Environment Variables & export

export Makes a Variable Inheritable

By default, a variable assigned in a script is only visible within that script's own shell, not to any programs it launches. export flags the variable so that it becomes part of the environment handed to every child process the script starts from that point on.

Example: export Makes a Variable Inheritable

#!/bin/bash
GREETING="hello from parent"
export GREETING
bash -c 'echo "Child sees: $GREETING"'

Unexported Variables Stay Local

A variable that is never exported is only visible in the current shell and any subshells that inherit its scope directly -- it will not be visible in a separately invoked child process. This distinction is easy to miss because both kinds of variables look identical when you just echo them in the same script.

Warning: The child process prints an empty value for LOCAL_ONLY because it was never exported.

Example: Unexported Variables Stay Local

#!/bin/bash
LOCAL_ONLY="not exported"
bash -c 'echo "Child sees LOCAL_ONLY as: [$LOCAL_ONLY]"'

Common Built-in Environment Variables

Bash and the operating system pre-populate several environment variables at login, such as HOME (the current user's home directory), USER (username), and PATH (the list of directories searched for commands). Scripts frequently read these rather than hard-coding values that differ between systems.

Example: Common Built-in Environment Variables

#!/bin/bash
echo "Home directory: $HOME"
echo "Current user: $USER"
echo "First PATH entry: $(echo "$PATH" | cut -d: -f1)"

Safely Extending PATH

When adding a directory to PATH, always include the existing value ($PATH) in the new assignment, otherwise you overwrite it entirely and the shell can no longer find standard commands. Appending or prepending with a colon separator keeps all previous search locations intact.

Example: Safely Extending PATH

#!/bin/bash
export PATH="/usr/local/mytools:$PATH"
echo "$PATH" | tr ':' '\n' | head -n 3