Shell Variables

Question 1

Shell variables are assigned using an..?

Answer 1

A=apple syntax

Question 2

Variables are examined (“dereferenced”) with the …?

Answer 2

$ character, as in echo $A

Question 3

At the kernel level, every process has a collection of environment variables, which are inherited by …?

Answer 3

child processes.

Question 4

The export command converts a shell variable into an…?

Answer 4

environment variable.

Question 5

The set and env commands list…?

Answer 5

shell variables and environment variables, respectively.

Question 6

The bash shell allows users to …?

Answer 6

set and reference shell variables.

Question 7

A shell variable is simply a …?

Answer 7

named value that the shell remembers.

Question 8

Shell variables can be used in…?

Answer 8

commands and shell scripts and can also be referenced by programs as configuration options.

(For example, the mutt email client runs an external editor when composing a message. By default this editor is vi. However, before running vi it will check to see if a variable called EDITOR has been set. If it has, then the command defined by EDITOR is used instead of vi. Most programs that launch external editors work the same way.)

Question 9

There are two types of shell variables…?

Answer 9

local variables and environment variables.

Question 10

A local variable exists only within…?

Answer 10

the shell which it is created.

Question 11

Environment variables are inherited by…?

Answer 11

child shells such as when a graphical terminal is launched after logging in.

Question 12

How to set a local variable…?

Answer 12

A = apple.

(don’t place any spaces on either side of the = sign. Now the shell will “hang on” to this association for as long as the shell exists (or until it is explicitly unset,)

Question 13

Whenever prince would like to use the value “apple”, he can use use the variable A instead, preceding the variable with a…?

Answer 13

dollar sign ($), as in echo $A

This is called dereferencing the variable A.

Question 14

The variable can be used anywhere on the command line (or in shell scripts). What if prince, waxing poetic, decided to write a few lines about apples, which he wanted to save in a file called ode_to_apple.txt. The following line could get him started…..?

Answer 14

[prince@station prince]$ echo “Oh, I like them squishy”&raquo_space; ode_to_$A.txt
[prince@station prince]$ ls
ode_to_apple.txt

When the bash shell examined the command line, it replaced $A with apple. These are the basics of shell variables. Variables are established, and set, with a VAR=value syntax, and dereferenced with a $VAR syntax.

Question 15

What can be used for variable names?

Answer 15

Variable names can be any string of alphanumeric characters (A-Z, a-z, 0-9), and the underscore (_), but cannot start with a number

Question 16

Shell variables are case…?

Answer 16

sensitive.

[prince@station prince]$ B=banana
[prince@station prince]$ echo $B is my favorite fruit
banana is my favorite fruit
[prince@station prince]$ echo $b is my favorite fruit
is my favorite fruit

Question 17

What can be a variable’s value?

Answer 17

Anything

Question 18

When assigning variables, the syntax is name=value, with no spaces. What if prince wanted the variable FRUIT to resolve to the phrase mushy bananas?

Answer 18

[prince@station prince]$ FRUIT=mushy bananas
-bash: bananas: command not found

We have stumbled into an advanced syntax for setting variables, namely name=value command, which sets the variable name only for the execution of the specified command. The bash shell dutifully set the variable FRUIT to the value mushy, and went to execute the command bananas, with expectable results. All of this is not the important bit. The important bit is that if you want to set a variable to a value which contains spaces, you must include the value in quotes.

[prince@station prince]$ FRUIT=”mushy bananas”
[prince@station prince]$ echo $FRUIT is my favorite fruit
mushy bananas is my favorite fruit

With this modification, prince gets the correct behavior from the bash shell, if not correct English grammar.

Question 19

When dereferencing variables, the variable name can be marked using…?

Answer 19

braces {}, if need be.

see https://academy.redhat.com/courses/rha030-6.1/rha030_bash_shvar.html for example.

Question 20

When finished with a variable, the variable may be unbound from its value with the…?

Answer 20

unset command.

[prince@station prince]$ unset A
[prince@station prince]$ echo $A

[prince@station prince]$

Question 21

Bash Read Only Variables:
Variable: ?
Expands to: ??????

Answer 21

The exit status of the most recently executed command.

Question 22

Bash Read Only Variables:
Variable: -
Expands to: ?????

Answer 22

Currently enabled shell option flags

Question 23

Bash Read Only Variables:
Variable: $
Expands to: ?????

Answer 23

Process id (pid) of current shell.

Question 24

Bash Read Only Variables:
Variables: !
Expands to:?????

Answer 24

Process id (pid) of most recent background command.

Question 25

Bash Read Only Variables:
Variables: _
Expands to: ?????

Answer 25

Last token of previous command.

Question 26

Bash Read Only Variables:
Variables: PPID
Expands to: ????

Answer 26

The process id (pid) of the shell’s parent.

Question 27

Bash Read Only Variables:
Variables: SHELLOPTS
Expands to: ?????

Answer 27

Colon separated list of currently enabled shell options, as reported by the set -o command.

Question 28

Bash Read Only Variables:
Variables: UID
Expands to: ???

Answer 28

The userid of the current user.

Question 29

Bash Preassigned Variables:
Variable: BASH_VERSION
Expands to: ??????

Answer 29

The current bash version

Question 30

Bash Preassigned Variables:
Variable: HOSTNAME
Expands to: ?????

Answer 30

The DNS hostname of the current machine.

Question 31

Bash Preassigned Variables:
Variable: OLDPWD
Expands to:????

Answer 31

The previous working directory.

Question 32

Bash Preassigned Variables:
Variable: PWD
Expands to:????

Answer 32

The current working directory.

Question 33

Bash Preassigned Variables:
Variable: RANDOM
Expands to:????

Answer 33

A random number between 0 and 32767

Question 34

Bash Preassigned Variables:
Variable: SECONDS
Expands to:????

Answer 34

The number of seconds since the shell was started.

Question 35

Just as the bash shell allows users to assign name-value pairs called shell variables, the Linux kernel allows any process to….?

Answer 35

define name-value pairs called environment variables.

These variables are a part of the process stored in the kernel, just as the process id, user id, and current working directory are part of the process. More importantly, whenever a process starts another process (such as the bash shell starting the ls command).

Question 36

Environment variables are…?

Answer 36

nherited by the child process. This allows users to use the bash shell to create or modify an environment variable, and then all commands started by the shell will inherit that variable.

Question 37

How do we create environment variables within the bash shell…?

Answer 37

First, a shell variable is created, and then the shell variable is “promoted” to an environment variable using the export command. (The variable will then be exported to any future child processes).

Examples:
https://academy.redhat.com/courses/rha030-6.1/rha030_bash_shvar.html

Question 38

Environment variables are often used to…?

Answer 38

configure commands with information about local configurations, or in other words, information about the local environment. As an example, many commands will look for an environment variable called LANG to determine the user’s language, and modify their output accordingly.

See : https://academy.redhat.com/courses/rha030-6.1/rha030_bash_shvar.html

Question 39

By setting the LANG environment variable to…?

Answer 39

de_DE, the abbreviation for the day “Friday” becomes the customary German abbreviation.

Question 40

By setting LANG to es_ES, the effects are…?

Answer 40

even more obvious, as the day’s and the month’s abbreviations have changed to Spanish (as well as capitalization conventions).

An important point deserves restating. The date command did not change behavior because the bash command had an environment variable called LANG (directly). The process running the date command modified its output because it had its own environment variable called LANG. It just happened to inherit this variable from the bash shell. All processes have environment variables, not just shells.

Why didn’t prince have to explicitly export the variable LANG? The variable is already an environment variable, set by startup scripts. Once a variable is an environment variable, it can be modified (and removed) using the same syntax as shell variables.
Often, users use a shorter syntax to both create and export an environment variable:
[prince@station prince]$ export EDITOR=nano
With this single command, prince has created, assigned, and exported the variable EDITOR.

Question 41

The bash shell provides two commands for listing which variables are defined….?

Answer 41

set and env.

Question 42

The set command, without arguments, lists…?

Answer 42

bothshell variables and environment variables associated with the shell

Question 43

the env command, again without arguments, lists only…?

Answer 43

variables which have been exported to the environment.

Question 44

Commonly Used Environment Variables:
Variable: TERM
Use: ??????

Answer 44

Specifies low level configuration of the user’s terminal. This variable is more relevant when using a serial line console (“dumb terminal”) to access the system.

Question 45

Commonly Used Environment Variables:
Variable: PATH
Use: ??????

Answer 45

Specifies directories to be searched for executable files.

Question 46

Commonly Used Environment Variables:
Variable: DISPLAY
Use: ??????

Answer 46

Specifies which X server clients should use in the graphical environment.

Question 47

Commonly Used Environment Variables:
Variable: LANG
Use: ??????

Answer 47

Specifies the preferred language for internationalized programs.

Question 48

Commonly Used Environment Variables:
Variable: EDITOR
Use: ??????

Answer 48

Many programs rely on an external editor for user input. Often, the default editor is vi. If the EDITOR environment variable is set, the specified editor will be used instead.

Question 49

Commonly Used Environment Variables:
Variable: PRINTER
Use: ??????

Answer 49

Most commands that submit or manage print jobs will examine this environment variable to determine the default printer.