I always forget about how to use shell environment variable correctly. I think because I did not grasp the key concept. For example, here is the wrong approach:

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$ FOO=foo BAR=bar echo foo is ${FOO} and bar is ${BAR}
foo is and bar is

The correct answer I was hoping for is:

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foo is foo and bar is bar

Because what I frequently do is use environment variable inside shell script:

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#!/usr/bin/env bash
#
# script.sh
echo foo is ${FOO} and bar is ${BAR}

then do:

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$ FOO=foo BAR=bar ./script.sh
foo is foo and bar is bar

This works as expected because the environment variables are passed into the subshell that executing the script.

But why not the first or the wrong approach?

The reason is that shell expands the variable before the command being executed. To shell, the first approach looks like:

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$ FOO=foo BAR=bar echo foo is and bar is

The environment variables FOO and BAR were expanded before the command was executed. Hence, no values were printed.

In Debian system with Vixie cron, environment variables can be defined in the crontab:

MESSAGE=Hello World
* * * * * /bin/sh ~/script.sh >> /tmp/script.log

Now, can the same environment variable MESSAGE be passed to the shell script ~/script.sh that being scheduled to run every minute? Let’s just give a try by adding the following line to script.sh:

echo "ECHO: ${MESSAGE}"

tail -f /tmp/script.log:

ECHO: Hello World
ECHO: Hello World
ECHO: Hello World

Therefore, the shell script will pick up the environment variables defined in crontab. This really is a convenience.