Linux Tactic

Simplify String Comparison in Bash with Case Statements

Introduction to Bash Case Statements

In Bash scripts, it is often necessary to check strings against variables. For example, when creating a script that prompts a user to choose between different options, the script must be able to recognize the user’s input and act accordingly.

Traditionally, this has been

done using if/elif statements. While this approach is effective, it can be clunky and take up a lot of space in the script.

Additionally, if the script involves a large number of options, it can become unwieldy to maintain. Enter case statements, a more organized and maintainable way to check strings against variables.

In this article, we will explore the basics of using case statements in Bash, including how to specify options and use wildcards. Disadvantages of using if/elif statements

Before delving into case statements, it is worth considering the disadvantages of using if/elif statements for string checking.

Firstly, if/elif statements can be quite long and take up a lot of space in a script. This can make the code difficult to read and maintain, especially if the script involves a large number of options.

Additionally, if/elif statements can be clunky. Because each option is checked individually, the script may have to execute multiple comparisons before finding a match.

This can be slow and inefficient. Overall, while if/elif statements are effective, they may not be the best choice for scripts that involve a large number of options or require efficient execution.to case statements and their advantages

Case statements are an alternative way to check strings against variables in Bash.

They offer several advantages over if/elif statements, including increased organization and maintainability. In a case statement, the script checks the variable against a series of options.

Each option is defined using the syntax “option1|option2|

option3)”. If the variable matches one of the options, the corresponding code block is executed.

Unlike if/elif statements, a case statement checks each option only once. This makes it more efficient, especially if there are many options to check.

Additionally, case statements are more organized than if/elif statements. Because each option is defined separately, the script is easier to read and maintain.

How to use Case Statements in Bash

Now that we have covered the basics of case statements, let’s explore how to use them in Bash scripts.

Basics of checking against a string

To use a case statement, the script must first set the variable to be checked. This variable is then referenced in the case statement using the “${1}” syntax, where “1” is the order of the argument passed to the script.

For example, suppose we are creating a script that prompts the user to choose between “option1”, “option2”, or “option3”. The script might start with the following code:

“`

#!/bin/bash

CHOICE=${1}

case ${CHOICE} in

option1)

# code block for option1

;;

option2)

# code block for option2

;;

option3)

# code block for option3

;;

*)

# code block for invalid input

;;

esac

“`

In this example, the user’s choice is set to the variable CHOICE, which is then checked against the options in the case statement. The code block corresponding to the user’s choice is executed, and if the input is invalid, the script executes the code block following the asterisk (*).

Specifying options and their order of checking

In a case statement, the options are checked in the order they are listed. If the user’s input matches an option, the script executes the corresponding code block and then exits the case statement.

For example, consider the following code:

“`

case ${CHOICE} in

option1)

# code block for option1

;;

option2)

# code block for option2

;;

option3)

# code block for option3

;;

esac

“`

In this case statement, the script checks for “option1” first, then “option2”, and finally “option3”. If the user’s input matches “option1”, the script executes the code block for “option1” and then exits the case statement.

Using wildcards in option checking

Wildcards can be used in case statements to match part of a string rather than an exact match. The syntax for using wildcards is to enclose the string to be matched in asterisks (*string*).

For example, suppose we want to create a script that responds differently to any input that starts with “option”. The script might use the following code:

“`

case ${CHOICE} in

option*)

# code block for any input starting with “option”

;;

*)

# code block for all other input

;;

esac

“`

In this case statement, the script uses the wildcard *option* to match any string that starts with “option”. If the user’s input matches the pattern, the script executes the code block for “option*” and then exits the case statement.

Ending the case statement with

esac

Every case statement in Bash must be concluded with the keyword

esac, which stands for “end case”. This syntax lets the script know that the case statement has ended and that it can return to normal execution.

For example, consider the following code:

“`

case ${CHOICE} in

option1)

# code block for option1

;;

option2)

# code block for option2

;;

option3)

# code block for option3

;;

*)

# code block for invalid input

;;

esac

“`

In this case statement, each option has its corresponding code block, and the script uses a wildcard for any input that does not match an option. The script terminates with

esac, which ensures that normal execution is resumed.

Conclusion

In conclusion, case statements are a more organized and maintainable way to check strings against variables in Bash scripts. Unlike if/elif statements, case statements are more efficient, especially for larger numbers of options.

Additionally, case statements are more organized, making the script easier to read and maintain. In this article, we covered the basics of using case statements in Bash, including how to specify options, the order of checking, using wildcards, and ending the case statement with

esac. By mastering these techniques, you can create more efficient and maintainable Bash scripts that are a pleasure to read and maintain.

3) Case Statement in Practice – Argument Checks

Now that we understand the basics of using case statements in Bash, let’s examine a practical example. We will create a script that checks arguments passed to it against an options list and performs a corresponding action depending on the given input.

Checking arguments against an options list

Suppose we want to create a script that takes in several arguments and performs different actions depending on the provided options. For example, we might create a script that allows us to manipulate text files in several ways, such as vertically displaying the text, adding text to a file, replacing text in a file, or verifying the contents of a file.

To start, let’s define the list of options and their corresponding actions:

“`

–vertical: displays file contents vertically

–add-txt: appends text to the end of the file

–replace: replaces old text with new text

–verify: confirms the contents of the file

“`

The script will check the arguments provided by the user against this list and execute the corresponding action.

Checking against provided options and setting variables

Our script will start with a case statement that checks for each option. We will use a while loop to iterate through the arguments given by the user and use shift 1 to remove each argument as it is processed.

Here is what our solution looks like:

“`

#!/bin/bash

while (( “$#” )); do

case “$1” in

–vertical)

VERTICAL=true

;;

–add-txt)

ADD_TEXT=true

NEW_TEXT=”$2″

shift 1

;;

–replace)

REPLACE=true

OLD_TEXT=”$2″

NEW_TEXT=”$3″

shift 2

;;

–verify)

VERIFY=true

;;

*)

echo “Invalid argument: $1”

exit 1

esac

shift 1

done

# file manipulation code

“`

In this script, we check each argument against the list of options using case statements. When we find a match, we set a corresponding variable that tells the script to perform the corresponding action.

For example, if the user passes in the –add-txt option, we set the variable ADD_TEXT to true and save the new text provided by the user in the variable NEW_TEXT. We use shift 1 to remove the processed option from the list of arguments.

By the end of the while loop, our script will have processed all arguments provided by the user and set the necessary variables. We can then perform the file manipulation based on the set variables and options.

For example, if the ADD_TEXT variable is true, we can use the following command to append the new text to the file:

“`

echo “$NEW_TEXT” >> file.txt

“`

Similarly, if the VERIFY variable is true, we can use the following command to confirm the contents of the file:

“`

cat file.txt

“`

Wrapping Up

In this article, we explored a practical example of using case statements in Bash to check arguments against a list of options and perform specific actions depending on the provided input. By utilizing case statements, we were able to create a more efficient and maintainable script that checks each argument only once.

Additionally, the script is more legible and easier to maintain, making it a valuable tool when working with Bash scripts. Furthermore, we highly recommend learning Bash from scratch if you plan to work with Bash scripts frequently.

There are many Bash Beginner Series available online that can teach you the fundamentals of Bash programming and equip you with the tools you need to create efficient and maintainable scripts. In conclusion, case statements provide an organized and maintainable way of checking strings against variables in Bash scripts.

They are more efficient than if/elif statements, especially when dealing with a large number of options. By setting variables for each option, you can quickly perform corresponding actions based on user inputs.

If you plan to work with Bash scripts frequently, it is essential to learn Bash from scratch. Creating efficient and maintainable scripts will save you time and simplify your work.

Remember that case statements are a valuable tool, and using them can streamline your coding process.

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