Check If Bash String Starts with Some Value [4 Methods]

While working with Bash scripts, sometimes you may need to check if a string starts with a specific value, character, or substring in Bash. It plays a crucial role in data processing or user input validation. You can use the following 4 methods to check if a string starts with a specific value, substring, or character:

  1. Using wildcard (*) operator: [[ "$string" =~ ^"$prefix" ]]
  2. Using “=~” operator: [[ $string =~ ^"$prefix" ]]
  3. Using the “grep” command: "$string" | grep -q "^$prefix"
  4. Using the “case” statement: case $string in

"$prefix"*)

4 Methods to Check If a Bash String Starts with Some Value

You can determine if a string begins with a particular value, character, or substring using various methods such as the wildcard operator, regex operator, “grep” command, or even a “case” statement. This involves comparing the string with a specified prefix, which can be a character, a specific value, or a substring. In this section, I’ll deep dive into the aforementioned 4 methods to check if a starts with some value.

1. Using Wildcard (*) Operator

Bash provides a wildcard * character which can be used to check if a string starts with a specific value. The wildcard operator, when placed after a prefix, permits any character to follow the prefix while comparing it with the string. Follow the script below to check if a string starts with a specific value:

#!/bin/bash

#Define the main string
string="LinuxSimply"
# Print the string
echo "The string: $string"
printf "\n"

# Check if the string starts with a character
prefix="L"
echo "The checking character: $prefix"
if [[ $string == "$prefix"* ]]; then
echo "The string starts with 'L'"
else
echo "The string doesn't start with 'L'"
fi
printf "\n"

# Check if the string starts with a substring
prefix="Linux"
echo "The checking substring: Linux"
if [[ $string == "$prefix"* ]]; then
echo "The string starts with 'Linux'"
else
echo "The string doesn't start with 'Linux'"
fi
printf "\n"

# Check if the string starts with a number
prefix="2"
echo "The checking number: $prefix"
if [[ $string == "$prefix"* ]]; then
echo "The string starts with '2'"
else
echo "The string doesn't start with '2'"
fi
EXPLANATION
In the initial part of the string, a string variable is assigned the value “LinuxSimply”. After that, the script prints the string variable.

Then, the script checks whether the string starts with a specific character. It sets the variable prefix to "L" and checks if the string begins with this character. The output of this check is then displayed. Following this, the script repeats a similar process but with the substring "Linux". It evaluates if the string begins with the particular substring and shows the result accordingly.

The final segment of the script checks if the starts with a numeric value. The variable prefix is set to "2", and the script checks whether the string starts with this numeric value. Finally, the script prints the result after examining the string.

Bash check if string starts with some value using wildcard.The output displays the outcomes of three scenarios of checking if the string starts with some value (character, substring, and numerical value) using the “wildcard” operator.

2. Using “=~” Operator

You can use the =~ operator with the caret ^ symbol in Bash to check if a string starts with a specific value using the syntax [[ $string =~ ^"$prefix" ]]. Specifically, this operator checks if the string starts with a specific value. The ^ symbol in the regular expression anchors the pattern to the start of the string, ensuring that the match occurs only at the beginning. Follow the provided script to check if a string starts with a specific value in Bash:

#!/bin/bash

#Define the main string
string="#LinuxSimply"

# Print the string
echo "The string: $string"
printf "\n"

# Check if the string starts with a character
prefix="#"
echo "The checking character: #"
if [[ $string =~ ^"$prefix" ]]; then
echo "The string starts with '#'"
else
echo "The string doesn't start with '#'"
fi
printf "\n"

# Check if the string starts with a substring
prefix="#Linux"
echo "The checking substring: #Linux"
if [[ $string =~ ^"$prefix" ]]; then
echo "The string starts with '#Linux'"
else
echo "The string doesn't start with '#Linux'"
fi
printf "\n"
# Check if the string starts with a number
prefix="2"
echo "The checking number: 2"
if [[ $string =~ ^"$prefix" ]]; then
echo "The string starts with '2'"
else
echo "The string doesn't start with '2'"
fi

EXPLANATION
This Bash script analyzes the string "#LinuxSimply" to check whether it starts with a specific character #, a substring #Linux, and a number 2. Code part [[ $string =~ ^"$prefix" ]] checks if the value of the string starts with the specified character or substring stored in the variable prefix. Here, the caret ^ indicates the matching position at the beginning of the string.

Checking if string stars with some value using "=~" operator.The output displays the result of checking if the string starts with a character, a substring, and a numerical value respectively.

3. Using the “grep” Command

In Bash, the grep command with the -q option is used to check if the string starts with the specified value. The caret ^ in the regex pattern indicates the beginning of the string line. Follow the script below to check three scenarios:

#!/bin/bash

#Define the main string
string="11#LinuxSimply"

# Print the string
echo "The string: $string"
printf "\n"

# Check if the string starts with a character
prefix="#"
echo "The checking character: #"
if echo "$string" | grep -q "^$prefix"; then
echo "The string starts with '#'"
else
echo "The string doesn't start with '#'"
fi
printf "\n"

# Check if the string starts with a substring
prefix="#Linux"
echo "The checking substring: #Linux"
if echo "$string" | grep -q "^$prefix"; then
echo "The string starts with '#Linux'"
else
echo "The string doesn't start with '#Linux'"
fi
printf "\n"

# Check if the string starts with a number
prefix="11"
echo "The checking number: 11"
if echo "$string" | grep -q "^$prefix"; then
echo "The string starts with '11'"
else
echo "The string doesn't start with '11'"
fi

EXPLANATION
In this Bash script, the variable string is set to "11#LinuxSimply". The script then checks various conditions related to the beginning of the string. It uses the grep command with the -q option to determine if the string starts with a specific character #, a substring #Linux, and a number 11.

Checking if string starts with some value using "grep" command.The output shows if the string begins with a character, a substring, and a numerical value, respectively using the “grep” command.

4. Using the “case” Statement

You can use a case statement to compare the string with the specific prefix to check if the string starts with that prefix. Follow the script below to get an example of that:

#!/bin/bash

#Define the main string
string="#LinuxSimply"

# Print the string
echo "The string: $string"
printf "\n"


# Check if the string starts with a character
Prefix="#"
echo "The checking character: #"
case $string in
"$prefix"* )echo "The string starts with '#'"
;;
* )
echo "The string doesn't start with '#'"
;;
esac
printf "\n"

# Check if the string starts with a substring
Prefix="#Linux"
echo "The checking substring: #Linux"
case $string in
"$prefix"*)
echo "The string starts with '#Linux'"
;;
* )
echo "The string doesn't start with '#Linux'"
;;
esac
printf "\n"

# Check if the string starts with a number
prefix="2"
echo "The checking number: 2"
case $string in
"$prefix"* )
echo "The string starts with '2'"
;;
* )
echo "The string doesn't start with '2'"
;;
esac

EXPLANATION
In the Bash script provided, the string variable string is set to #LinuxSimply. Using a case statement, the script checks if the string begins with a specific character #, a substring #Linux, and a number 2.

The output shows the result of checking if the string starts with a character, a substring, and a number respectively.

Note: So far, the 4 methods discussed in this article, all check if a string starts with some value in a case-sensitive manner.

Checking if string begins with some value using case statement.In the upcoming section, I’ll explain how to check if a string starts with a certain value in a case-insensitive manner.

Case-Insensitive Check If Bash String Starts with Value

To conduct a case-insensitive check for a string’s initial match with a specific prefix, convert both the string and the prefix to the same case, either lowercase or uppercase, before performing the comparison. The script provided below demonstrates an approach, which converts the string and prefix to lowercase before the comparison operation. Follow the script below:

#!/bin/bash
#define the string and a prefix
string="LinuxSimply"
prefix="LiNuX"

#Print the string and the prefix
echo "The string: $string"
echo "The prefix to match: $prefix"
printf "\n"

# Convert both the string and prefix to lowercase for case-insensitive comparison
string_lower=$(echo "$string" | tr '[:upper:]' '[:lower:]')
prefix_lower=$(echo "$prefix" | tr '[:upper:]' '[:lower:]')


# Check if the lowercase string starts with the lowercase prefix
if [[ "$string_lower" == "$prefix_lower"* ]]; then
echo "The string starts with "$prefix" in a case-insensitive manner."
else
echo "The string does not start with the specified value in a case-insensitive manner."
fi
EXPLANATION
The script compares whether the string "LinuxSimply" starts with the prefix "LiNuX" in a case-insensitive manner. It achieves this by converting both the string and prefix to lowercase using the tr command. The [[ "$string_lower" == "$prefix_lower"* ]] condition checks if the lowercase string begins with the lowercase prefix, and based on the result, it prints an appropriate message.

Bash check if string starts with some value in case insensitive manner.The output reveals the outcome of verifying whether the string initiates with the specified prefix in a manner that ignores case sensitivity.

Conclusion

Checking if a string starts with a specific value in Bash is a straightforward yet essential task. Whether you opt for the above approach, understanding these techniques will enable you to handle string manipulations efficiently. This article explores four techniques for checking if a string starts with a specific prefix, primarily focusing on case-sensitive methods. Additionally, a case-insensitive approach is elaborated for added convenience. It is anticipated that this article will enhance your understanding and proficiency in this aspect.

People Also Ask

How to check if a string starts with either of two specific patterns.

To check if a string starts with either of two specific patterns in Bash, you can use a regular expression with the =~ operator. In the following example,  the condition [[ "$string" =~ ^(pattern1|pattern2) ]] checks whether the value of the string environment variable matches the specified regular expression patterns (pattern1 or pattern2).

#!/bin/bash

#Define string
string="user123"

#Defining the matching patterns
pattern1="user"
pattern2="123"

if [[ "$string" =~ ^(pattern1|pattern2) ]]; then
echo "The string starts with either pattern1 or pattern2."
elseecho "The string does not start with either pattern1 or pattern2."
fi

#Degfine the string
string="123user"
if [[ "$string" =~ ^(pattern1|pattern2) ]]; thenecho "The string starts with either pattern1 or pattern2."
else
echo "The string does not start with either pattern1 or pattern2."
fi

##Output
The string starts with either pattern1 or pattern2.
The string starts with either pattern1 or pattern2.

How to check if a string contains only digits?

To check if a string contains only digits in Bash, you can use a combination of parameter expansion and pattern matching. For example, check the following script:

#!/bin/bash

#Define the string
string="12345"

#Check if the string contains only number
if [[ "$string" =~ ^[0-9]+$ ]]; then
echo "The string contains only digits."
else
echo "The string does not contain only digits."
fi

##Output
The string contains only digits.

What is RegEx in Linux?

In Linux, RegEx (Regular Expression) refers to a powerful and flexible sequence of characters that define a search pattern. Regular expressions are used in various Linux commands and utilities to perform pattern matching, searching, and text manipulation tasks.

How do you check strings equal in Bash?

In Bash, you can check if two strings are equal in various approaches. The simplest one is using the == operator within the [[ ... ]] construct. Here’s an example:

#!/bin/bash

#Defin the stirng
string1="Hello"
string2="World"

#Check strings equality
if [[ "$string1" == "$string2" ]]; then
echo "The strings are equal."
else
echo "The strings are not equal."
fi

##output
The strings are not equal.

Related Articles


<< Go Back to Check String in Bash | Bash String | Bash Scripting Tutorial

Rate this post
LINUX
FUNDAMENTALS
A Complete Guide for Beginners Enroll Course Now
icon linux
Auhona Islam

Auhona Islam is a dedicated professional with a background in Electronics and Communication Engineering (ECE) from Khulna University of Engineering & Technology. Graduating in 2023, Auhona is currently excelling in her role as a Linux content developer executive at SOFTEKO to provide a more straightforward route for Linux users. She aims to generate compelling materials for Linux users with her knowledge and skills. She holds her enthusiasm in the realm of Machine Learning (ML), Deep Learning (DL), and Artificial Intelligence (AI). Apart from these, she has a passion for playing instruments and singing. Read Full Bio

Leave a Comment