Why env python

In this usage, the environment is typically not changed More on env is to be found here , and, as always, in man env from a terminal. Additional information on the shebang; why doesn't! Specifically the lines: An interpreter script is a text file that has execute permission enabled and whose first line is of the form:!

Improve this answer. Community Bot 1. Jacob Vlijm Jacob Vlijm Just as a tip, you should always do! You should always make your Python scripts use a specific version of the interpreter rather than putting just 'python'.

Why can't we use! BharadwajRaju see my updated answer. Show 3 more comments. Jacob's answer explains it well. However there is one more point I would like to mention. What is python's "virtual environment" all about? Isac Casapu Isac Casapu 3 3 bronze badges. However in , a "Python launcher" was developed which to some degree mimics this Linux behaviour for Windows. This is limited just to choosing which Python interpreter is run — e.

The launcher is optionally installed as py. This was even true of different Linux distributions. This could make for some really long paths. I have memories of the stuff from Sunfreeware. Then you could write! Of course, reasonable meant for Python and Perl that you had also set the appropriate environmental variables. But on a Sparc station it might not run at all. I miss my sparc station. But not a lot. Ok, now you've got me trolling around on E-Bay.

And no, I did not wear either a turnip OR an onion on my belt. If you're running your script in a virtual environment, say venv , then executing which python while working on venv will display the path to the Python interpreter:. Note that the name of the virtual environment is embedded in the path to the Python interpreter.

Therefore, hardcoding this path in your script will cause two problems:. Therefore, to add to Jonathan 's answer, the ideal shebang is!

The line! You might have multiple python versions installed. For example, a. For giving execution permission,. Considering the portability issues between python2 and python3 , you should always specify either version unless your program is compatible with both. Some distributions are shipping python symlinked to python3 for a while now - do not rely on python being python2.

This is emphasized by PEP :. In order to tolerate differences across platforms, all new code that needs to invoke the Python interpreter should not specify python, but rather should specify either python2 or python3 or the more specific python2. This distinction should be made in shebangs, when invoking from a shell script, when invoking via the system call, or when invoking in any other context. It tells the interpreter which version of python to run the program with when you have multiple versions of python.

It allows you to select the executable that you wish to use; which is very handy if perhaps you have multiple python installs, and different modules in each and wish to choose. How are we doing? Please help us improve Stack Overflow. Take our short survey. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Why do people write! Ask Question. Asked 11 years, 8 months ago. Active 1 month ago. Viewed k times. I see this at the top of Python files: For Python 2 files! Improve this question. Gabriel Staples The answer below that states that it is just a comment line. That's not always the case. I have a "Hello, World!

To create a virtual environment, decide upon a directory where you want to place it, and run the venv module as a script with the directory path:. A common directory location for a virtual environment is. This name keeps the directory typically hidden in your shell and thus out of the way while giving it a name that explains why the directory exists.

It also prevents clashing with. This script is written for the bash shell. If you use the csh or fish shells, there are alternate activate. For example:. You can install, upgrade, and remove packages using a program called pip. You can browse the Python Package Index by going to it in your web browser. I use an installer script that modifies the! I haven't had to use anything other than! A minor point: the! Neither of these is likely to be a significant concern.

It might be an issue if you want your script to run on a really old system, but then you're likely to need to modify it anyway. Another possible issue, thanks to Sopalajo de Arrierez for pointing it out in comments is that cron jobs run with a restricted environment.

For that particular environment and perhaps others like it , the! I haven't used virtualenv myself. Expected results: that print sys. Each time. Although it's TRUE that! Indeed, developers don't care which interpreter is found " first ", all they care about is that their code is executed using the specified interpreter they know to be compatible with their code to ensure consistent results- wherever that may live in the filesystem Specifying the absolute path is more precise on a given system.

The downside is that it's too precise. Suppose you realize that the system installation of Perl is too old for your scripts and you want to use your own instead: then you have to edit the scripts and change! Attempting to operate with a bad PATH is very rarely useful, and indicates that you know very little about the system the script is running on, so you can't rely on any absolute path anyway.

For example, a bash script that only works on Linux can safely use! A script that is only meant to be used in-house can rely on house interpreter location conventions. It's more flexible than specifying an absolute path but still requires knowing the interpreter name.

In such cases, you can often make a polyglot script that can be interpreted both by the standard shell and by your desired interpreter. For example, to make a Python 2 script portable both to systems where python is Python 3 and python2 is Python 2, and to systems where python is Python 2 and python2 doesn't exist:.

First, it's not portable. Second, as Keith Thompson has noted, it can cause trouble with passing arguments on the shebang line. The maximally portable solution is this:. This is because the! It's part of the kernel's binary loader. This can be tested. Put this in a file and mark it executable:. If a file is marked executable and begins with a! It doesn't solve the problem of specifying the full path to the interpreter, it just moves it to env.

This prevents your script's name from appearing in, e. Changing your PATH is more work than just editing the first line of a script, especially when scripting such edits is trivial.

Changing the order of PATH directories is significantly more difficult If you need to do that, it's much easier to just have several! In vi , that would be as simple as moving the cursor to the! Even with an editor as trivial as nano , it would take seconds to use the mouse to copy and paste.

Overall, the advantages of using! Even the "convenience" advantage is largely illusory. IMO, it's a silly idea promoted by a particular kind of programmer who thinks that operating systems are not something to be worked with, they are a problem to be worked around or ignored at best.

Run it as, e. Using env is also useful when you want to share scripts between multiple rvm environments for example. Running this on the cmd line, shows which ruby version will be used when! Therefore, when you use env , you can use different ruby versions through rvm, without changing your scripts. If you're writing purely for yourself or for your job and the location of the interpreter you're calling is always in the same place, by all means use the direct path.