How to Use Bash Function

Introduction

Bash functions are reusable blocks of commands that enhance script readability and prevent task duplication by allowing other elements to reference them. A function in Bash allows you to group commands that perform a specific task. You can call and execute functions by referencing their names in any part of a script or a Bash shell environment.

This article explains how to use functions in Bash to efficiently write reusable commands in scripts.

The Bash Function Syntax

You can define a function in Bash using two formats depending on your scripting style. You can use the function name and parenthesis () or the function keyword followed by a function name as detailed below.

• Bash function syntax with parenthesis ().

  function_name () {
      #commands-to-run
  }

  Inline parenthesis format:

  function_name () { #commands-to-run; }

• Bash function syntax with the function keyword.

  function function_name {
      #commands-to-run
  }

  Inline function format:

  function function_name { #commands-to-run; }

Create Bash Functions

You can create a function in a Bash shell environment or a script and reuse it by calling its name. Follow the steps below to create Bash functions using the parenthesis and function keyword methods.

1. Create a new inline greet function using the parenthesis method in a shell environment.

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   $ greet () { echo "Hello World! Greetings from Vultr"; }; greet

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   Output:

   Hello World! Greetings from Vultr

2. Create an inline greet function using the function keyword method.

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   $ function greet () { echo "Hello World! Greetings from Vultr"; }; greet

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   Output:

   Hello World! Greetings from Vultr

3. Create a new greet.sh script.

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   $ nano greet.sh

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4. Add the following contents to the greet.sh file.

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   #!/bin/bash
   
   function greet () {
      echo 'Hello World! This is a function'
   }
   
   greet

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   Save and close the file.

   In the above script, the greet function outputs the Hello World! This is a function message using the echo command when the script calls the greet function.

5. Run the script using Bash.

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   $ bash greet.sh

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   Output:

   Hello World! This is a function

Call Bash Functions

Calling functions invokes the commands or code within them. You can call a function in Bash using its name and include optional arguments to invoke specific commands within the function. You can pass the following arguments when calling functions in Bash:

• $0: References the function name.
• $1, $2, etc: Refers to positional function parameters.
• ${!variable}: Accesses the value of variable with indirect referencing.
• $#: Stores the number of positional parameters to pass to a function.
• $*: Expands all positional parameters into a single string separated by spaces.
• $@: Expands positional parameters as separate quoted strings.

Follow the steps below to create, call, and pass function arguments to invoke specific commands.

1. Create a new call-functions.sh script.

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   $ nano call-functions.sh

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2. Add the following contents to the call-functions.sh file.

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   #!/bin/bash
   
   greet="Hello World!"
   
   function greeting () {
     echo $greet
   }
   
   echo "Calling the function"
   
   greeting

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   Save and close the file.

   In the above script, the greeting function imports the greet variable value and outputs it using the echo command when the script calls the function.

3. Run the script using Bash.

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   $ bash call-functions.sh

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   Output:

   Calling the function
   Hello World!

4. Create another call-function-args.sh script.

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   $ nano call-function-args.sh

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5. Add the following contents to the call-function-args.sh file.

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   #!/bin/bash
   
   function calculate() {
     local a=$1  
     local b=$2 
     local sum=$((a + b)) 
     echo "The sum of $a and $b is: $sum"  
   }
   
   calculate 5 10

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   Save and close the file.

   In the above code, the script calls the calculate function using the $1 and $2 arguments to forward the input result as values to the local a and b variables. The sum variable stores the sum of the a and b variables while the echo command outputs the final result.

6. Run the script using Bash.

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   $ bash call-function-args.sh 

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   Output:

   The sum of 5 and 10 is: 15

Return Status Using Functions

Unlike other programming languages, Bash functions do not return status codes automatically. A function returns the status of the last command by default, 0 for success, or a non-zero number between 1 to 255 for failure. Follow the steps below to use the return statement to terminate a function and display a specific custom status codes.

1. Create a new return-function.sh script.

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   $ nano return-function.sh

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2. Add the following contents to the return-function.sh file.

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   #!/bin/bash
   
   function greetings() {
     echo "Hello World!"
     return 200
   }
   
   greetings
   func_status=$?
   echo "The Return status is: $func_status"

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   Save and close the file.

   In the above code, the script calls the greetings function to output a Hello World message, passes the status code to the $? variable, and returns a non-zero 200 status code.

3. Run the script using Bash.

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   $ bash return-function.sh

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   Output:

   Hello World!
   The Return status is: 200

Exit Command in Bash Functions

The exit command terminates an entire function and returns the exit status code. exit is similar to return and allows exiting a function in case of any errors or unmet conditions to stop all additional executions. Follow the steps below to use the exit command to return the exit status in a function.

1. Create a new exit-function.sh script.

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   $ nano exit-function.sh

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2. Add the following contents to the exit-function.sh file.

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   #!/bin/bash
   
   function greetings() {
     echo "Hello World!"
     exit 2
     echo "This can't run, the script has exited"
   }
   
   greetings

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   Save and close the file.

   In the above script, the exit command terminates the function from executing any further tasks and returns a non-zero exit code of 2.

3. Run the script using Bash.

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   $ bash exit-function.sh

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   Output:

   Hello World!

Use Variables with Functions

Variables in Bash store specific data that you can use with functions to hold repeated values or manipulate data. Functions support global and local variables:

• Global Variables: Declared outside a function body and are accessible in any part of a script.
• Local Variables: Declared inside a function using the local keyword and are only accessible inside the function.

Follow the steps below to create and reference variables in functions.

1. Create a new var-function.sh script.

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   $ nano var-function.sh

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2. Add the following contents to the var-function.sh file.

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   #!/bin/bash
   
   greeting="Hello" 
   read username
   
   function greetings() {
     local name="$1"  
     echo "$greeting, $name!"  
   }
   
   greetings $username

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   Save and close the file.

   In the above script, the greetings function uses a local name variable and imports the global variable greeting. The function inputs user data from the username variable using the read command value as an argument. The echo command combines the variables into a single result and displays a message when the script calls the function.

3. Run the script using Bash.

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   $ bash var-function.sh 

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   Enter a name such as John Doe when prompted to test the function in Bash.

   John Doe
   Hello, John!

Use Bash Functions with Loops

You can use Bash functions with loops to continuously execute commands or tasks over several times. Bash functions work with for, while, and until loops to execute repetitive tasks within or outside a function. Follow the steps below to use loops within and outside a Bash function to execute repeated tasks.

1. Create a new loop-in-function.sh script to use internal loops.

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   $ nano loop-in-function.sh

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2. Add the following contents to the loop-in-function.sh file.

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   #!/bin/bash
   
   function count() {
     local compare="$1" 
     local var=0  
   
     for ((var=0; var<compare; var++)); do
       echo "Current value: $var"
     done
   }
   
   count 10

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   Save and close the file.

   In the above script, the count function uses a for loop to continuously increment the value of the var variable and print its current value if it's less than the input argument value of 10 stored in the compare local variable.

3. Run the script using Bash.

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   $ bash loop-in-function.sh 

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   Output:

   Current value: 0
   Current value: 1
   Current value: 2
   Current value: 3
   Current value: 4
   Current value: 5
   Current value: 6
   Current value: 7
   Current value: 8
   Current value: 9

4. Create another loop-out-functions.sh script to use external loops.

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   $ nano loop-out-functions.sh

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5. Add the following contents to the loop-out-functions.sh file.

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   #!/bin/bash
   
   function greetings() {
     local username="$1" 
     echo "Hello, $username!" 
   }
   
   names=("John" "Doe" "Admin" "Jane" "root" ) 
   for username in "${names[@]}"; do
     greetings "$username" 
   done

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   Save and close the file.

   In the above script, the greetings function imports the username variable from the input argument when the external for loop calls the function. The for loop continuously sets a new username value from the names array.

6. Run the script using Bash.

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   $ bash loop-out-functions.sh 

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   Output:

   Hello, John!
   Hello, Doe!
   Hello, Admin!
   Hello, Jane!
   Hello, root!

Use Bash Functions With Conditional Statements

Bash Functions use conditional statements to execute specific commands or tasks when a condition evaluates to true. You can use conditional statements such as if, elif, if-then, and case inside or outside functions to execute specific commands. Follow the steps below to use conditional statements with functions in Bash.

1. Create a new if-elif-in-function.sh script.

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   $ nano if-elif-in-function.sh

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2. Add the following contents to the if-elif-in-function.sh file.

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   #!/bin/bash
   
   function count() {
     read var
     if (( var > 0 )); then
       echo "$var is greater than zero."
     elif (( var < 0 )); then
       echo "$var is less than zero."
     else
       echo "$var is zero."
     fi
   }
   
   count

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   Save and close the file.

   In the above script, the count function imports an argument from the user input using the read command when the script calls the function and assigns it to the local var variable . The if-elif conditional statement evaluates if the var value is greater or less than zero to output a message using the echo command.

3. Run the script using Bash.

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   $ bash if-elif-in-function.sh 

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   Enter a value such as 1 and verify the script's output:

   1
   1 is greater than zero.

4. Create a new case-out-function.sh script.

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   $ nano case-out-function.sh

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5. Add the following contents to the case-out-function.sh file.

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   #!/bin/bash
   
   function help_page() {
     echo "Available options:"
     echo "1. apache"
     echo "2. mysql"
     echo "3. php"
     echo "Please enter a valid command or enter 'help' to view this page"
   }
   
   read -p "Enter a command (apache, mysql, php or help): " user_input
   
   case "$user_input" in
     apache)
       echo "Use dnf install httpd to install the Apache Web server..."
       ;;
     mysql)
       echo "Use dnf install mysql to install the MySQL database..."
       ;;
     php)
       echo "Use dnf install php to install PHP..."
       ;;
     help | -h)
       help_page
       ;;    
     *)
       help_page
       ;;
   esac

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   Save and close the file.

   In the above code, the script calls the help_page function in a case conditional statement when the user input fails to match any pattern to display the help information page.

6. Run the script using Bash.

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   $ bash case-out-function.sh

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   Enter help when prompted to match the function's pattern to display the help page.

   Enter a command (apache, mysql, php or help): 

   Output:

   Enter a command (apache, mysql, php or help): help
   Available options:
   1. apache
   2. mysql
   3. php
   Please enter a valid command or enter 'help' to view this page

Conclusion

You have used functions in Bash to create reusable commands and run tasks. You can use functions with arrays, loops, expressions, and other elements in Bash to enable code reusability. In addition, you can use functions in Bash to automate scripts or run system administration tasks that require reusable commands.