Variable Substitution in Bash [Replace Character & Substring]

Bash programmers do the replacement, the substitution, and the removal of the bash variable too often. It helps them print necessary text on the terminal, customize a command with the necessary option to get their necessary output, replace unnecessary words from a text, concatenate two or more variables and print, and do arithmetic operations. In this article, I will demonstrate to you these types of tasks which will help you to get a holistic idea of this topic.

Free Downloads to work with Variable Substitution in Bash:

Syntax to Substitute, Replace, Remove Variable and Substring

1. To substitute the Bash variable:

   echo "String, $variable_name!"

2. To replace substring from string:

   new_string="${original/old_substring/new_substring}"

3. To remove a substring from a variable:

   result="${original_string/$substring_to_remove/}"

Variable Substitution in Bash

Bash variable substitution is a process where the value of a variable substitutes or replaces another variable. The $(..) syntax is commonly used for variable substitution. I have listed some examples related to this topic below. The examples are simple variable substitution, concatenation of string using variable substitution, arithmetic substitution using arithmetic expansion, and command substitution by creating customized commands.

Example 01: Bash Variable Substitution

Here, I will develop a Bash script where a text will be printed where the value of a variable will be substituted. To know more, follow the below steps.

Steps to Follow >

1. At first, launch an Ubuntu Terminal.
2. Write the following command to open a file in Nano:

   nano script1.sh

   EXPLANATION

   • nano: Opens the nano text editor.
   • script1.sh: Bash script name.
3. Copy the script mentioned below:

   !#/bin/bash
   
   #assigning the value of the name variable
   name="John"
   
   #substituting name variable
   echo "Hello, $name!"

   EXPLANATION

   #! /bin/bash ‘#!’, is called shebang or hashbang. It indicates the interpreter to be used for executing the script, in this case, it’s bash. After that, the name=”John” command has assigned value to the name variable. Finally, the echo “Hello, $name!” command has printed a text on the terminal where the name variable is replaced by the assigned value.

4. Press CTRL+O and ENTER to save the file; CTRL+X to exit.
5. Use the following command to make the file executable:

   chmod +x script1.sh

   EXPLANATION

   • chmod: Changes the permissions of files and directories.
   • +x: Argument with the chmod command to add the executable permission.
   • script1.sh: File that you want to make executable.
6. Run the script by the following command:

   ./script1.sh

   The image shows that the bash script has printed a text on the terminal where the name variable is substituted by its assigned value.

Example 02: Concatenation of String Utilizing Variable Substitution

Here, I will develop a bash script that will concatenate two variables utilizing variable substitution. To know more, follow the below script.

Script (script2.sh) >

#/bin/bash

#assigning first_name and last_name variable
first_name="John"
last_name="Doe"

#concatenating two variables utilizing variable substitution
full_name="$first_name $last_name"

#printing the concatenated result
echo "Full Name: $full_name"

Run the script by executing the following command

./script2.sh

The Bash script has concatenated two variables using variable substitution and printed on the terminal. Here John and Doe from the output line come from the first_name and the last_name variable respectively. After concatenation, I got Jogn Doe on the terminal.

Example 03: Arithmetic Substitution Using Arithmetic Expansion

In this example, I will develop a bash script that will do a mathematical addition utilizing the arithmetic substitution. To know more, follow the below script.

Script (script3.sh) >

#!/bin/bash

#assigning value to the num1 and the num2 variable
num1=5
num2=3

#arithmetic Substitution
result=$((num1 + num2))

#printing the result
echo "Sum of $num1 and $num2:$result"

Run the script by executing the following command.

./script3.sh

The bash script has calculated the summation of the num1 and the num2 variables utilizing the arithmetic substitution method.

Example 04: Command Substitution by Creating Customized Command

Here, I will create a customized command and then print the output of the customized command on the terminal. To know more, follow the below script.

Script (script4.sh) >

#!/bin/bash

#creating a customized date
current_date=$(date +%Y-%m-%d)

#printing the output of the customized command
echo "Current Date: $current_date"

Run the script by executing the following command

./script4.sh

The bash script has created a customized date command and printed the output of that command on the terminal.

Replacing Substring in a Variable

Bash variable replacement is a process where the value of a variable replaces or variable. I have listed some examples related to this topic below.

Example 01: Replace a Single Occurrence Using Single Slash (/) Operator

In this example, I will develop a bash script that will look for a match of a specific substring for a single time and will replace it with another substring. To know more, follow the below script.

Script (script5.sh) >

#!/bin/bash

#original variable is assigned
original="Hello, World!"

#”World” will be replaced by “Everyone” from the original variable and kept on the new_string
new_string="${original/World/Everyone}"

#new_string will be printed
echo "$new_string"

Run the script by executing the following command

./script5.sh

The image shows that the bash script has replaced the “World” with “Everyone” and printed the updated string on the terminal.

Example 02: Replace All Occurrences Using Double Slash (//) Operator

Following the previous method, I will develop another bash script in a different way that will look for a match of a specific substring and replace it with another substring. This is just another approach to using a single slash(/). To know more, follow the below script.

Script (script6.sh) >

#!/bin/bash

#string assigning
original="She sells seashells by the seashore. She shells seashells for sure."

#sh will be replaced by SH
new_string="${original//sh/SH}"

#printing the result
echo "$new_string"

Run the script by executing the following command.

./script6.sh

The image shows that the Bash script has replaced the character “SH” with the character “sh” and printed the new_string on the terminal.

Example 03: Replace One Character with Another Using tr Command

One might use one character in place of another character. In such a situation, the programmer can replace one character with another using the tr command. To know more, follow the below script.

Script (script11.sh) >

#!/bin/bash

#assigning main string
Main_string="Linux Simply"

#replacing n by p and save it to New_string
export New_string=$(echo "$Main_string" | tr 'n' 'p')

#printing the value of Main_string
echo "Main_string:" $Main_string

#printing the value of Modified_string
echo "Modified_string:" $New_string

Run the script by executing the following command.

./script11.sh

The image shows that the Bash script has replaced the n character with the p character with the help of the tr command.

An alternative to using the tr command is parameter expansion. The syntax for parameter expansion is:

New_string="${Main_string//(replaced character)/(new character)}"

Removing Substring from a Variable

Bash variable removal is a process where some words or sub-strings from the main string is deleted. I have listed some examples related to this topic below.

Example 01: Using Single Slash (/) Operator to Remove a Specific Substring From a Given String

In this example, I will develop a Bash script that will remove a substring from a sentence utilizing the single slash(/). To know more, follow the below script.

Script (script7.sh) >

#!/bin/bash

# Original sentence
sentence="Hello, world! This is an example."

# Substring to remove
substring="world"

# Remove substring using parameter expansion
result="${sentence/$substring/}"

# Print the result
echo "Original Sentence: $sentence"
echo "Substring to Remove: $substring"
echo "Result after removal: $result"

Run the script by executing the following command.

./script7.sh

The Bash script has replaced the sub-string from the main string by utilizing the single slash(/) and printed the new string.

Example 02: Using Double Slash(//) Operator to Remove a Specific Substring from a Given String

Here, I will develop a Bash script that will remove a substring from a sentence utilizing the double slash(//). This is an alternative approach of the single slash(/) approach. To know more, follow the below script.

Script (script8.sh) >

#!/bin/bash

#original and substring_to_remove string assigned
original_string="Hello, world!"
substring_to_remove="world"

#sub-string will be removed from the original string and kept on the result
result="${original_string//$substring_to_remove/}"

#result will be printed
echo "$result"

Run the script by executing the following command.

./script8.sh

The Bash script has replaced the sub-string from the main string with the utilizing the double slash(//) and printed the new string.

Example 03: Using sed (Stream Editor) Command to Remove Substring

Here, I will develop a Bash script that will remove a substring from a original string utilizing the sed command. To know more, follow the below script.

Script (script9.sh) >

#!/bin/bash

#value assigning
original_string="Hello, world!"
substring_to_remove="world"

#removing substring_to_remove from original_string using sed command
result=$(echo "$original_string" | sed "s/$substring_to_remove//g")

#printing the result on the terminal
echo "$result"

Run the script by executing the following command.

./script9.sh

The Bash script has removed the sub-string “world” from the main string with the help of the sed command and printed the new string Hello on the command line.

Example 04: Using the awk Command to Remove Substring

In this example, I will develop a Bash script that will remove a substring from an original string utilizing the awk command. To know more, follow the below script.

Script (script10.sh) >

#!/bin/bash

# Remove "world" from the variable
original_string="Hello, world!"
substring_to_remove="world"

#removing substring_to_remove from original_string using awk command
result=$(echo "$original_string" | awk -v var="$substring_to_remove" '{gsub(var,"")}1')

#printing the result on the terminal
echo "$result"

Run the script by executing the following command.

./script10.sh

The Bash script has removed the sub-string from the main string with the help of the awk command and printed the new string.

Conclusion

In this article, I have tried to explore the full process of bash variable substitution and replace characters. I believe, after going through this article, you will be productive enough to remove or a sub-string from the main string, customize a command, or replace a variable by its value.

People Also Ask

Which expression is used in bash for command substitution?

The $(..) syntax is the basic syntax for command substitution where the command to be executed is enclosed between the parentheses. This form is the recommended syntax of command substitution.

Can you use substitution with 3 variables?

Yes, you can use substitution with 3 variables. Concatenating strings from 3 variables which is the basic demonstration of the usage of variable substitution with three variables.

Are bash variables case-sensitive?

Yes, bash variables are case-sensitive. Variables in Bash Scripts are untyped and declared on the definition. Bash also supports some basic type declaration using the declare option, and bash variables are case-sensitive.

Can Bash variables have numbers?

Yes, Bash variables can have numbers. A variable in Bash can contain a number, a character, or a string of characters. Variable names can contain uppercase, and lowercase letters, numbers, underscores, and digits.

How do I replace a character in a variable in bash?

The basic syntax for replacing any character in a variable in bash is: ${original/substring_to_replaced/new_substring} Here, the original is the main string, substring_to_replaced is the substring to be replaced, and finally the new_substring is the new substring which will be in place of substring_to_replaced.

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