lecture03-your-environment.md

lecture3

Your Environment

Below, I have listed examples of the many common commands and practices of the shell. If you want to know more about specific commands, you can use man to find out more information.

man find  # will return the man page about find. Press 'q' to get out of this page

Or just do what I did for this entire lesson and search The Google.

Relevant Terminal Commands

which - locate where a program lives in the file system

# Instructions
# which <program>

which ls
# /bin/ls

which wget
# /usr/bin/ls

which bash
# /usr/local/bin/bash

Environment Variables

Environment variables are temporary variables used inside your shell. Some are set by the operating system ($USER, $CWD, etc). Some are user specified. They're helpful storing information that you might use a lot ($ANDROID_IP), or information that is needed for your programs to run, but is too sensitive to put in your code ($DATABASE_PASSWORD).

Getting and Setting

# Print all of the environment variables to the screen
printenv

# Create and assign an environment variable
# <Variable Name>="<Value>"
NAME="kirby"

# Reference an environment variable
# $<Variable Name>
echo My name is $NAME
# My name is kirby

Important Environment Variables

Here are some important environment variables you should know about.

Variable	Description
$HOME	The home directory of the current user
$PATH	Indicates search path for commands
$PWD	The current working directory
$USER	The current user

The PATH

The $PATH variable is especially important. It is how your terminal knows where your programs are. When you type ls in the terminal the OS searches through your $PATH variable for possible directories for it.

Here is an example $PATH variable:

PATH=/homes/kkohlmor/bin/.amd64-linux:/homes/kkohlmor/bin:/usr/local/bin:/bin:/usr/bin:/sbin:/usr/sbin:/usr/x86_64-pc-linux-gnu/gcc-bin/4.6.3:/usr/games/bin:.

Colons are used to delimitate each directory for searching. The directories are ordered with highest precedence first and lowest last.

Here is the same $PATH, but with each directory on a line

PATH=
/homes/kkohlmor/bin/.amd64-linux  # the OS will look for commands here first
/homes/kkohlmor/bin               # this is where you should install apps
/usr/local/bin                    # because it's in your home directory
/bin                              # unlike the other directories
/usr/bin
/sbin
/usr/sbin
/usr/x86_64-pc-linux-gnu/gcc-bin/4.6.3
/usr/games/bin
.                                 # and will look here last

Backticks (Subshell)

Text between backticks is executed and replaced by the output of the command.

For example:

# if hello.txt contains "Hello World"
echo The contents of hello.txt are `cat hello.txt`.
# The contents of hello.txt are Hello World.

# you can do a lot more useful and complex things too

# if list_of_files.txt contains some files
# you can delete all of them this way
rm `cat list_of_files.txt`

# store files in an environment variable
FILES=`find . -name "Shape.java"`