Bash command can run in the shell, but not via python suprocess.run

Question

If I run this command in ubuntu shell:

debconf-set-selections <<< 'postfix postfix/mailname string server.exmaple.com'

It runs successfully, but if I run it via python:

>>> from subprocess import run
>>> run("debconf-set-selections <<< 'postfix postfix/mailname string server.exmaple.com'", shell=True)
/bin/sh: 1: Syntax error: redirection unexpected
CompletedProcess(args="debconf-set-selections <<< 'postfix postfix/mailname string server.exmaple.com'", returncode=2)

I don't understand why python is trying to interpret whether there is redirection etc. How does one make the command successfully run so one can script installation of an application, e.g. postfix in this case via python (not a normal bash script)?

I have tried various forms with double and single quotes (as recommended in other posts), with no success.

heemayl · Accepted Answer · 2019-06-24 21:10:42Z

1

subprocess uses /bin/sh as shell, and presumably your system's one does not support here-string (<<<), hence the error.

From subprocess source:

if shell:
    # On Android the default shell is at '/system/bin/sh'.
    unix_shell = ('/system/bin/sh' if
                  hasattr(sys, 'getandroidapilevel') else '/bin/sh')

You can run the command as an argument to any shell that supports here string e.g. bash:

run('bash -c "debconf-set-selections <<< \"postfix postfix/mailname string server.exmaple.com\""', shell=True)

Be careful with the quoting.

Or better you can stay POSIX and use echo and pipe to pass via STDIN:

run("echo 'postfix postfix/mailname string server.exmaple.com' | debconf-set-selections", shell=True)

edited Jun 24, 2019 at 21:10

answered Jun 24, 2019 at 21:05

heemayl

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3 Comments

run_the_race Over a year ago

Wow, don't know how you guys know this stuff. It almost worked, I just had to tweak the quotes a little: run('bash -c "debconf-set-selections <<< \'postfix postfix/mailname string server.exmaple.com\'"', shell=True)

heemayl Over a year ago

@run_the_race /bin/sh: 1: Syntax error: redirection unexpected is the hint ;) BTW please check the POSIX option i added.

Gordon Davisson Over a year ago

You could skip a quote level if you remove shell=True, and pass the command as a list: run(["bash", "-c", "debconf-set-selections <<< 'postfix postfix/mailname string server.exmaple.com'"])

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