Track 2 · Lesson 3 of 6
Environment variables and PATH
Read, set and export environment variables in Termux, and learn how PATH decides which command runs when you type a name.
Every time you open Termux you get a fresh shell. That shell keeps a set of named values called environment variables. They hold your home directory, the folders searched for commands, your terminal type and your locale. Change one and you change how your commands behave, without editing the commands themselves.
What you'll be able to do
- Print any environment variable with
echoorprintenv. - Set your own shell variables and reuse them later in the session.
- Export a variable so the programs you launch inherit it.
- Explain what
PATHis and why the order of its entries matters. - Add a folder to
PATHfor the current session.
Step 1: List what the shell already knows
The env command prints every environment variable in the current session. Termux sets a few dozen. This is a trimmed view.
envANDROID_DATA=/data
ANDROID_ROOT=/system
COLORTERM=truecolor
HOME=/data/data/com.termux/files/home
LANG=en_US.UTF-8
PATH=/data/data/com.termux/files/usr/bin:/data/data/com.termux/files/usr/bin/applets
PREFIX=/data/data/com.termux/files/usr
PWD=/data/data/com.termux/files/home
SHELL=/data/data/com.termux/files/usr/bin/bash
TERM=xterm-256color
TMPDIR=/data/data/com.termux/files/usr/tmpprintenv prints the same values, one per line. Pipe it through sort to put them in alphabetical order.
Step 2: Read a single variable
Put a $ in front of a name and the shell replaces it with the value before running the command. echo is the quickest way to see that value.
echo $HOME/data/data/com.termux/files/homeprintenv does the same job but takes the bare name, with no $.
printenv HOME/data/data/com.termux/files/homeWrap the name in double quotes when it sits inside a longer string, as in echo "home is $HOME". Double quotes still expand the variable. Single quotes do not, so echo 'home is $HOME' prints that text literally.
Step 3: Set your own variable
Assign a value with the name on the left, an equals sign, then the value on the right. Keep the spaces away from the equals sign.
project=notes
echo $projectnotesThis variable lives only in the current shell session. Close the session and it is gone.
Step 4: Export it so child processes see it
A variable you set stays private to your shell until you export it. A child process, such as another bash, starts with a fresh copy of the environment. It cannot see private shell variables.
local_only=hello
bash -c 'echo $local_only'The output is empty. The child shell has no idea what local_only is. The parent still does.
echo $local_onlyhelloNow export a variable and run the same test.
export project
bash -c 'echo $project'notesBecause project is exported, every child process inherits it. That is the whole difference. An exported variable is copied into the environment of the programs you run. A plain shell variable is not.
You can set and export in one line.
export EDITOR=nano
printenv EDITORnanoNever put a password, an API key or a token into a variable that you export in a script you share. Exported variables are copied into every child process and can be read from the environment of those processes. Keep secrets in a file that only you can read, and load them only when a command needs them.
Step 5: Understand PATH and why order matters
PATH is a list of folders separated by colons. When you type a command name, the shell walks that list from left to right. It runs the first matching program it finds.
echo $PATH/data/data/com.termux/files/usr/bin:/data/data/com.termux/files/usr/bin/appletsBecause the shell stops at the first match, the leftmost folder wins. If two folders both hold a program called python, the copy in the earlier folder runs.
To make your own folder take priority, put it at the front.
export PATH="$HOME/bin:$PATH"
echo $PATH/data/data/com.termux/files/home/bin:/data/data/com.termux/files/usr/bin:/data/data/com.termux/files/usr/bin/appletsAnything in $HOME/bin now runs ahead of the system copies. To search your folder last instead, swap the order.
export PATH="$PATH:$HOME/bin"
echo $PATH/data/data/com.termux/files/usr/bin:/data/data/com.termux/files/usr/bin/applets:/data/data/com.termux/files/home/binWhen you change PATH, write the full path, such as $HOME/bin, rather than a relative one. PATH is read from whatever directory you happen to be in, so a relative entry changes meaning every time you change directory.
The change lasts only for this session. To keep it, put the export line at the end of your shell startup file.
echo 'export PATH="$HOME/bin:$PATH"' >> ~/.bashrcThat line prints nothing when it succeeds. Open a new session and the change is there.
Quick reference
| Command | What it does |
|---|---|
env | Print all environment variables |
printenv | Print all, or one named, environment variable |
echo $NAME | Print the value of a variable |
NAME=value | Set a shell variable for this session |
export NAME | Mark a variable so child processes inherit it |
export NAME=value | Set and export in one step |
unset NAME | Remove a variable |
echo $PATH | Show the command search path |
export PATH="$HOME/bin:$PATH" | Prepend a folder to PATH |
Next steps
- Shell scripting basics to put these values to work inside a script.
- Text processing with sed and awk to filter the output you generate.
- Pipe Dreams to practise combining commands.
Test yourself on this
4 questions and 1 task where you run a real command, with the reasoning explained either way.
