Check If a Variable Exists in Bash Using If Statement

In Bash, the “if else” statement is used to control the flow of execution of the bash script statements. Within the “if else” conditional statement, a variable is an essential factor to keep the input values that help to run the script based on the variables set. Therefore, checking if a variable exists is an important task in bash scripting. There are different options including -z, -v, -n, -p, and modifiers that assist in accomplishing this task.

This article delves into four methods to check if a variable exists in Bash using the “if else” statement.

What is Variable in Bash?

A variable is a vital component of any programming language. It is basically a storage that stores values temporarily. These values can be any type including integers, floating numbers, strings, or any characters. In bash scripting, you can define a variable by any name you wish for or keep any value. You can access the stored values by prefixing $ using with the variable name in bash. Check the syntax below:

variable_name=value
Let’s see an example of it:

number=10
website_name="Linuxsimply.com"

Here, the number and website_name are the variable names that store integer 10 and a string Linuxsimply.com respectively.

Types of Variables

Different types of variables store different types of data in bash. Primarily, there are three types:

1. User-defined Variables

   The user defines this type of variable to assign a value. Users can assign integers, numbers, strings, or arrays as a variable. For example,

   name="Hello, I am mou"

2. Environment Variables

   Environment variables are predefined by the system. Linux bash shell controls the environment variable. These variables are always written in capital letters. Let’s see an example of an environment variable:

   echo "$USER"

   

   It shows the current user name mou.

3. Special VariablesSpecial variables store predefined values and it helps users to write the bash script easily.Now, see an example of using a special variable:

   #!/bin/bash
   echo "number of arguments:$#"

   Here, the $# special variable represents the number of arguments. You can see the number of arguments 3 in the place of the $# sign.

4 Methods to Check if a Variable Exists in Bash

To check if a variable exists in Bash, there are four approaches utilizing the “if else” statement along with various options. You can use different options like -v, -z, -n, and -p to check the presence of variables. In addition to the “-z” and “-p” options, each method includes two distinct scenarios for checking the existence of a variable in Bash. Depending on the conditions set using the options, the output returns as true or false.

Method 01: Using “-v” Operator

To check if a variable exists in the bash, you can use the -v operator that checks the variable’s existence regardless the variable is empty. To check if a variable exists in Bash, follow the procedure:

1. Open the Ubuntu terminal and create a Bash file with Nano text editor (for example: nano v_opt.sh)
   
2. Now, write the following script in the Bash file.

   #!/bin/bash
   number=10
   if [[ -v number ]]; then
   echo "variable exists."
   else
   echo "variable does not exist."
   fi 

   EXPLANATION

   The -v operator is used to check if a variable exists. Here, variable number=10. If you run the script, it’ll show that the “variable exists.” If the variable was not set, the script would show “variable does not exist.”
   
3. After that, make the script executable. and run it.
   In this image, you can see that the variable exists.

Method 02: Using “-z” Operator

You can check if a variable exists using the -z option within the “if else” statement. The “if else” statement returns true when the variable is empty. It checks whether the variable is empty or not. With the help of -z operator, two distinct conditions can be built to check the existence of integer variables and string variables.

Case 01: Check for Integer Variable

To check the existence of an integer variable, you can use the -z operator within the if else statement. The below script is an example:

#!/bin/bash
number=10
if [ -z "$number" ]; then
    echo "variable does not exist."
else
    echo "variable exists."
fi

Now, run the script.  You can notice that the variable exists.

Case 02: Check for String Variable by Parameter Expansion

To check if a string variable is set or not, you can adopt parameter expansion with the help of the -z operator. It checks If a variable is set but does not give any information about the type of value the variable holds. Now write the following script to check if a string variable is set:

#!/bin/bash
a="Hello,how are you?"

# Check if the variable is set and not empty using parameter expansion
if [ -z "${a+x}" ]; then
    echo "String variable is unset and empty"
else
    echo "String variable is set and not empty"
fi

Now, run the script. As the variable was not empty, the above script executed the else block of the script.

Case 03: Check for Variables Using Modifiers

• Using “+set” Modifier

Using the -z option along with the +set modifier within the “if else” statement aids in finding variables in bash. If you don’t declare the variable in the bash script, it will return true. Check the example below:

#!/bin/bash
x=10
if [ -z "${x+set}" ]; then
    echo "variable x is unset."
else
    echo "variable x is set."
fi

Now, run the script.

As the variable x is declared in the bash script, the “else” block of the script runs and the output shows that the “variable is set”.

• Using “-unset” Modifier

This -unset modifier with the -z option checks if the variable is unset and null. When you declare any variable containing no value, It returns as true. That means it will execute the else block code if you define any variable that contains a value.

#!/bin/bash
x=
if [ -z "${x-unset}" ]; then
    echo "variable x is unset."
else
    echo "variable x is set."
fi

Now, run the script. As the variable x doesn’t hold any value, the “if” block executes and the output shows that the “variable x is unset”.

Method 03: Using “-n” Operator

The -n operator in Bash is utilized to check if a variable exists by verifying whether its length is nonzero. When combined with an “if else” statement, it allows you to create conditional logic based on whether a variable has been defined or not.

Case 01: Checking for an Integer Variable is Set in Bash

Write the following script in your Bash file to check if an integer variable is set or not:

#!/bin/bash
number=10
if [ -n "$number" ]; then
    echo "The variable exists."
else
    echo "The variable does not exist."
fi

Now, run the script.

As the length of the variable  number is nonzero, the “if” block executes and the output shows that “The variable exists.”

Case 02: Check for Environment Variable Using “env” Command.

To check if an environment variable exists in Bash, you can use this env command. This command lists all the environment variables. Hence, the grep command is used along with env to find the defined variable in the environment variable’s list.

Here’s an example script you can write and run:

#!/bin/bash
variable="Hello,How are you?"
if [ -n "$(env | grep -o '^variable=')" ]; then
    echo "variable is set."
else
    echo "variable is unset."
fi

If you run the script, then the output will be like below. As the environment variable is set, the “if” block of the script runs and the output shows that the “variable is set.”

Case 03: Check for Variable Using “printenv” Command

While dealing with a lot of external variables and other methods fail to check the particular environment variable, implementing this method is the best possible option. However, it is limited to environment variables, in that case, check other methods if you want to check other types of variables. To check if a variable is set using printenv command, write the following script in your Bash file:

#!/bin/bash
variable="Hello,How are you?"
if [ -n "$(printenv variable)" ]; then
    echo "variable is set."
else
    echo "variable is unset."
fi

Before running the script, you have to set the variable to the environment variable by writing the following code in your Ubuntu terminal:

export variable="Hello,How are you?"

Now, run the script.

You can see that the environment variable is set in your system.

Case 04: Check for Variable Using “echo” Command in Bash

It’s a beginner-friendly method to verify the presence of a variable. However, if the variable contains values separated by spaces, this method is not appropriate to use here. You can check the following script:

#!/bin/bash
a="Hello"
if [ -n "$(echo $a)" ]; then
    echo "variable exists."
else
    echo "variable does not exist."
fi

Now, run the script.

You can see that the output shows the defined variable a exists.

Case 05: Using the “isset” Function to Check If a Variable is Set

The isset function checks the variable existence and if they are null. When it finds that the defined variable in the bash script and is not null, it returns the “True” as output. Check out the following script:

#!/bin/bash
isset() {
    [ -n "${!1}" ]
}
a="Hello"
if isset a; then
    echo "variable is set."
else
    echo "variable is unset."
fi

Now, run the script. Since the a variable is non-empty, the output shows that the variable is set.

Case 06: Check for Variable Using Modifiers

• Using “+set” Modifier

You can employ modifiers with the -n option too. The +set modifier returns the empty variable as “true”. Otherwise, the script will execute the “False” code block. Now, you can follow the steps given below:

#!/bin/bash
x=10
if [[ -n ${x+set} ]];then
    echo "variable x is set"
else
    echo "variable x is not set"
fi

Now, run the script. After running the script, the output shows that the “variable x is set”.

• Using “-unset” Modifier

Within the “if else” statement, the -n operator with -unset modifier gives the output by checking the string length of the assigned variable. Here, I have explained an example to check if the string variable is empty or not set. Let’s check it.

#!/bin/bash
website="Linuxsimply.com"
if [[ -n ${website-unset} ]];then
    echo "String is set"
else
    echo "String is not set"
fi

Now, run the script. Since I have set the Linuxsimply.com string in the bash script. Therefore, it shows ‘string is set’.

Method 04: Using “declare” Command

To check if a variable exists in Bash, you can use the declare command incorporating the -p option. You can check an example below to know more about it:

#!/bin/bash
a="hello"
if declare -p a &>/dev/null; then
    echo "The variable is set."
else
    echo "The variable is unset."
fi

Now, run the script. The output shows that the variable is set as the variable a defined in the script.

Summary of the Modifiers and Options

Check the following chart of the options and modifiers used above to have a better understanding:

Conclusion

I hope this article helps you understand the whole procedure of how to check a variable’s existence. Here, I have described 4 methods to verify the variable’s existence. In addition, you can employ two modifiers +set and -unset along with -z and -n options to check if a variable exists in bash. Read the article attentively and follow every step to finish your task successfully.

People Also Ask

How to check if a character exists in a string in bash?

You can check it with different methods such as pattern matching or looping through the characters. The common way is to use the grep command within the if else statement. Here is an example script that checks whether ‘o’ exist in ‘Hello, World’

#!/bin/bash

string="Hello, world!"
char="o"

if grep -q "$char" <<< "$string"; then
echo "Character '$char' exists in the string."
else
echo "Character '$char' does not exist in the string."
fi

What is ‘${}’ in bash?

You can utilize this ‘${}’ sign-in parameter expansion method in bash. It allows you to manipulate variables or access specific attributes of variables.

• ${variable} is used to access the value of the variable named variable.
• ${variable:-default} will use the value of variable if it’s set, otherwise, it will use default.
• ${#variable} will return the length of the value stored in variable.
• ${variable%pattern} will remove the shortest match of pattern from the end of variable.
• ${variable/pattern/replacement} will replace the first occurrence of pattern in variable with replacement.

How to read a variable in bash?

You can use the read command to read the variable in bash. read name will take the value entered by the user and store it in name variable. However, this command can read multiple values at once. For example, read first_name last_name will read two values entered by the user and store the values to first_name and last_name variable respectively.

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