Unix Executable Scripts (#!)

If you are going to use Python on a Unix, Linux, or Unix-like system, you can also turn files of Python code into executable programs, much as you would for programs coded in a shell language such as csh or ksh. Such files are usually called executable scripts. In simple terms, Unix-style executable scripts are just normal text files containing Python statements, but with two special properties:

 

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  • Their first line is special. Scripts usually start with a line that begins with the characters #! (often called “hash bang”), followed by the path to the Python interpreter on your machine.
  • They usually have executable privileges. Script files are usually marked as executable to tell the operating system that they may be run as top-level programs. On Unix systems, a command such as chmod +x file.py usually does the trick.

Let’s look at an example for Unix-like systems. Use your text editor again to create a file of Python code called brian:

#!/usr/local/bin/python
print('The Bright Side ' + 'of Life...')        # + means concatenate for strings

The special line at the top of the file tells the system where the Python interpreter lives. Technically, the first line is a Python comment. As mentioned earlier, all comments in Python programs start with a # and span to the end of the line; they are a place to insert extra information for human readers of your code. But when a comment such as the first line in this file appears, it’s special because the operating system uses it to find an interpreter for running the program code in the rest of the file.

Also, note that this file is called simply brian, without the .py suffix used for the module file earlier. Adding a .py to the name wouldn’t hurt (and might help you remember that this is a Python program file), but because you don’t plan on letting other modules import the code in this file, the name of the file is irrelevant. If you give the file executable privileges with a chmod +x brian shell command, you can run it from the operating system shell as though it were a binary program:

% brian
The Bright Side of Life...

A note for Windows users: the method described here is a Unix trick, and it may not work on your platform. Not to worry; just use the basic command-line technique explored earlier. List the file’s name on an explicit python command line:[5]

C:\misc> python brian
The Bright Side of Life...

In this case, you don’t need the special #! comment at the top (although Python just ignores it if it’s present), and the file doesn’t need to be given executable privileges. In fact, if you want to run files portably between Unix and Microsoft Windows, your life will probably be simpler if you always use the basic command-line approach, not Unix-style scripts, to launch programs.

The Unix env Lookup Trick

On some Unix systems, you can avoid hardcoding the path to the Python interpreter by writing the special first-line comment like this:

#!/usr/bin/env python
...script goes here...

When coded this way, the env program locates the Python interpreter according to your system search path settings (i.e., in most Unix shells, by looking in all the directories listed in the PATH environment variable). This scheme can be more portable, as you don’t need to hardcode a Python install path in the first line of all your scripts.

Provided you have access to env everywhere, your scripts will run no matter where Python lives on your system—you need only change the PATH environment variable settings across platforms, not in the first line in all your scripts. Of course, this assumes that env lives in the same place everywhere (on some machines, it may be in /sbin, /bin, or elsewhere); if not, all portability bets are off!

 

[5] As we discussed when exploring command lines, modern Windows versions also let you type just the name of a .py file at the system command line—they use the Registry to determine that the file should be opened with Python (e.g., typing brian.py is equivalent to typing python brian.py). This command-line mode is similar in spirit to the Unix #!, though it is system-wide on Windows, not per-file. Note that some programs may actually interpret and use a first #! line on Windows much like on Unix, but the DOS system shell on Windows simply ignores it.