Using the =~ Operator in Bash
The regular expression is a very useful tool to match any content or search and replace the content of a file or in a string using a regex pattern. It can be used with the Bash script in different ways. The =~ symbol is used in the “if” statement of Bash to search any string. Many types of expression can be used to define the matching regex patterns. Some of the commonly used regular expressions and the uses of some expressions with the =~ operator are explained in this tutorial.
Commonly Used Regular Expressions
| Expression | Purpose |
| . | It is used to search the characters without a newline (\n). |
| ^ | It is used to search for characters at the start of the string. |
| $ | It is used to search for characters at the end of the string. |
| [0-9] | It is used to search any number from the range of 0-9 in the string. |
| [A-Z] | It is used to search for any character from the range of A-Z in the string. |
| [a-z] | It is used to search for any character and number from the range of a-z in the string. |
| [^A-Z0-9] | It is used to search all characters except the capital alphabets and digits in the string. |
| [a-zA-z0-9] | It is used to search for any character and number from the range of a-z, A-Z, and 0-9 in the string. |
| \n | It is used to search for the newline character. |
| \t | It is used to search the tab character. |
Different Examples of =~ Operators
The different ways of searching a particular string inside a text using the =~ operator and regular expression pattern are shown in this part of the tutorial.
Example 1: Search a Particular String Using the “*” Symbol
Create a Bash file with the following script that takes the main string value where the string is searched and the search string value is searched in the main string value. Next, the “=~” operator is used with the search string to check if the search string exists in the main string or not. Here, the “*” symbol is used to denote any number of characters.
#Take the main string
read -p "Enter the main string value: " strValue
#Take the search string
read -p "Enter the search string value: " search
#Check whether the search string exists in the main string or not
if [[ $strValue =~ .*$search.* ]]; then
echo "The string exists in the text."
else
echo "The string doesn't exist in the text."
fi
The following output appears after executing the script with the main string value of “Learn regular expression” and the search string value of “regular”. Here, the search string exists in the main string:
Example 2: Check the Extension of the Particular File
Create a Bash file with the following script that takes the filename from the command-line argument and check whether the file is a Bash file or not.
#Take the filename from the argument
filename=$1
#Define the searching extension value
extension='bash'
#Check if the extension matches with the file extension or not
if [[ "$filename" =~ \.$extension$ ]]; then
echo "$filename is a bash file."
else
echo "$filename is not a bash file."
fi
The following output appears for the “ping1.bash” filename which is a Bash file:
The following output appears for the “hello.txt” filename which is not a Bash file:
Example 3: Search the Particular Characters in a String
Create a Bash file with the following script that takes a string value and search the range of characters from “a” to “e” in the string.
#Take the main string
read -p "Enter the main string value: " strValue
#Check if the string contains any character from a to e or not
if [[ $strValue =~ [a-e] ]]; then
echo "The string contains characters from 'a' to 'e'"
else
echo "The string does not contain any character from 'a' to 'e'"
fi
The following output appears after executing the script with the “LinuxHint” input value:
The following output appears after executing the script with the “Hello World” input value:
Example 4: Validate the Mobile Number
Create a Bash file with the following script that takes a mobile number of the particular format and check whether the number is valid or invalid using the regular expression pattern and the =~ operator.
read -p "Enter a mobile number [880-XXXX-XXXXXX]: " mobile
#Set the pattern for matching
regexPattern='^880-[0-9]{4}-[0-9]{6}'
#Check the mobile number is valid or invalid
if [[ $mobile =~ $regexPattern.* ]]; then
echo "Mobile number is valid."
else
echo "Mobile number is Invalid."
fi
The following output appears after executing the script with the “880-1922-032970” input value which is valid:
The following output appears after executing the script with the “880-15677-67345” input value which is invalid:
Conclusion
The methods of using the “=~” operator to search the string values with different types of regular expressions are shown in this tutorial.
Source: linuxhint.com