Bash Shell Script Function Examples

How do I create a shell script function using Bash under UNIX / Linux operating systems?

Functions are nothing but small subroutines or subscripts within a Bash shell script. You need touse to break up a complex script into separate tasks. This improves overall script readability and ease of use. However, shell function cannot return value. They return a status code.

Declare Shell Function

All functions must be declared before they can be used. The syntax is:

function name(){
  Commands
}

 
name(){
 Commands
 return $TRUE
}

You can call function by typing its name:

 
name

Example

Create a shell script called file.sh:

#!/bin/bash
# file.sh: a sample shell script to demonstrate the concept of Bash shell functions
# define usage function
usage(){
	echo "Usage: $0 filename"
	exit 1
}
 
# define is_file_exits function
# $f -> store argument passed to the script
is_file_exits(){
	local f="$1"
	[[ -f "$f" ]] && return 0 || return 1
}
# invoke  usage
# call usage() function if filename not supplied
[[ $# -eq 0 ]] && usage
 
# Invoke is_file_exits
if ( is_file_exits "$1" )
then
 echo "File found"
else
 echo "File not found"
fi

Run it as follows:
chmod +x file.sh ./file.sh ./file.sh /etc/resolv.conf

Task: Export functions

You need to use export command:

 
fname(){
  echo "Foo"
}
 
export -f fname

Task: Make readonly functions

You create functions at the top of the script and set the readonly attribute with the readonly command:

 
fname(){
  echo "Foo"
}
 
usage(){
  echo "Usage: $0 foo bar"
  exit 1
}
 
readonly -f usage
readonly -f fname

Task: Local variables functions

Use the local command to create local variables:

#!/bin/bash
# gloabal x and y
x=200
y=100
 
math(){
  # local variable x and y with passed args
  local x=$1
  local y=$2
  echo $(( $x + $y ))
}
 
echo "x: $x and y: $y"
# call function 
 
echo "Calling math() with x: $x and y: $y"
math 5 10
 
# x and y are not modified by math()
echo "x: $x and y: $y after calling math()"
echo $(( $x + $y ))

Task: Recursion

A recursive function call itself. Recursion is a useful technique for simplifying some complex algorithms, and breaking down complex problems.

 
#!/bin/bash
foo(){
  # do something
  # if not false call foo
  foo
}
 
# call foo
foo

See Recursive function for more details.

Recommend readings:

Chapter 9: Functions from the Linux shell scripting wiki

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