Bash shells with different environment variables

If it is an sh script - as in, it explicitly references #!/bin/sh - which might still be bash but would be like invoking it with --posix --no-rc --no-profile - then you can specify the ENV file with the ENV environment variable:

ENV=/path/to/rcfile sh

Specific variables need either to be declared on the command-line - as above for $ENV - or else with export. For example, for $PATH you do:

export "PATH=$PATH:/some/more/paths"; sh

The $BASH_ENV variable you reference is not a file you need to source - and it isn't interpreted anyway when bash is invoked as sh - but is rather a path to a file that is sourced when a non-interactive bash shell is invoked - such as with a script that specifies the:

#!/bin/bash

...or whatever bang line.

Another way you might like to invoke your script/shell is with the env utility. It can be used to explicitly remove values from the environment, or else, as is usually easiest, to wipe it clean from the start:

env - BASH_ENV=/path/to/rcfile /usr/bin/bash /some/script/file

That will tell env to invoke the /usr/bin/bash command - with all its arguments appended - with the $BASH_ENV environment variable specified, but otherwise with a clean environment entirely.

bash has special syntax for setting environment variables, while fish uses a builtin. I would suggest writing your .env file like so:

setenv VAR1 val1
setenv VAR2 val2

and then defining setenv appropriately in the respective shells. In bash (e.g. .bashrc):

function setenv() { export "$1=$2"; }
. ~/.env

In fish (e.g. config.fish):

function setenv; set -gx $argv; end
source ~/.env

Note that PATH will require some special handling, since it's an array in fish but a colon delimited string in bash. If you prefer to write setenv PATH "$HOME/bin:$PATH" in .env, you could write fish's setenv like so:

function setenv
    if [ $argv[1] = PATH ]
        # Replace colons and spaces with newlines
        set -gx PATH (echo $argv[2] | tr ': ' \n)
    else
        set -gx $argv
    end
 end

This will mishandle elements in PATH that contain spaces, colons, or newlines.

The awkwardness in PATH is due to mixing up colon-delimited strings with true arrays. The preferred way to append to PATH in fish is simply set PATH $PATH ~/bin.