When I use the shebang
#!/usr/bin/env python to run a script, how does the system know which
python to use? if I look for a
python bin path in the environment variables I find nothing.
env | grep -i python
The shebang expects a full path to the interpreter to use so the following syntax would be incorrect:
Setting a full path like this might work:
but would be non portable as python might be installed in
/opt/python/bin, or wherever other location.
is a method allowing a portable way to specify to the OS a full path equivalent to the one where
python is first located in the
The shebang line (from “sharp bang”, i.e.
#!) is processed by the kernel. The kernel doesn't want to know about environment variables such as
PATH. So the name on the shebang line must be an absolute path to an executable. You can also specify an additional argument to pass to that executable before the script name (with system-dependent restrictions I won't go into here). For example, for a Python script, you can specify
on the first line, and when you execute the script, the kernel will in fact execute
/usr/bin/python /path/to/script. But that's not convenient: you need to specify the full path of the command. What if you have
/usr/bin on some machines and
/usr/local/bin on others? Or you want to set your
/home/joe/opt/python-2.5/bin so as to use a specific version of Python? Since the kernel won't do the
PATH lookup for you, the idea is to make the kernel run a command that in turns looks up the desired interpreter in the
path-lookup-command must take the name of an executable as an argument and look it up in
PATH and execute it: the kernel will run
/fixed/path/to/path-lookup-command python /path/to/script. As it happens, the
env command does just that. Its main purpose is to run a command with a different environment, but since it looks up the command name in
$PATH, it's perfect for our purpose here.
Although this is not officially guaranteed, historic Unix systems provided
/usr/bin, and modern systems have kept that location precisely because of the widespread use of
#!/usr/bin/env. So, in practice, the way to specify that a script must be executed by the user's favorite Python interpreter is
Right, so run:
env | grep PATH
Your $PATH is a list of directories. Unix will go through that list of directories, in order, until it finds "python".
You can see which directory it finds with the 'which' command: