Load environment variables from a file with key=value pairs and optionally execute a program

defaultenv.sh

#!/bin/sh
# defaultenv.sh - Load environment variable defaults and run programs

set -e
[ x"$DEBUG" = "x1" ] && set -x

_load_envrc () {
    local file="$1"; shift
    if [ -r "$file" ] ; then
        # Bourne shell limits how we can test and set env vars, so here I'm
        # just writing the .env file to a new file, then reading the file into
        # a while loop, and using `env` and `grep` to see if the KEY might already
        # be set. It's pretty terrible, but this is the best way I could figure
        # to do this with Bourne shell.
        TMP=$(mktemp)
        cat "$file" | grep -v '^$\|^[[:space:]]*#' | sed -e 's/^export[[:space:]]\+//' | grep -i '^[a-z0-9_]\+=' > $TMP
        while read LINE ; do
            KEY="${LINE%%=*}"
            VALUE="${LINE#*=}"
            if ! env | grep -q -e "^$KEY=" ; then
                echo "Setting $file env '$KEY=$VALUE'" 2>&1
                export "$KEY"="$VALUE"
            fi
        done < $TMP
        rm -f $TMP
    else
        echo "Error: cannot read envrc '$file'" ; exit 1
    fi
}

if  [ $# -eq 0 ] \
    && [ -n "$ENVRC" ] \
    && [ -r "$ENVRC" ]
then
    _load_envrc "$ENVRC"

elif [ $# -gt 0 ] ; then
    _load_envrc "$1"
    shift

else
    echo "Usage: $0 [ENVRC [ARGS [..]]]"
    echo ""
    echo "Pass a file ENVRC (made up of KEY=VALUE lines) and they will be set as environment variables,"
    echo "but only if an existing KEY isn't already set in the environment."
    echo "If ARGS are passed, they are executed by the shell."
    echo "If no arguments are passed, the ENVRC environment variable will be checked for the file to read."
    exit 1
fi

if [ $# -gt 0 ] ; then
    exec "$@"
fi