Table of contents


How to define functions

  1. Syntax

     function func_name {
     # nothing to do
 }

    OR

     func_name() {
     # nothing to do
 }

  2. For example

     #!/bin/bash

 function is_dir_git_repo {
     if git rev-parse --git-dir > /dev/null 2>&1; then
         echo 'using git repo'
         return 1
     else
         echo 'not using git repo'
         return 0
     fi 
 }

 is_directory() {
     if [ -d "$1" ]; then
         return 1
     else
         return 0
     fi
 }

 is_directory() {
     [ -d "$1" ]
 }


How to pass values to a function

  1. Parameter variables

    If no parameters are passed, the environment variable $# exists but has a value of 0.

    Belows are some parameter variables that we need to know.

    Parameter variable Description
    $0 The file name of the current script
    $1, $2, … The parameters given to the script
    $# The number of arguments supplied to a script
    $* A list of all the parameters, in a single variable, seperated by the first character in the environment variable IFS. If IFS is modified, then the way $* seperates the command line into parameters will change.
    $@ A subtle variation on $*; it does not use the IFS environment variable, so parameters are not run together even if IFS is empty
    $$ The process number of the current shell.
    $? The exit status of the last command
    $! The process number of the last background command

  2. Pass arguments and return value to function

    We supply the arguments directly after the function name. In function, we can access the value of arguments by using $1, $2, …

    We will use keyword return to return our something.

      print_value() {
   echo "Hello $1"
   return 10
 }

 print_value world

 # Use #? contains the return status of the previously run command or function.
 echo "The returned value from above function is $?"

 # or
 value=$( print_value VietNam )


Some ways to return value from functions

  1. Using global variables

     function useGlobalVar() {
     myresult='hello, world'
 }

 useGlobalVariable
 echo $myresult

  2. Define local variables and using command substitution

    • Command Substitution

      A command substitution means storing the output of a command execution in a variable.

      There are two ways to perform a command substitution:

      • Using the backtick character (‘)

          dir='pwd'
  echo $dir

      • Using the dollar sign format - $()

          dir=$(pwd)
  echo $dir

    • How to use

        function useLocalVar() {
      local localVar="Fuck you with local variables"
      echo "$localVar"
  }

  result=$(useLocalVar)
  echo $result

  3. Using parameters as variables that receive result

     function passVarAsParam() {
     local _resultVar=$1
     local localVar='Hello, world! What do you want?'

     eval $_resultVar="'$localVar'"  # lack single quote in double quote string --> it is not working.
 }

 passVarAsParam result
 echo $result

  4. Out of all above ways, we can use $? symbol to get the result from a command.

    $? symbol is the exit status of the last command. Exit status is a numerical value that is returned by a command.

     0 - successful
 1 - fail

     run_some_command
 EXIT_STATUS=$?

 if [ "$EXIT_STATUS" -eq "0" ]
 then
     # Do work when command exists on success
 else
     # Do work for when command has a failure exit
 fi

    Or we can embedded a command in a condition of if, while statements.

     if run_some_command
 then
     # Do work when command exists on success
 else
     # Do failure exit work
 fi

    Use the second way is faster because we does not use temporary variable to receive our result.


Wrapping up

  • Understanding about how to pass value to a function, use these parameters in a function, and return values.