environment variables

Question 1

What command displays the currently set CLI environment variables?

Answer 1

env

Question 2

What does the term “session” mean?

Answer 2

refers to the state of being logged in to a computer’s command line interface.

When you log in, you start a new session, in which your commands will be recorded and other contextual information will be maintained.

When you close Terminal or type “exit”, your session is closed and that context and data is lost.

Question 3

How do you set an environment variable on the fly to only affect the current session?

Answer 3

Set the variable on its own line, then use it anywhere:
$ SOMETHING=’some value’
$ echo $SOMETHING
some value

Question 4

Name some environment files

Answer 4

.bashrc, .bash_profile, .bash_login, .login, .profile

Question 5

What happens when you open a Terminal or connect to a server?

Answer 5

1. You are logged into the command line with a user account. and put in the home dir for that user.
2. One or more environment files are executed. These files set environment variables.
   E.g. .bashrc, .bash_profile, .bash_login, .login, .profile

Question 6

what do environment files do?

Answer 6

Store environment variables and allow you to modify or create environment variables.

Question 7

How do you set an environment variable to temporarily change a variable before it gets used in a command?

Answer 7

Set the variable immediately preceding the command on the same line:

$ SOMETHING='a value' env
...
SOMETHING=a value
...
NOTE: that var is set ONLY for the executed command

Question 8

What is the correct syntax to set a variable?

Answer 8

that there should be no spaces between the variable and the equal sign or the equal sign and the value.

E.g.
SOMETHING=’some value’

Question 9

When setting a value for a variable, are quotations required?

Answer 9

No.

But you must do it when the value has special characters.

Question 10

what does the export command in an environment files like .bashrc or .bash_profile mean?

Answer 10

It means “make this variable available globally,” thus the variable is available and can be invoked from anywhere in CLI

A variable assignment that starts with export in your .bashrc will be available in the CLI session when you log in.

Note that if an environment variable has been exported once, you don’t need to continually put export before it when you set its value.

Question 11

Can you use variables as a command?

Answer 11

Yes.

E.g.
$ MESSAGE='Hello, world!'
$ COMMAND="echo"
$ $COMMAND $MESSAGE
Hello, world!

Question 12

When Interpolating Strings (aka displaying variable values in other strings), what must you do to the string that is invoking the variables?

Answer 12

The string must be surrounded in double quotation marks (“), not the single quotes (‘).

E.g.
$ MESSAGE1=”This is message 1.”
$ MESSAGE2=”This is message 2.”
$ MESSAGE=”$MESSAGE1 $MESSAGE2”

Question 13

What variable determines which directories are searched when a command is entered?

Answer 13

PATH

It tells the CLI session where to find executable files for common commands, such as echo or pwd.

Note: When you type a word into the command line, and it doesn’t start with a /, ~, or a . (because those would indicate a path to an actual file), the command line will search each of the directories listed in the PATH environment variable for that command.

Question 14

what does bin in /usr/local/bin stand for?

and what is the bin directory?

Answer 14

binary

a standard directory name that contains executable files, or programs.

Question 15

what command reveals which executable file is executed when a command is run?

Answer 15

which
[which][command]

E.g. which pwd
/bin/pwd

It searches the paths for the named command and displays the first one it finds

Question 16

How do you make a custom executable file executable like a built-in command?

Answer 16

1. make sure it has executable permissions
2. add the path to the directory it is contained in, to the PATH variable in ~/.bashrc:

E.g.
PATH=”/path/to/my/executables-directory:$PATH”

Question 17

When adding a path to a directory, like so:
PATH=”/path/to/my/executables-directory:$PATH”

why is $PATH needed at the end?

Answer 17

This preserves the old PATH locations while making your directory the highest priority.

Question 18

What is the PATH environment variable?

Answer 18

an ordered, colon-delimited, list of directories that contain executables

Question 19

Does the order of the paths set in PATH matter?

Answer 19

Yes.

The order of the directories in the PATH variable is first-found-first-execute

Question 20

What happens if you use a /, ., or ~ before a command?

Answer 20

The command line will interpret that as an actual path to a file, and will not use the PATH variable

Question 21

where should permanent modifications to environment variables be stored?

Answer 21

in environment files such as .bashrc, .bash_profile, .bash_login, .login, .profile

Question 22

what is this command?&raquo_space;

Answer 22

redirection operator

Question 23

what operator appends text to a file?

Answer 23

> >

Question 24

what will this do?

echo ‘blah’&raquo_space; ~/some_unique_file_name.txt

Answer 24

creates the file and appends ‘blah’ to its contents

Question 25

what will this do?

echo ‘blah’&raquo_space; ~/pre_existing_file.txt

Answer 25

appends ‘blah’ to the contents of the file

Question 26

____ provides context for the commands you run.

Answer 26

environment variables

Question 27

How do you permanently set environment variables?

Answer 27

exporting the variables in an environment file like ~/.bashrc