Example | Description |
---|---|
fruit=”apple” | Define a variable named fruit containing the name of a fruit. |
case $fruit in | Start a case statement in Bash with the in keyword. |
“apple”) | Check if the value of fruit matches the pattern “apple”. |
echo “This is an apple.” | If the value of fruit matches the pattern “apple”, execute the echo statement that says “This is an apple.” |
“banana”) | Check if the value of fruit matches the pattern “banana”. |
echo “This is a banana.” | If the value of fruit matches the pattern “banana”, execute the echo statement that says “This is a banana.” |
“orange”) | Check if the value of fruit matches the pattern “orange”. |
echo “This is an orange.” | If the value of fruit matches the pattern “orange”, execute the echo statement that says “This is an orange.” |
*) | Use the * symbol as the default case if none of the patterns match. |
echo “This is not a valid fruit.” | If the value of fruit does not match any of the patterns, execute the echo statement that says “This is not a valid fruit.” |
In the above table, we have provided an example of using a switch statement in Bash to execute different code based on the value of a particular variable. The example defines a variable named fruit
containing the name of a fruit, and uses a switch statement to check the value of fruit
against three patterns (“apple”, “banana”, and “orange”). Since the value of fruit
is “apple”, the code will execute the echo
statement that says “This is an apple.” The table shows each step of the example, including the patterns being checked and the code being executed for each pattern.
Are you a Unix shell programmer looking to execute different code based on a particular value or expression? If yes, then you have come across the need to use switch statements in Bash. In this article, we will provide you with everything you need to know about switch statements in Bash, and how to master them.
What are Bash Switch Statements?
Bash switch statements, also known as case statements, are control statements used in programming languages to execute different code based on the value of a particular variable or expression. The case statement in Bash is used to test a value against a series of patterns or conditions.
The syntax for a case statement in Bash is as follows:
case expression in
pattern1)
# Code to be executed if expression matches pattern1
;;
pattern2)
# Code to be executed if expression matches pattern2
;;
pattern3)
# Code to be executed if expression matches pattern3
;;
*)
# Code to be executed if expression does not match any of the patterns
;;
esac
Here, expression
is the variable or expression being tested, and pattern
is the value being checked against the expression. The ;;
symbol indicates the end of each pattern, and the *
symbol is used as the default case if none of the patterns match.
How to Use Bash Switch Statements
To use a switch statement in Bash, you first need to define the variable or expression you want to test. For instance, let’s say you have a variable named fruit
that contains the name of a fruit. You can use a switch statement to execute different code based on the value of the fruit
variable.
fruit="apple"
case $fruit in
"apple")
echo "This is an apple."
;;
"banana")
echo "This is a banana."
;;
"orange")
echo "This is an orange."
;;
*)
echo "This is not a valid fruit."
;;
esac
In the above example, the code checks the value of the fruit
variable against the three patterns (“apple”, “banana”, and “orange”). Since the value of fruit
is “apple”, the code will execute the echo
statement that says “This is an apple.”
Advanced Bash Switch Statements
Switch statements in Bash can be made more advanced by using regular expressions, ranges, and logical operators. Let’s take a look at some examples:
Using Regular Expressions
You can use regular expressions in Bash switch statements by enclosing the patterns in quotes and using the =~
operator. For example:
fruit="apple"
case $fruit in
"ap*")
echo "This is a fruit starting with 'ap'."
;;
"b[a-z]*")
echo "This is a fruit starting with 'b'."
;;
*)
echo "This is not a valid fruit."
;;
esac
In the above example, the code checks the value of the fruit
variable against the two patterns using regular expressions. The first pattern will match any fruit that starts with “ap”, and the second pattern will match any fruit that starts with “b” and is followed by any letter from “a” to “z”.
Using Ranges
You can use ranges in Bash switch statements by enclosing the patterns in brackets and using the ==
operator. For example:
number=10
case $number in
[0-9])
echo "This is a one-digit number."
;;
[1-9][0-9])
echo "This is a two-digit number."
;;
*)
echo "This is not a valid number."
;;
esac
In the above example, the code checks the value of the number
variable against the two patterns using ranges. The first pattern will match any number between 0 and 9, and the second pattern will match any number between 10 and 99.
Using Logical Operators
You can use logical operators in Bash switch statements by combining multiple patterns using the |
symbol. For example:
fruit="apple"
case $fruit in
"apple" | "banana" | "orange")
echo "This is a fruit."
;;
"carrot" | "potato" | "tomato")
echo "This is a vegetable."
;;
*)
echo "This is not a valid fruit or vegetable."
;;
esac
In the above example, the code checks the value of the fruit
variable against two sets of patterns using logical operators. The first set of patterns will match any fruit (“apple”, “banana”, or “orange”), and the second set of patterns will match any vegetable (“carrot”, “potato”, or “tomato”).
Drawbacks and Limitations of Bash Switch Statements
While Bash switch statements are useful, they have some limitations. They are not as efficient as other programming languages like C or Java, and they cannot use variables as labels. Additionally, they cannot be used for complex conditions or expressions, and they can become difficult to read and maintain if there are too many patterns.
Real-life Example: Converting File Formats with Switch Statements
I recently had to convert a large number of image files from .jpg to .png format for a project. Instead of manually converting each file, I decided to use a Bash script with a switch statement to automate the process.
First, I created a new directory and moved all of my .jpg files into it. Then, I wrote the following script:
#!/bin/bash
for file in /path/to/directory/*.jpg
do
case "$file" in
*.jpg)
filename=$(basename "$file")
extension="${filename##*.}"
filename="${filename%.*}"
convert "$file" "/path/to/new/directory/$filename.png"
;;
*)
echo "Skipping $file, not a .jpg file"
;;
esac
done
Here’s how the switch statement works in this script:
- The
case
statement checks each file in the specified directory and matches it against the pattern*.jpg
. - If the file matches the pattern, the script extracts the filename and extension using the
basename
command and the${}
parameter expansion syntax. - The script then uses the
convert
command to convert the .jpg file to .png format and saves the new file in the specified directory. - If the file doesn’t match the pattern, the script simply skips it and moves on to the next file.
Thanks to this switch statement, I was able to quickly and easily convert all of my .jpg files to .png format with just one script.
Conclusion
Switch statements in Bash are a powerful tool for Unix shell programmers, enabling them to execute different code based on the value of a particular variable or expression. In this article, we have discussed the syntax and usage of switch statements in Bash, as well as advanced techniques such as using regular expressions, ranges, and logical operators. We have also highlighted some drawbacks and limitations of using Bash switch statements. By mastering switch statements in Bash, you can take your Unix shell programming skills to the next level and become a more efficient and effective programmer.
Insider Tips
- Use a switch statement when you need to execute different code based on a particular value or expression.
- Remember to enclose patterns in quotes when using regular expressions and brackets when using ranges.
- Use the
|
symbol to combine multiple patterns using logical operators.- Always include a default case using the
*
symbol to handle situations where none of the patterns match.- Be aware of the limitations of Bash switch statements and use them judiciously.