A Brief Primer on Unix Environment Variables

The shell used will not make a significant difference, assuming one adheres to the Bourne—or ideally a Bourne derived—shell. At present, I favor ZSH.

$ echo $SHELL is being used /bin/zsh is being used $ echo this shell uses $(tty), process id $$ this shell uses /dev/ttyp8, process id 19741 $ echo $MY_ENV_VAR $ MY_ENV_VAR="something of value" $ echo $MY_ENV_VAR something of value $ $SHELL $ echo this shell uses $(tty), process id $$ this shell uses /dev/ttyp8, process id 19875 $ echo $MY_ENV_VAR $ exit $ echo $MY_ENV_VAR something of value

Note that though MY_ENV_VAR shows a value in the first shell, it does not in the subshell (opened via the $SHELL command). After closing the subshell, the custom MY_ENV_VAR is still defined in the original shell. Moving on, under the same session:

$ export MY_ENV_VAR $ echo $MY_ENV_VAR something of value $ $SHELL $ echo this shell uses $(tty), process id $$ this shell uses /dev/ttyp8, process id 22585 $ echo $MY_ENV_VAR something of value $ exit $ echo $MY_ENV_VAR something of value

The export builtin ensures child processes inherit the custom environment setting. The export only needs to be done once on the variable. In modern Bourne shells, this can either be done via a single export SOME_ENV=some_value command, or, as shown above, separate commands.

Processes may strip environment variables, usually for security reasons. This would account for otherwise exported variables not being present in child processes. For confirmation, run ktrace or strace on the process, and determine what—such as setenv(3), or the %ENV hash in Perl—manipulate the evironment.

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A Brief Primer on Unix Environment Variables