Newbie getting desperate with for

Cameron Simpson cs at
Thu Feb 17 22:40:37 CET 2011

On 17Feb2011 18:40, Alister Ware <alister.ware at> wrote:
| On Thu, 17 Feb 2011 16:42:05 +0800, Werner wrote:
| > On 17/02/11 16:39, Chris Rebert wrote:
| >> On Thu, Feb 17, 2011 at 12:27 AM, Werner <wdahn at> wrote:
| >>> I have a trivially simple piece of code called
| >>> ____________________________________________________
| >>> while True:
| >>> It runs fine with Eric but when I try to run it from shell...
| >>>> ./
| >>> ./ line 4: syntax error near unexpected token `('
| >>> ./ line 4: `    for i in range(10):'
| >> Looks like it's being run as a shell script rather than through the
| >> Python interpreter (hence why the error is not in the form of an
| >> exception with a traceback).
| >> 
| >> Try adding:
| >> #!/usr/bin/env python
| >> 
| >> as the first line in your file. This tells the shell to run the script
| >> using Python.
| may I ask what is the purpose of this code segment, it does not look like 
| it would achieve much?

When you run a command from the shell, it first tries to directly execv()
it, which calls the kernel to load and execute the file. If the file is
not a kernel compatibly file, the shell then presumes the file is a
shell script and interprets it as shell commands.

Thus the original error message, since it is Python code and not shell

The shebang line:

  #!/usr/bin/env python

is a special piece of script syntax recognised _by_the_kernel_, which
makes the script something the kernel will handle. If a file's first two
characters are:


then the kernel reads the remainder of that line as a command to be used
to execute the script by appending the script pathname. So the line
supplied causes the kernel to execute the command:

  /usr/bin/env python the-path-to-the-script ...

where "..." would be any additional arguments you supplied on the
command line. In this way, many scripting languages can be "directly"

These two characters were chosen because "#" is a comment character in
many/most UNIX scripting languages and because "!" is the traditional
UNIX indicator for a "shell escape" it tools like editors, where one
indicates that one wants to run an external shell comand by preceeding
it with a "!" "(bang"), eg:

  ! ls

and variations.

Now, it is usual to provide the direct path to the interpreter, eg:


etc. However, some some things that path may vary from system to system.
On POSIX systems, /bin/sh is guarenteed to be present at that location,
but python or perl may be in /usr/bin or /usr/local/bin etc.
The #! syntax _requires_ an absolute path to the executable.
If the executable may be in different places (because the script author
is shipping a script that will run on some unknown system) this is a

The "env" command is actually a special hook for setting environment
variables before running a command, but without extra arguments is can
be used to rely on $PATH to find the command. So the command:

  /usr/bin/env python

incantation finds "the python that would normally be run", without
needing to know the install path of the python executable.

Cameron Simpson <cs at> DoD#743

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