, and with the #!/bin/bash shebang, it would execute as a Bash script.
Determining executable types by file extension is more of a Windows thing.
Question #6)
That goes back the stuff in question #1 - if the shebang claims the interpreter is at some OTHER path than where it is, this particular script can't be executed until the shebang is fixed, or the interpreter is moved. Shebangs are very handy, but not infallible.
Thankfully, most interpreters are installed in fairly standard locations these days, so it'd be somewhat unusual to find (say) Perl installed at /some/wonky/weird/path instead of /usr/bin
From the manpage:
-t Issue a warning when a source file mixes tabs and spaces
for indentation in a way that makes it depend on the worth of a tab
expressed in spaces. Issue an
error when the option is given twice.
The right form of the line is the one you want to use.
It's the interpreter that reads this line known as shebang. If you write a python script with first line as "#!/usr/bin/python" & invoke it using bash, it's the /bin/sh interpreter that reads first line and starts the proper interpreter.
It's a shebang. The syntax of feature consists of the character sequence #!, i.e. the number sign and an exclamation point character
File extensions are not relevant in linux generally. You can have a python script that doesn't have a .py extension.
For ex.
shadyabhi#archlinux ~ $ cat a
print "Hello World"
shadyabhi#archlinux ~ $ python2 a
Hello World
shadyabhi#archlinux ~ $
Even the shebangs are only necessary if you want to start a script using $./script as in this case you didn't mention the interpreter you want to use.
#!/usr/bin/env python
issue errors about inconsistent tab usage
Kernel
#!/path_to_the_interpreter or /usr/bin/env
*nix does not check extensinon at all(except some DE could do that)
This is why you should use #!/usr/bin/env
More info at wiki
The different paths are to where the python interpreter has been installed. Different flavours of Linux install it in different places.
Linux doesn't care for extensions its a Windows thing.
The bash session uses the line to call the correct interpreter for the script your running.
The different places to where the files are stored, called and used are all based on defined places where files should be and located by software. Dev for devices, home for user stored area, bin for programs. But as time has gone by, different systems require different locations.
I would suggest getting a book on Linux/Unix and learning the basics of the file system. It does help a lot.
This is called a shebang. It tells the system that it should pass the file as an argument to the specified program instead of trying to execute it per se.
First of all, what is the right form of this line? and why?
The correct path is wherever your python interpreter is installed. The arguments (-tt) will depend on what you want. Some people insist on #!/usr/bin/env in case the interpreter happens to be elewhere.
What does -tt key means in #!/usr/bin/python -tt ?
I don't use python, so someone else will have to answer this.
When I launch any script in Linux (not exact Python script) what program parses and uses this line? I consider that it's not bash because even for bash scripts this line is needed.
I've heard (and am pretty sure) it's the kernel. Even if it were bash, it would need the line to tell bash it is supposed to be a script it should interpret instead of passing to another program. /usr/bin/env is a command that searches the PATH for the specified argument and passes the script through the program it finds.
What syntax of this line for any script? And what's the name of interpeter that parses it?
The syntax is the same as a command line, #!command arguments, but command has to be an absolute path, the PATH doesn't get searched.
Why this line is so necessary if each file have it's extension?
Extensions mean nothing in *nix. I could name a bash script script.pl, script.exe, or even script without an extension. If the script has the right shebang line, it gets passed through the right interpreter, otherwise the kernel tries to execute it as an executable and fails. The system doesn't know about extensions. They're a convention for users, nothing more.
And what about that in each computer interpreter for some kind of scripts will be stored in different place than in another? And script couldn't be run.
If I understand this correctly, you're saying different systems / distributions keep the interpreters in different places (e.g. /usr/bin/python and /usr/local/bin/python), and asking how the system knows which to use?
The answer is, it uses the one which is at the absolute path you gave it. This is actually a slight problem with executable scripts, and the reason why /usr/bin/env has come into vogue. As I said, env searches the PATH for the correct interpreter, so as long as your system has a /usr/bin/env, you're set, you don't need to look up or guarantee the location of the interpreter.
What does this line of code mean? Without it, my python3 http server can't understand and let the browser download an empty .py file (depend on the link to the .py file)
#! /usr/local/bin/python3
It's not a Python thing, it a hashbang (or shebang) line which indicates which interpreter should process the file.
The rules vary but, in its simplest form, a file with the name xyz (containing that as the first line), when run from the command line with xyz, will run it using that interpreter, similar to:
/usr/local/bin/python3 xyz
This is not a python specific notion, see http://en.wikipedia.org/wiki/Shebang_(Unix)
It's the shebang/hashbang line and a Linux/UNIX thing, not Python-related at all.
When executing the file, the kernel will see the #! magic and use whatever comes after it to execute the script. The actual program that gets launched by the kernel will be program-from-shebang script-file-path [script-args]
Note that it's usually not a good thing to include a .../local/... path but rather use e.g. #!/usr/bin/env python3 which will result in python3 being looked up in the current PATH which is much more portable.
That is not python-specific but is called Shebang and tells the operating system with which program to run this script.
UNIX Shebang? See http://en.wikipedia.org/wiki/Shebang_(Unix). The space between ! and the first / probably shouldn't be there.