Purpose of usr bin python3 shebang

Question

I have noticed this in a couple of scripting languages  but in this example  I am using python  In many tutorials  they would start with    usr bin python3 on the first line  I don t understand why we have this    Shouldn t the operating system know it s a python script  obviously it s installed since you are making a reference to it  What if the user is using a operating system that isn t unix based The language is installed in a different folder for whatever reason The user has a different version  Especially when it s not a full version number Like Python3 vs Python32    If anything  I could see this breaking the python script because of the listed reasons above

User · Answer

That s called a hash-bang  If you run the script from the shell  it will inspect the first line to figure out what program should be started to interpret the script   A non Unix based OS will use its own rules for figuring out how to run the script  Windows for example will use the filename extension and the   will cause the first line to be treated as a comment   If the path to the Python executable is wrong  then naturally the script will fail  It is easy to create links to the actual executable from whatever location is specified by standard convention

User · Answer

This line helps find the program executable that will run the script  This shebang notation is fairly standard across most scripting languages  at least as used on grown-up operating systems    An important aspect of this line is specifying which interpreter will be used  On many development-centered Linux distributions  for example  it is normal to have several versions of python installed at the same time   Python 2 x and Python 3 are not 100  compatible  so this difference can be very important  So     usr bin python and     usr bin python3 are not the same  and neither are quite the same as     usr bin env python3 as noted elsewhere on this page

User · Answer

usr bin python3 is a shebang line  A shebang line defines where the interpreter is located  In this case  the python3 interpreter is located in  usr bin python3  A shebang line could also be a bash  ruby  perl or any other scripting languages  interpreter  for example     bin bash  Without the shebang line  the operating system does not know it s a python script  even if you set the execution flag  chmod  x script py  on the script and run it like   script py  To make the script run by default in python3  either invoke it as python3 script py or set the shebang line  You can use    usr bin env python3 for portability across different systems in case they have the language interpreter installed in different locations

User · Answer

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 path  usr bin python3 and current file as the first argument   usr bin python3  path to script py  and this works for any scripting language that uses   as a comment character  And analogously  if you decide to use env instead  which you likely should always do to work on systems that have the python3 in a different location  notably pyenv  see also this question  the shebang     usr bin env python3  ends up calling analogously   usr bin env python3  path to script py  which does what you expect from env python3  searches PATH for python3 and runs  usr bin python3  path to script py  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  python3 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  See also  Why do people write    usr bin env python on the first line of a Python script

User · Answer

To clarify how the shebang line works for windows  from the 3 7 Python doc    If the first line of a script file starts with     it is known as a    shebang    line  Linux and other Unix like operating systems have native support for such lines and they are commonly used on such systems to indicate how a script should be executed  The Python Launcher for Windows allows the same facilities to be used with Python scripts on Windows To allow shebang lines in Python scripts to be portable between Unix and Windows  the launcher supports a number of    virtual    commands to specify which interpreter to use  The supported virtual commands are     usr bin env python   The  usr bin env form of shebang line has one further special property  Before looking for installed Python interpreters  this form will search the executable PATH for a Python executable  This corresponds to the behaviour of the Unix env program  which performs a PATH search    usr bin python  usr local bin python python

User · Answer

And this line is how  It is ignored  It will fail to run  and should be changed to point to the proper location  Or env should be used  It will fail to run  and probably fail to run under a different version regardless

User · Answer

Actually the determination of what type of file a file is very complicated  so now the operating system can t just know   It can make lots of guesses based on -   extension UTI MIME   But the command line doesn t bother with all that  because it runs on a limited backwards compatible layer  from when that fancy nonsense didn t mean anything  If you double click it sure  a modern OS can figure that out- but if you run it from a terminal then no  because the terminal doesn t care about your fancy OS specific file typing APIs   Regarding the other points  It s a convenience  it s similarly possible to run   python3 path to your script  If your python isn t in the path specified  then it won t work  but we tend to install things to make stuff like this work  not the other way around   It doesn t actually matter if you re under  nix  it s up to your shell whether to consider this line because it s a shellcode   So for example you can run bash under Windows   You can actually ommit this line entirely  it just mean the caller will have to specify an interpreter   Also don t put your interpreters in nonstandard locations and then try to call scripts without providing an interpreter

[python] Purpose of #!/usr/bin/python3 shebang

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