Why do people write usr bin env python on the first line of a Python script

Question

It seems to me like the files run the same without that line

User · Answer

That is called the shebang line  As the Wikipedia entry explains      In computing  a shebang  also called a hashbang  hashpling  pound bang  or crunchbang  refers to the characters      when they are the first two characters in an interpreter directive as the first line of a text file  In a Unix-like  operating system  the program loader takes the presence of these two characters as an indication that the file is a script  and tries to execute that script using the interpreter  specified by the rest of the first line in the file    See also the Unix FAQ entry   Even on Windows  where the shebang line does not determine the interpreter to be run  you can pass options to the interpreter by specifying them on the shebang line  I find it useful to keep a generic shebang line in one-off scripts  such as the ones I write when answering questions on SO   so I can quickly test them on both Windows and ArchLinux   The env utility allows you to invoke a command on the path      The first remaining argument specifies the program name to invoke  it is searched for according to the PATH environment variable  Any remaining arguments are passed as arguments to that program

User · Answer

It probably makes sense to emphasize one thing that the most have missed  which may prevent immediate understanding  When you type python in terminal you don t normally provide a full path  Instead  the executable is up looked in PATH environment variable  In turn  when you want to execute a Python program directly   path to app py  one must tell the shell what interpreter to use  via the hashbang  what the other contributors are explaining above    Hashbang expects full path to an interpreter  Thus to run your Python program directly you have to provide full path to Python binary which varies significantly  especially considering a use of virtualenv  To address portability the trick with  usr bin env is used  The latter is originally intended to alter environment in-place and run a command in it  When no alteration is provided it runs the command in current environment  which effectively results in the same PATH lookup which does the trick   Source from unix stackexchange

User · Answer

You can try this issue using virtualenv  Here is test py      usr bin env python import sys print sys version    Create virtual environments  virtualenv test2 6 -p  usr bin python2 6 virtualenv test2 7 -p  usr bin python2 7   activate each environment then check the differences  echo  PATH   test py

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Perhaps your question is in this sense   If you want to use   python myscript py  You don t need that line at all  The system will call python and then python interpreter will run your script   But if you intend to use     myscript py  Calling it directly like a normal program or bash script  you need write that line to specify to the system which program use to run it   and also make it executable with chmod 755

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If you have several versions of Python installed   usr bin env will ensure the interpreter used is the first one on your environment s  PATH  The alternative would be to hardcode something like    usr bin python  that s ok  but less flexible   In Unix  an executable file that s meant to be interpreted can indicate what interpreter to use by having a    at the start of the first line  followed by the interpreter  and any flags it may need    If you re talking about other platforms  of course  this rule does not apply  but that  shebang line  does no harm  and will help if you ever copy that script to a platform with a Unix base  such as Linux  Mac  etc

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The line    bin bash python3 or    bin bash python specifies which python compiler to use  You might have multiple python versions installed  For example  a py       bin bash python3 print  Hello World     is a python3 script  and b py       bin bash python print  Hello World    is a python 2 x script In order to run this file   a py or   b py is used  you need to give the files execution privileges before hand  otherwise executing will lead to Permission denied error  For giving execution permission   chmod  x a py

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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 394      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 x and python3 x versions  see the Migration Notes   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

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It just specifies what interpreter you want to use  To understand this  create a file through terminal by doing touch test py  then type into that file the following      usr bin env python3 print  test    and do chmod  x test py to make your script executable  After this when you do   test py you should get an error saying     File    test py   line 2     print  test                   SyntaxError  Missing parentheses in call to  print    because python3 doesn t supprt the print operator   Now go ahead and change the first line of your code to      usr bin env python2   and it ll work  printing test to stdout  because python2 supports the print operator  So  now you ve learned how to switch between script interpreters

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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  e g      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  ALTERNATIVE 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

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The exec system call of the Linux kernel understands shebangs      natively When you do on bash    something  on Linux  this calls the exec system call with the path   something  This line of the kernel gets called on the file passed to exec  https   github com torvalds linux blob v4 8 fs binfmt script c L25 if   bprm- gt buf 0              bprm- gt buf 1            It reads the very first bytes of the file  and compares them to     If the comparison is true  then the rest of the line is parsed by the Linux kernel  which makes another exec call with   executable   usr bin env first argument  python second argument  script path  therefore equivalent to   usr bin env python  path to script py  env is an executable that searches PATH to e g  find  usr bin python  and then finally calls   usr bin python  path to script py  The Python interpreter does see the    line in the file  but   is the comment character in Python  so that line just gets ignored as a regular comment  And yes  you can make an infinite loop with  printf     a n    sudo tee  a sudo chmod  x  a  a  Bash recognizes the error  -bash   a   a  bad interpreter  Too many levels of symbolic links     just happens to be human readable  but that is not required  If the file started with different bytes  then the exec system call would use a different handler  The other most important built-in handler is for ELF executable files  https   github com torvalds linux blob v4 8 fs binfmt elf c L1305 which checks for bytes 7f 45 4c 46  which also happens to be human readable for  ELF   Let s confirm that by reading the 4 first bytes of  bin ls  which is an ELF executable  head -c 4  quot   which ls  quot    hd   output  00000000  7f 45 4c 46                                         ELF  00000004                                                                   So when the kernel sees those bytes  it takes the ELF file  puts it into memory correctly  and starts a new process with it  See also  How does kernel get an executable binary file running under linux  Finally  you can add your own shebang handlers with the binfmt misc mechanism  For example  you can add a custom handler for  jar files  This mechanism even supports handlers by file extension  Another application is to transparently run executables of a different architecture with QEMU  I don t think POSIX specifies shebangs however  https   unix stackexchange com a 346214 32558   although it does mention in on rationale sections  and in the form  quot if executable scripts are supported by the system something may happen quot   macOS and FreeBSD also seem to implement it however  PATH search motivation Likely  one big motivation for the existence of shebangs is the fact that in Linux  we often want to run commands from PATH just as  basename-of-command  instead of   full path to basename-of-command  But then  without the shebang mechanism  how would Linux know how to launch each type of file  Hardcoding the extension in commands   basename-of-command py  or implementing PATH search on every interpreter  python basename-of-command  would be a possibility  but this has the major problem that everything breaks if we ever decide to refactor the command into another language  Shebangs solve this problem beautifully

User · Answer

It seems to me like the files run the same without that line    If so  then perhaps you re running the Python program on Windows  Windows doesn t use that line   instead  it uses the file-name extension to run the program associated with the file extension   However in 2011  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 g  to select between Python 2 and Python 3 on a system where both are installed  The launcher is optionally installed as py exe by Python installation  and can be associated with  py files so that the launcher will check that line and in turn launch the specified Python interpreter version

User · Answer

this tells the script where is python directory        usr bin env python

User · Answer

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     Envs venv bin python  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    If you upload the script to a repository  you re forcing other users to have the same virtual environment name  This is if they identify the problem first  You won t be able to run the script across multiple virtual environments even if you had all required packages in other virtual environments    Therefore  to add to Jonathan s answer  the ideal shebang is    usr bin env python  not just for portability across OSes but for portability across virtual environments as well

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This is meant as more of historical information than a  real  answer    Remember that back in the day you had LOTS of unix like operating systems whose designers all had their own notion of where to put stuff  and sometimes didn t include Python  Perl  Bash  or lots of other GNU Open Source stuff at all    This was even true of different Linux distributions  On Linux--pre-FHS 1 -you might have python in  usr bin  or  usr local bin   Or it might not have been installed  so you built your own and put it in   bin   Solaris was the worst I ever worked on  partially as the transition from Berkeley Unix to System V  You could wind up with stuff in  usr    usr local    usr ucb   opt  etc  This could make for some really long paths  I have memories of the stuff from Sunfreeware com installing each package in it s own directory  but I can t recall if it symlinked the binaries into  usr bin or not    Oh  and sometimes  usr bin was on an NFS server 2      So the env utility was developed to work around this   Then you could write    bin env interpreter and as long as the path was proper things had a reasonable chance of running  Of course  reasonable meant  for Python and Perl  that you had also set the appropriate environmental variables  For bash ksh zsh it just worked    This was important because people were passing around shell scripts  like perl and python  and if you d hard coded  usr bin python on your Red Hat Linux workstation it was going to break bad on a SGI   well  no  I think IRIX put python in the right spot  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  Bastages     1  File-system Hierarchy Standard  https   en wikipedia org wiki Filesystem Hierarchy Standard   2  Yes  and sometimes people still do stuff like that  And no  I did not wear either a turnip OR an onion on my belt

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It s recommended way  proposed in documentation      2 2 2  Executable Python Scripts      On BSD   ish Unix systems  Python scripts can be made directly   executable  like shell scripts  by putting the line      usr bin env python3 2    from http   docs python org py3k tutorial interpreter html executable-python-scripts

User · Answer

This is a shell convention that tells the shell which program can execute the script      usr bin env python  resolves to a path to the Python binary

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The main reason to do this is to make the script portable across operating system environments     For example under mingw  python scripts use       c python3k python    and under GNU Linux distribution it is either      usr local bin python    or      usr bin python   and under the best commercial Unix sw hw system of all  OS X   it is      Applications MacPython 2 5 python   or on FreeBSD      usr local bin python   However all these differences can make the script portable across all by using      usr bin env python

User · Answer

Technically  in Python  this is just a comment line   This line is only used if you run the py script from the shell  from the command line   This is know as the  Shebang    and it is used in various situations  not just with Python scripts   Here  it instructs the shell to start a specific version of Python  to take care of the rest of the file

User · Answer

Expanding a bit on the other answers  here s a little example of how your command line scripts can get into trouble by incautious use of  usr bin env shebang lines      usr local bin python -V Python 2 6 4    usr bin python -V Python 2 5 1   cat my script py     usr bin env python import json print  hello  json    PATH  usr local bin  usr bin     my script py  hello  json   PATH  usr bin  usr local bin     my script py  Traceback  most recent call last     File    my script py   line 2  in  lt module gt      import json ImportError  No module named json   The json module doesn t exist in Python 2 5   One way to guard against that kind of problem is to use the versioned python command names that are typically installed with most Pythons     cat my script py     usr bin env python2 6 import json print  hello  json    If you just need to distinguish between Python 2 x and Python 3 x  recent releases of Python 3 also provide a python3 name     cat my script py     usr bin env python3 import json print  hello  json

User · Answer

In order to run the python script  we need to tell the shell three things    That the file is a script Which interpreter we want to execute the script The path of said interpreter   The shebang    accomplishes  1    The shebang begins with a   because the   character is a comment marker in many scripting languages  The contents of the shebang line are therefore automatically ignored by the interpreter   The env command accomplishes  2   and  3    To quote  grawity       A common use of the env command is to launch interpreters  by making   use of the fact that env will search  PATH for the command it is told   to launch  Since the shebang line requires an absolute path to be   specified  and since the location of various interpreters  perl  bash    python  may vary a lot  it is common to use          usr bin env perl   instead of trying to guess whether it is    bin perl   usr bin perl   usr local bin perl   usr local pkg perl     fileserver usr bin perl  or  home MrDaniel usr bin perl on the user s   system         On the other hand  env is almost always in  usr bin env   Except in   cases when it isn t  some systems might use  bin env  but that s a   fairly rare occassion and only happens on non-Linux systems

User · Answer

It tells the interpreter which version of python to run the program with when you have multiple versions of python

[python] Why do people write #!/usr/bin/env python on the first line of a Python script?

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