Environment Variables, PATH and .bashrc

Introduction

Every program on Linux starts with a set of named values called environment variables: who you are, where your home folder is, which folders contain commands. Applications use them for configuration too — API keys, database addresses, whether to run in development or production mode. Understanding them explains countless “command not found” errors and is essential for deploying software.

Reading Variables

A $ in front of a name gives its value. By convention, environment variables are written in capitals:

echo "Home: $HOME"
echo "User: $USER"
echo "Shell: $SHELL"

printenv prints variables by name, and on its own lists all of them (env does the same):

printenv HOME USER SHELL

Shell vs Environment Variables

Create a variable by writing NAME=value — with no spaces around the =. A plain variable only exists in the current shell; programs you start from it cannot see it:

GREETING="Hello"
echo "$GREETING from the shell"
bash -c 'echo "child process sees: [$GREETING]"'

(bash -c starts a new program — a child process — that runs one command.) export turns it into an environment variable, which is copied to every program started from this shell:

export GREETING="Hello"
bash -c 'echo "child process sees: [$GREETING]"'

You can also set a variable for a single command by writing it in front. This is how apps are often started in a particular mode:

APP_ENV=production bash -c 'echo "APP_ENV is $APP_ENV"'
echo "after: [$APP_ENV]"

How PATH Finds Commands

When you type ls, how does the shell know where the program is? It looks in each folder listed in the PATH variable, in order, separated by colons:

echo $PATH

echo $PATH | tr ':' '\n'

The first folder that contains a matching program wins. If none of them does, you get “command not found” — even if the program exists somewhere else on the disk.

Adding Your Own Commands

Priya writes a small script called greet in a folder ~/bin and makes it executable with chmod +x ~/bin/greet:

#!/bin/bash
echo "Hi, $USER! Today is $(date +%A)."

With a PATH that does not include that folder, the shell cannot find it:

greet

Add the folder to the front of PATH, and greet becomes a command like any other:

export PATH="$HOME/bin:$PATH"
greet
which greet

Making It Permanent: .bashrc

Variables set in a terminal disappear when you close it. To apply settings to every new terminal, add them to ~/.bashrc, a script Bash runs each time it starts an interactive shell. Ubuntu’s default file already defines some aliases — shortcuts for longer commands:

grep -n "^alias l" ~/.bashrc

So ll is really ls -alF. Priya appends her own settings to the end of the file (with nano, or with a here-document as shown in lesson 5):

tail -n 5 ~/.bashrc

Changes apply to new terminals. To load them into the current one, run source ~/.bashrc. In a new shell, everything is in place:

type update
echo EDITOR=$EDITOR
greet

Common Mistakes

Frequently Asked Questions

Key Takeaways

Summary

You now understand how the shell is configured and how programs receive settings. Next you will combine everything so far into shell scripts: files of commands that automate real work.