[shell] How to call a function in shell Scripting?

I have a shell script which conditionally calls a function.

For Example:-

if [ "$choice" = "true" ]
then 
  process_install
elif [ "$choice" = "false" ]
then 
  process_exit
fi

process_install()
{
  commands...
  commands...
}

process_exit()
{
  commands...
  commands...
}

Please let me know how to accomplish this.

The functions need to be defined before being used. There is no mechanism is sh to pre-declare functions, but a common technique is to do something like:

main() {
  case "$choice" in
    true)  process_install;;
    false) process_exit;;
  esac
}

process_install()
{
  commands...
  commands...
}

process_exit()
{
  commands...
  commands...
}

main()

You can create another script file separately for the functions and invoke the script file whenever you want to call the function. This will help you to keep your code clean.

Function Definition : Create a new script file
Function Call       : Invoke the script file

#!/bin/bash  
# functiontest.sh a sample to call the function in the shell script  

choice="true"  
function process_install  
{  
  commands...
}  

function process_exit  
{
  commands...  
}  

function main  
{  
  if [[ "$choice" == "true" ]]; then  
      process_install
  elif [[ "$choice" == "false" ]]; then  
      process_exit  
  fi  
}  

main "$@"  

it will start from the main function

#!/bin/bash

process_install()
{
    commands... 
    commands... 
}

process_exit()
{
    commands... 
    commands... 
}


if [ "$choice" = "true" ] then
    process_install
else
    process_exit
fi

Summary:

• Define functions before using them.
• Once defined, treat them as commands.

Consider this script, called funcdemo:

#!/bin/bash

[ $# = 0 ] && exhort "write nastygram"

exhort(){
    echo "Please, please do not forget to $*"
}

[ $# != 0 ] && exhort "write begging letter"

In use:

$ funcdemo
./funcdemo: line 3: exhort: command not found
$ funcdemo 1
Please, please do not forget to write begging letter
$

Note the potential for a missing function to lie undiscovered for a long time (think 'by a customer at the most critical wrong moment'). It only matters whether the function exists when it is executed, the same as it only matters whether any other command exists when you try to execute it. Indeed, until it goes to execute the command, the shell neither knows nor cares whether it is an external command or a function.

Example of using a function() in bash:

#!/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_exists function
# $f -> store argument passed to the script
is_file_exists(){
    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_exists "$1" )
then
 echo "File found: $1"
else
 echo "File not found: $1"
fi