Let us see how to pass parameters to a Bash function.

• A shell function is nothing but a set of one or more commands/statements that act as a complete routine.
• Each function must have a unique name.
• Shell functions have their own command line argument or parameters.
• Use shell variable $1, $2,…$n to access argument passed to the function.

Contents

• 1Passing parameters to a Bash function
• 1.1Invoke function
• 2Examples
• 2.1Function shell variables
• 2.1.1How do I display function name?
• 2.1.1.1FUNCNAME in action
• 3Return values
• 3.1Examples
• 3.2How to return custom value from a user-defined function?
• 4Listing functions
• 4.1Show the known functions and their code/definitions
• 4.2How to remove or unset functions

Passing parameters to a Bash function

• The syntax is as follows to create user-defined functions in a shell script:

function_name(){
   command_block
}
## OR ##
function function_name_here(){
   command_line_block
}
## passing parameters to a Bash function ##
my_function_name(){
  arg1=$1
  arg2=$2
  command on $arg1
}

Invoke function

• To invoke the the function use the following syntax:

 my_function_name foo bar

Where,

1. my_function_name = Your function name.
2. foo = Argument # 1 passed to the function (positional parameter # 1).
3. bar = Argument # 2 passed to the function.

Examples

Create a function called fresh.sh:

#!/bin/bash

# write a function
fresh(){
   # t stores $1 argument passed to fresh()
   t=$1
   echo "fresh(): \$0 is $0"
   echo "fresh(): \$1 is $1"
   echo "fresh(): \$t is $t"
   echo "fresh(): total args passed to me $#"
   echo "fresh(): all args (\$@) passed to me -\"$@\""
   echo "fresh(): all args (\$*) passed to me -\"$*\""
}

# invoke the function with "Tomato" argument
echo "**** calling fresh() 1st time ****"
fresh Tomato

# invoke the function with total 3 arguments
echo "**** calling fresh() 2nd time ****"
fresh Tomato Onion Paneer

Save and close the file. Run it as follows:

chmod +x fresh.sh
./fresh.sh

Sample outputs:

**** calling fresh() 1st time ****
fresh(): $0 is ./fresh.sh
fresh(): $1 is Tomato
fresh(): $t is Tomato
fresh(): total args passed to me 1
fresh(): all args ($@) passed to me -"Tomato"
fresh(): all args ($*) passed to me -"Tomato"
**** calling fresh() 2nd time ****
fresh(): $0 is ./fresh.sh
fresh(): $1 is Tomato
fresh(): $t is Tomato
fresh(): total args passed to me 3
fresh(): all args ($@) passed to me -"Tomato Onion Paneer"
fresh(): all args ($*) passed to me -"Tomato Onion Paneer"

Let us try one more example. Create a new shell script to determine if given name is file or directory (cmdargs.sh):

#!/bin/bash
# Name - cmdargs.sh
# Purpose - Passing positional parameters to user defined function 
# -----------------------------------------------------------------
file="$1"

# User-defined function
is_file_dir(){
        # $f is local variable
	local f="$1"
        # file attributes comparisons using test i.e. [ ... ]
	[ -f "$f" ] && { echo "$f is a regular file."; exit 0; }
	[ -d "$f" ] && { echo "$f is a directory."; exit 0; }
	[ -L "$f" ] && { echo "$f is a symbolic link."; exit 0; }
	[ -x "$f" ] && { echo "$f is an executeble file."; exit 0; }
}

# make sure filename supplied as command line arg else die
[ $# -eq 0 ] && { echo "Usage: $0 filename"; exit 1; }

# invoke the is_file_dir and pass $file as arg
is_file_dir "$file"

#function