I am using /bin/tcsh as my default shell.
However, the tcsh style command os.system('setenv VAR val') doesn't work for me. But os.system('export VAR=val') works.
So my question is how can I know the os.system() run command under which shell?
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4 Answers
Was just reading Executing BASH from Python, then 17.1. subprocess — Subprocess management — Python v2.7.3 documentation, and I saw the executable argument; and it seems to work:
$ python
Python 2.7.1+ (r271:86832, Sep 27 2012, 21:16:52)
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> print os.popen("echo $0").read()
sh
>>> import subprocess
>>> print subprocess.call("echo $0", shell=True).read()
/bin/sh
>>> print subprocess.Popen("echo $0", stdout=subprocess.PIPE, shell=True).stdout.read()
/bin/sh
>>> print subprocess.Popen("echo $0", stdout=subprocess.PIPE, shell=True, executable="/bin/bash").stdout.read()
/bin/bash
>>> print subprocess.Popen("cat <(echo TEST)", stdout=subprocess.PIPE, shell=True).stdout.read()
/bin/sh: Syntax error: "(" unexpected
>>> print subprocess.Popen("cat <(echo TEST)", stdout=subprocess.PIPE, shell=True, executable="/bin/bash").stdout.read()
TEST
Hope this helps someone,
Cheers!
2 Comments
Baskaya
I am that someone. This is what I was looking for. Thank you for noting this :)
CheeHow
Yes, this is so true when one is piping stdout from a single command to multiple commands using tee, something like
command out=stdout | tee >(command_1 in=stdin) >(command_2 in=stdin). Thanks for posting this, thumbs up~These days you should be using the Subprocess module instead of os.system(). According to the documentation there, the default shell is /bin/sh. I believe that os.system() works the same way.
Edit: I should also mention that the subprocess module allows you to set the environment available to the executing process through the env parameter.
1 Comment
Nicholas Knight
Indeed, /bin/sh (which is almost always some form of bourne shell) is almost always what is meant when anything *nix-related says "the shell" without qualification. It may also be useful to note that if you really need to execute some snippet under a specific non-bourne shell, you could pass the function something like '/path/to/tcsh -c "your tcsh snippet here"'.
os.system() just calls the system() system call ("man 3 system"). On most *nixes this means you get /bin/sh.
Note that export VAR=val is technically not standard syntax (though bash understands it, and I think ksh does too). It will not work on systems where /bin/sh is actually the Bourne shell. On those systems you need to export and set as separate commands. (This will work with bash too.)
answered May 25, 2009 at 4:17
Laurence Gonsalves
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1 Comment
Ben L
Confirm this behavior. So the
os.system() is no longer useful. I am running some scripts that require bash, and this os.system is giving me hard time. So in the future I will always use the subprocess : subprocess.call(cmd, shell=True, executable="/bin/bash")If your command is a shell file, and the file is executable, and the file begins with "#!", you can pick your shell.
#!/bin/zsh
Do Some Stuff
You can write this file and then execute it with subprocess.Popen(filename,shell=True) and you'll be able to use any shell you want.
Also, be sure to read this about os.system and subprocess.Popen.
2 Comments
Matt Joiner
i was going to point out that shell=True is not necessary, but then it occurred to me: is it the the shell that's responsible for interpreting shebangs and acting accordingly?
S.Lott
Correct. The shell interprets the "magic" bytes "#!" to see what other shell should really be using this file.