Bash Shell Script Function Examples

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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
}

OR

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:

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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9 comment

  1. fun1(){ x=100000;  echo " In fun() x = $x " ; }
fun2(){ y=200000;  echo " In fun() y = $y " ; }
  
x=100 ; y=200
echo "before calling d fun1() x=$x" 
echo -e $(fun1)
echo " after calling d fun1() x=$x"
echo "before calling d fun2() y=$y" 
fun2
echo " after calling d fun() y=$y"

    output :

    before calling d fun1() x=100
In fun() x = 100000
 after calling d fun1() x=100
before calling d fun2() y=200
 In fun() y = 200000
 after calling d fun() y=200000

    I m a beginner in Shell programing…., anybody can explain me what is the diff. b/w calling a function as $(fun) or fun….., why the value of x is not changed n value of is changed… what is diff. b/w both invocation.

    Reply Report comment

  2. @shweta

    $(command) – causes the command to be run in a subshell (child process of the parent) and any changes made in the subshell do not affect the parent execution context. See the section COMMAND EXECUTION ENVIRONMENT in the bash manpage.

    e.g.
    x=1; echo “before x=$x”; echo “$x” | while read x; do echo “during x=$x”; ((x++)); echo “during x=$x”; done; echo “after x=$x”
    before x=1
    during x=1
    during x=2
    after x=1

    x=1; echo “before x=$x”; ( x=2; echo “during x=$x”; ); echo “after x=$x”
    before x=1
    during x=2
    after x=1

    Reply Report comment

  7. Hello, i seem to be having this issue whenever i try to do bash on centos. I don’t know why as my bash is /bin/bash and ive tried it on many new installs in this code… i used the code from this section: Task: Local variables functions.

    [[email protected] /]# sh install.sh
    :command not found
    `nstall.sh: line 6: syntax error near expected token `{
    `nstall.sh: line 6: `math(){

    Its really annoying.. i always get :command not found with any new lines… and functions always create an error aswell… why is this?

    Thanks
    Lewis

    Reply Report comment

  8. how can I pass the argument to the loop? I have the following code:

    download(){
      rm $HOME/.netrc
      echo "machine server login [email protected] password password" >$HOME/.netrc      

      echo "macdef Ka" >>$HOME/.netrc
      echo "prompt" >>$HOME/.netrc
      
      for yyyy in {$1..$2}
      do
      for mm in {$3..$4}
      do
      for day in {$5..$6}
      do
      
      echo "cd /gpmdata/$yyyy/$mm/$day/radar" >> $HOME/.netrc
     done
      done
      done
      echo "bye" >> $HOME/.netrc
      echo "" >> $HOME/.netrc
      chmod 600 $HOME/.netrc
      a=$7
      echo '$a' | ftp arthurhou.pps.eosdis.nasa.gov  #I don't know if this line is correct
}
GPM_download $1 $2 $3 $4 $5 $6 $7
exit 1

    then I execute

    ./GPM_download.sh 2014 2014 05 05 27 27 Ka
    but the created file .netrc looks like this

    "cd /gpmdata/{2014..2014}/{05..05}/{27..27}/radar"

    not as I wish “cd /gpmdata/20140527/radar”

    Reply Report comment

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