[Bug 1516300] Re: dash command variable assignments remain in the shell after command execution completed

[Seth Arnold]

I gave this a test with Ubuntu 14.04, 16.04, 18.04, 20.04, LTS releases,
and Debian 10 and Debian 11, in lxd.

Ubuntus before 20.04 all showed the described behaviour.

Ubuntu 20.04 LTS worked the same as both Debian releases.

The versions of dash in each release make this make some sense:

$ for h in u14 u16 u18 u20 d10 d11 ; do echo $h ; lxc exec $h -- dpkg -l dash  | awk '/^ii/ {print $2, $3;}' ; done
u14
dash 0.5.7-4ubuntu1
u16
dash 0.5.8-2.1ubuntu2
u18
dash 0.5.8-2.10
u20
dash 0.5.10.2-6
d10
dash 0.5.10.2-5
d11
dash 0.5.10.2-7


What's even more surprising to me is that the functionality of passing a variable in to the function doesn't even work in dash. Compare the bash with the dash:

u20-bash$ foo () { printenv | grep SHELL; }
u20-bash$ echo $SHELL
/bin/bash
u20-bash$ foo
u20-bash$ SHELL=foo foo
SHELL=foo
u20-bash$ echo $SHELL
/bin/bash

u20-dash$ foo () { printenv | grep SHELL; }
u20-dash$ foo
u20-dash$ SHELL=foo foo
u20-dash$ echo $SHELL

u20-dash$


I'm surprised at such a fundamental difference in behaviour between the two, but it's now been five years with no other reports, so perhaps no one depends upon the behaviour and changes which shell they use to execute scripts.

It might be best to just leave it alone.

Thanks

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https://bugs.launchpad.net/bugs/1516300

Title:
  dash command variable assignments remain in the shell after command
  execution completed

Status in dash package in Ubuntu:
  Confirmed

Bug description:
  If a shell function is invoked with variable assignments preceding it,
  the assignments remain in the shell after the command execution
  completed. This is unexpected behavior and might be a potential
  security issue, since it allows to modify the user environment in a
  subtle unexpected way. For example, consider the following commands
  that shouldn't change the SHELL value outside function foo, yet it
  does in Ubuntu 14.04

  echo $SHELL # check our default shell, gives /bin/bash

  foo () { printenv | grep SHELL; } # no side effects, can be anything
  SHELL=/bin/sh foo

  echo $SHELL # now gives /bin/sh, but expected to give /bin/bash as
  before

  I checked bash and zsh, none of them have this problem. sh in freebsd
  and debian handle this case correctly. So far, it seems the issue is
  limited to Ubuntu dash.

  lsb_release -rd
  Description:	Ubuntu 14.04.3 LTS
  Release:	14.04

  apt-cache policy dash
  dash:
    Installed: 0.5.7-4ubuntu1
    Candidate: 0.5.7-4ubuntu1
    Version table:
   *** 0.5.7-4ubuntu1 0
          500 http://us.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
          100 /var/lib/dpkg/status

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