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.

Answer 1

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 
    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")