Python subprocess, subshells, and redirection

Question

I want to use the magic of subshells and redirection with the python subprocess module, but it doesn't seem to work, complaining about unexpected tokens are the parenthesis. For example, the command

cat <(head tmp)

when passed to subprocess gives this

>>> subprocess.Popen("cat <(head tmp)", shell=True)
<subprocess.Popen object at 0x2b9bfef30350>
>>> /bin/sh: -c: line 0: syntax error near unexpected token `('
/bin/sh: -c: line 0: `cat <(head tmp)'

Notice that your errors are coming from `/bin/sh`. I think that syntax may only with in `bash`. Do you want subprocess to run `bash`? Is that the question? — S.Lott, Sep 13 '11 at 19:59
@S.Lott I thought that on my installation, sh was bash. Certainly, if I did `man sh`, the man page that came up was bash. Scott Lamb pointed out that when invoked as `sh`, this feature is unavailable, so in a sense, yes, I wanted it run as `bash`, even though I thought it already was. — pythonic metaphor, Sep 13 '11 at 20:08
And why are you shelling out to the shell when Python can do what you want? — bash-o-logist, Sep 13 '11 at 23:45

score 11 · Accepted Answer · answered Sep 13 '11 at 20:01

The <(head tmp) syntax is a bash feature called "process substitution". The basic/portable /bin/sh doesn't support it. (This is true even on systems where /bin/sh and /bin/bash are the same program; it doesn't allow this feature when invoked as plain /bin/sh so you won't inadvertently depend on a non-portable feature.)

>>> subprocess.Popen(["/bin/bash", "-c", "cat <(head tmp)"])
<subprocess.Popen object at 0x1004cca50>

Python subprocess, subshells, and redirection

1 Answers1