Environment and PATH

Environment Variables, PATH and Startup Files

NAME=value creates a shell variable, visible only to the current shell. export NAME makes it an environment variable that child processes inherit, which is how APP_ENV reaches PHP's getenv(). NAME=value command sets it for one command. PATH lists the directories searched, left to right, for a command; prepend your own with export PATH="$HOME/bin:$PATH". Ubuntu 225 's ~/.profile does this for ~/bin and ~/.local/bin if they exist.

Shell variables, environment variables and the PATH search orderShell

GREETING=hello
bash -c 'echo "child sees: [$GREETING]"'   # a shell variable stays in this shell
export GREETING
bash -c 'echo "child sees: [$GREETING]"'   # an environment variable is inherited
APP_ENV=staging php -r 'echo getenv("APP_ENV"), PHP_EOL;'   # for one command only
echo "$PATH" | tr ':' '\n' | head -n 6     # searched left to right

Output

child sees: []
child sees: [hello]
staging
/usr/local/sbin
/usr/local/bin
/usr/sbin
/usr/bin
/sbin
/bin

A login shell (an SSH session) reads /etc/profile, then the first of ~/.bash_profile, ~/.bash_login and ~/.profile. An interactive non-login shell reads ~/.bashrc, which Ubuntu's ~/.profile also sources, so aliases and PS1 belong there. Scripts, cron jobs and systemd 142,543 services read none of these files.

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