Environment Variables and PATH

Environment variables are a set of key-value pairs maintained at the operating-system level, inherited by the current process and every child process it spawns. The shell itself, and every command run inside it, can read these variables for configuration — which is why the behavior of many command-line tools can be adjusted through environment variables.

Viewing and setting environment variables

echo $HOME              # view the value of a single environment variable
env                      # list all environment variables in the current session
export MY_VAR="value"    # set an environment variable, inheritable by child processes
MY_VAR="value"           # without export, valid only in the current shell — child processes can't see it
unset MY_VAR             # remove a previously set environment variable

export is the key word here: an assignment without export is just an ordinary shell variable, scoped to the current shell process. Adding export writes the variable into the environment table, after which it’s readable by any child process this shell spawns — a script it runs, a program it calls, and so on.

PATH: how commands get found

PATH is one of the environment variables encountered most directly — its value is a colon-separated list of directory paths:

echo $PATH

As covered in the shell basics section, when a command that isn’t a built-in is entered, the shell searches the directories listed in PATH, in order, for a matching executable, and runs the first match it finds — it doesn’t keep looking even if a later directory has another file with the same name. This explains two common situations:

“It’s installed, so why does it say command not found?”: the program’s executable isn’t in any directory listed in PATH. Either add the directory containing it to PATH, or call it directly with its absolute path.
“Two versions are installed, so why does it always run the old one?”: an earlier directory in PATH contains an older executable with the same name, which gets matched first. Running which <command-name> shows which file’s path would actually be executed.

Temporarily adding a directory to PATH (appended to the end, keeping the existing directories):

export PATH="$PATH:/new/directory"

Configuration files: where variables get set persistently

A variable exported directly in a terminal only lasts for that session — it’s gone once the terminal is closed. To have it take effect every time a terminal opens, the export command needs to go into a configuration file the shell reads automatically at startup. For bash:

~/.bashrc: read when an interactive non-login shell starts — this is usually what’s triggered by simply opening a new terminal window day to day
~/.profile (or ~/.bash_profile, depending on the distribution): read when a login shell starts — for example, logging in over SSH, or the start of a graphical login session

Common built-in environment variables

Variable	Purpose
`HOME`	the current user’s home directory path
`USER`	the currently logged-in username
`SHELL`	the default shell path for the current login session
`PWD`	the current working directory
`LANG`	the system’s language and locale setting