Shell to Python – How to Replace Bash with Python

A script can check its Python version and, if that's Python 3, re-start itself using Python 2. Add the following near the head of the script:

if sys.version > '3':
  python2 = os.popen('which python2 2> /dev/null').read().rstrip()
  if python2:
    args = sys.argv[:]
    args.insert(0,python2)
    os.execv(python2,args)
  else:
    sys.exit("%s requires Python Version 2 (python2 not in PATH)" % os.path.basename(__file__))

This uses the system's which command to locate python2 in the environment's PATH. It then relaunches itself with that (or aborts if unable to find it).

Note that the script does need to be valid Python 3 syntax for it to start in Python 3.

Also, any output should be flushed before the execv call or it will be lost. Adding, for example, sys.stdout.flush() just before the call to execv will flush any print statements.

Early shells had only a single data type: strings. But it is common to manipulate lists of strings, typically when passing multiple file names as arguments to a program. Another common use case for splitting is when a command outputs a list of results: the command's output is a string, but the desired data is a list of strings. To store a list of file names in a variable, you would put spaces between them. Then a shell script like this

files="foo bar qux"
myprogram $files

called myprogram with three arguments, as the shell split the string $files into words. At the time, spaces in file names were either forbidden or widely considered Not Done.

The Korn shell introduced arrays: you could store a list of strings in a variable. The Korn shell remained compatible with the then-established Bourne shell, so bare variable expansions kept undergoing word splitting, and using arrays required some syntactic overhead. You would write the snippet above

files=(foo bar qux)
myprogram "${files[@]}"

Zsh had arrays from the start, and its author opted for a saner language design at the expense of backward compatibility. In zsh (under the default expansion rules) $var does not perfom word splitting; if you want to store a list of words in a variable, you are meant to use an array; and if you really want word splitting, you can write $=var.

files=(foo bar qux)
myprogram $files

These days, spaces in file names are something you need to cope with, both because many users expect them to work and because many scripts are executed in security-sensitive contexts where an attacker may be in control of file names. So automatic word splitting is often a nuisance; hence my general advice to always use double quotes, i.e. write "$foo", unless you understand why you need word splitting in a particular use case. (Note that bare variable expansions undergo globbing as well.)