Bash Script – Define Bash Variables and its types

Variables are an important aspect of any programming language. Without variables, you will not be able to store any required data. With the help of variables, data is stored at a particular memory address and then it can be accessed as well as modified when required. In other words, variables let you store, read, access, and manipulate data. 

Working of Bash variables

When you write a bash script, you either want to manipulate the already defined variable, set the value of the variable, or simply read the predefined variables. There are various ways to set values of a variable:

1) Directly assigning the value 

myvar="Gfg"

2) Set its value in accordance with the value of the result obtained from the program or a command (Check command substitution topic below)

If you are assigning the value of a variable to another variable then use the “$” sign otherwise it will show an error:

Note that when we define the value of newvar as myvar without the “$” sign it prints the value as “myvar” rather than getting the value of variable “myvar”.

Output:

Example script:

 #! /bin/bash 
var1=5
Add () {
    var1=$(($var1 + 10))
    echo $var1 

}
Add 
#Assigning the output of function Add to another variable 'var2'.
var2=$var1
echo $var2

Output:

15
15

3) The variable can be read simply by using the $ sign before the variable name. Example:

echo "The value of variable, myvar is $myvar"

Now that we understand how the variables can be read or assigned a value, let us see what happens after we run the bash script:

• It checks for the variables which are defined in the script.
• Interprets code line by line.
• It then replaces the variable names with their assigned values.
• Finally, execute the code line by line by repeating the above process.

Naming variables

There are a few rules to keep in mind while naming a variable:

• Variables can begin with an alphanumeric character or underscore, followed by a letter, number, or underscore.
• Variables are case sensitive, so “gfg” and “Gfg” are two different variables.
• Variables cannot start with a number.
• Do not use special characters while defining a variable name.

Example of rightly defined variable names

gfg
GFG
Gfg
_gfg
g_f_g
March6

Examples of wrongly defined variable names

6March
567
!gfg
@Gfg
gfg*GFg

Syntax to declare the variable or set its value

Declaring or setting the value of a variable is fairly simple. The basic syntax is as follows:

variableName=value

Example:

myvar=geeks

There should not be any space between variable name, equal sign, and the value assigned. Otherwise, you will encounter an error:

If the value of the variable contains spaces then use quotes to define them.

myvar="Geeks for geeks"

You can also declare a variable using declare command:

declare myvar="geeks"

Using Local and global variables in bash

When you define a variable in your shell, it is only accessible during that particular session. Because that is a local variable. In the below example you can see that when we tried to access the variable “var1” in the new session (session number-1692), it does not print our variable value.

Note: echo $$ is used to print the process ID of the current session (See special variables below)

To make the variable globally accessible to any child sessions/processes (of the current session), we can make use of the ‘export’ command. Using the export command you can access the variable defined in your previous session in another session as well. Example:

Let us now see how to define local and global variables while writing a shell script. It is no different than any other programming language.

Create a file var.sh, using the command:

touch var.sh

Open it in vim editor using the command:

vim var.sh

Go to the insert mode by pressing Esc and ‘i’.

In this script, we will define a global variable ‘myvar’ and a local variable ‘myvar’. Then we will try changing the value of the variable ‘myvar’ inside our function. Finally, we try to call the function and print the value of both the local variables as well as the global variables. We will see that only the value of the local variable will be changed.

#! /bin/bash 
myvar=5

function calc(){

# use keyword 'local' to define a
# local variable 
local myvar=5
(( myvar=myvar*5 )) 

# print the value of local variable 
echo $myvar 
}

# call the function calc 
calc 

# print the value of global variable and
# observe that it is unchanged.
echo $myvar 

Save the file using “:wq!”.

And run it using the command:

bash var.sh

The output:

The first value which is printed is of our local variable and the second value is of our global variable which did not change, regardless of the operation we performed inside the function ‘calc’.

Types of bash variables

There are two types of variables in bash:

• The system defined variables or Environment variables
• User-defined variables

System defined variables  

These are predefined variables or the variables which are created and maintained by Linux bash shell. These variables are loaded when you open a new bash session. These variables are used to define system properties. They are defined in capital letters.  Let’s now see some of the system-defined variables:

1. BASH_VERSION
2. BASH
3. PWD
4. OSTYPE
5. HOME
6. LANG
7. HOSTNAME
8. PATH
9. COLUMNS
10. USER

Note: There are more system-defined variables. Above are the common system-defined variables.

You can see the list of system-defined variables using the following commands:

• set
• env
• printenv

The output looks like this:

1. BASH_VERSION: Displays the name of your current bash version.
2. BASH: Displays the shell name.
3. PWD: It stands for “Present working directory”. It displays the path to your present working directory.
4. OSTYPE: Simply displays the name of the operating system.
5. HOME: Displays the home directory of the current user.
6. LANG: Displays the language of the Linux system. Example: LANG: en_IN
7. HOSTNAME: Displays the name of the host. Example: kali
8. PATH: It displays an ordered list of paths (separated by colons). Example:

        9. COLUMNS: Displays the length of the column used to show output on the terminal.

        10. USER: Displays the current user. Example: root

User-defined variables  

These are the variables that you define and assign a value.  

Syntax:

variable_name=value

Example:

myvar=gfg

If the value of the variable contains spaces then use quotes to define them.

myvar="Geeks for geeks"

Example script:

• Create a file named – gfg.sh.

• Open the vim editor and go to the insert mode by pressing Esc and typing ‘i’.
• Write the script:

#! /bin/bash
myvar=Hello
newvar=”welcome to gfg”
echo “$myvar, $newvar”
#variables are case sensitive
Myvar=”Hi again!”
echo “$Myvar”

• Save changes by pressing Esc and “:w”.
• And then quit by pressing Esc and “:q”.
• Run the script by using the command: “bash gfg.sh”

Note that it is recommended (but not required) to define your variable in lowercase. To make sure that the defined variable name doesn’t coincide with the system-defined variable name.  

User-defined variables are deleted after the shell script executes. In contrast, system-defined variables can be accessed by any other application and are not stored in any file.

Built-in special variables

Bash comes with some built-in special variables. These variables store some important data that can come in handy when you write a shell script. They are available for use to all users. Below is the list of these special variables and the result they store:

Special variable- $$

This special variable gives the PID or Process Identifier of the current shell. In the case of a shell script, it gives the PID under which the script is running.

Special variable-$?

Every command returns an exit status. Thus the exit status can be used to identify if the command is terminated successfully or there was some error.

Some common exit statuses:

Using array variables in Bash

Arrays are used to store data and in bash, you can store values of different types or the same type in an array.

There are various ways to declare an array and its elements. In this tutorial, however, we will declare an array by defining elements that are space-separated.

Syntax:

arrayName=(element1 element2 element3 element4)

Note: If your element includes whitespace then enclose that element in quotes.

Syntax to get the length of the array:

{#arrayName[@]}

Syntax to get a particular array element:

{arrayName[index]}

Syntax to get the length of a particular array element:

{#arrayName[index]}

Example script: 

In this example, we will write a script to iterate through the array elements, print their names and respective lengths.

Create a file named “myarray.sh”. Open the file in vim editor. 

#! /bin/bash

# declare an array with pre-defined values 
declare -a my_data=(Learning "Bash variables" from GFG); 

# get length of the array 
arrLength=${#my_data[@]}

# print total number of elements in the array
echo "Total number of elements in array is: $arrLength"

# iterate through the array and print length of 
# each element and their values
echo "Below are the elements and their respective lengths:"
for (( i=0; i<arrLength; i++ ));
do
    echo "Element $((i+1)) is=> '${my_data[$i]}'; and its length is ${#my_data[i]}"
done

# print the whole array at once
echo "All the elements in array : '${my_data[@]}'"

Output:

Explicitly declaring datatype of a variable

In bash, we do not need to declare data types, like any other programming languages (int, str, etc..). The data type is identified by the shell script/shell itself (the variables defined in bash are termed as untyped). But there can be some circumstances when you will want to explicitly define the data type. In such cases, you can utilize the declare command to create a variable with a particular data type. Declare commands offer many flags to explicitly declare data types. Below is the list of those flags:

 

Integer

You can declare an integer using the below command:

declare -i myvar

Now that you defined the data type of variable “myvar” as an integer, if you try assigning its value as a floating number you will get an error.

Notice that if you try to assign the value of “myvar” as a string. It evaluates the string “xyz” as an integer and hence prints 0. Whereas the floating number gives an error.  

Array 

Declaring array using flag ‘-a’.

declare -a myarray

or

declare -a myarray=([element1 element2 element3])

Associative array 

Declare an associative array using flag ‘-A’ 

declare -A newArray=([key1]=value1 [key2]=value2 [key3]=value3)

Print all the values of an array using the command:

 ${newArray[@]}

or

${newArray[*]}

Example:

Print all the keys of an array using the command:

${!newArray[@]}

or

${!newArray[*]}

To print both the key, value pair you can use for loop:

# ! /bin/bash
# declare array
declare -A newArray=([Ron]=10 [John]=30 [Ram]=70 [Shyam]=100)

# iterate through the array
for key in "${!newArray[@]}";
do
    echo "$key scored : ${newArray[$key]} in Math test";
done

Output:

Command-line arguments

Command-line arguments or positional parameters are used to pass input to a shell script or a command. Using command-line arguments makes the script more dynamic.

Syntax:

bash script.sh argument1 argument2 argument3 argument4 .......

You can pass in the special variables described above as an argument to your shell script.

Example:

Create a script named “example1.sh”. 

#! /bin/bash 
# declare a variable which will store all
# the values of our arguments, in doing so
# we will make use of the special variable 
# "$@". Alternatively, "$*" can also be used. 

myvar=("$@")

# Lets store the length of the number of arguments 
# (currently unknown) in another variable.
# Here we can make use of the special variable '$#' 

le="$#"
echo $le
# let us now print the arguments that user 
# passed by looping through the length of
# the number of arguments

for (( i=0; i<le; i++ ))
do
    echo "Argument $((i+1)) is => ${myvar[i]}"
done

In the above script, we made use of special variables “$@” and “$#”. 

Let us now try to make use of other special variables. 

Create another script “example2.sh”

#! /bin/bash 

echo "The process ID of current shell is: $$" 
echo "The exit status of last executed command was: $?"
echo "The name of this script is: $0"
echo "First argument passed to this script is: $1"
echo "Second argument passed to this script is: $2"
echo "Total number of arguments passed to this script is: $#"
echo "His full name is: $1 $2"

You can notice that using command line arguments helps you reduce the number of variables to be defined in a script.

Command substitution 

Sometimes, while writing a shell script you will encounter some instances when you want to assign the output of a particular command to a variable. This can be done with the help of command substitution.  Command substitution typically works by running a particular shell command and storing its result in a variable for further use or to display.

Syntax:

varname=`command-name`
Example: myvar=`ls`

or

varname=$(command-name)
Example: myvar=$(ls)

In the above examples, the output of the command: ‘ls’ will be stored in the variable ‘myvar’. Let’s now see the output we get after we print the variable ‘myvar’:

It is evident from the results of the first terminal (i.e by simply running the  ‘ls’ command) that the output is extended to multiple lines. Whereas if we use command substitution the output is printed in a single line (space separated) .

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