(debian-policy.info)Environment variables

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9.9 Environment variables
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Programs installed on the system PATH (‘/bin’, ‘/usr/bin’, ‘/sbin’,
‘/usr/sbin’, or similar directories) must not depend on custom
environment variable settings to get reasonable defaults.  This is
because such environment variables would have to be set in a system-wide
configuration file such as a file in ‘/etc/profile.d’, which is not
supported by all shells.

If a program usually depends on environment variables for its
configuration, the program should be changed to fall back to a
reasonable default configuration if these environment variables are not
present.  If this cannot be done easily (e.g., if the source code of a
non-free program is not available), the program must be replaced by a
small “wrapper” shell script that sets the environment variables if they
are not already defined, and calls the original program.

Here is an example of a wrapper script for this purpose:

     #!/bin/sh
     BAR=${BAR:-/var/lib/fubar}
     export BAR
     exec /usr/lib/foo/foo "$@"