bash script and exporting variables into a subshell

spork · 10-11-2008, 06:49 PM

Hi there,
I have a bash script that I grabbed from work which checks that the user running the script is a specific user (we'll call the user foo). If the user is not foo then the current user name is exported to an environment variable and the script re-executes through sudo as foo (sudo -u foo $0 $*).

So far so good, however when the script gets re-run the environment variable no longer exists. It runs fine at work (REL, but I don't know which version), but not locally (Fedora 8).

My suspicion is that sudo as a different user is causing me to lose the environment variable, but that doesn't sound right either. Any hints?

Code:

#!/bin/bash

echo "0--owner=$OWNER"
echo "0--user=$USER"
if [ "$1" = "" ]; then
    echo "Usage: foo <bar>"
    exit;
fi

base=/foo/bar
if [ ! -d "$base" ]; then
    echo "Base directory '$base' does not exist"
    exit 1
fi

if [ "$USER" != "foo" ]; then
    export OWNER=$USER
    echo "1--owner=$OWNER"
    sudo -u foo $0 $*
    echo "DONE"
    exit;
fi

echo "2--owner = $OWNER"
if [ "$OWNER" = "" ]; then
    OWNER=$USER
fi

echo "3--owner = $OWNER"

...
<meat of the script>

Results of execution:
0--owner=
0--user=john
1--owner=john
0--owner=
0--user=foo
2--owner =
3--owner = foo

chrism01 · 10-11-2008, 07:42 PM

I think the env at work isn't what you think.
Its a fundamental of shells that you can only create an env var into the current env, and optionally export it to all child shells created from the current shell.
However, as soon as the 'current'/parent shell exits(!), all new env vars created are destroyed.
IOW, you can't export env vars 'upwards'.
Every time you run a shell prog it crates a new (sub) shell to run in. It doesn't run in the env its called from.
You can get env vars to remain if you source the code instead of running it normally ie

source some_shell_prog

OR

. some_shell_prog

http://www.tldp.org/LDP/abs/html/subshells.html

HTH

spork · 10-11-2008, 08:15 PM

Thanks Chris. That confirms what I was thinking.

What is odd is that it works at work. OWNER is used later on in the script and is available with the original caller's name.

I also realize I made a typo in the code section where the username in the sudo line is wrong. I will correct that now.

Thank you for the link.

osvaldomarques · 10-14-2008, 11:38 PM

Hi Spork,

The sudo had recently improved its security rules. One of the more devastating things to me was the clear of the environment variables as part of this policy. You can do "man sudo" and look for "preserve environment" and "setenv".

RHEL 4 and/or 4 is perhaps a relatively old distribution and uses the old sudo behavior. Fedora 8 is the distribution I use and it's where the things are broken.

Osvaldo.