logo
search-iconinfo-icon

Functions - Shell Script

What is called a function in shell might be called in some languages procedures, and functions.

Simple Function

#!/bin/sh

# Definition
hello () { 
  echo "Hello World!"
}

# Invocation
hello

Run file:

$ ./example.sh
Hello World!

Function with parameters

We can also define a function with parameters. The parameters work like the parameters of the shell file, $1, $2 and so on.

#!/bin/sh

add () {
   echo $1 + $2 = $(($1 + $2))
}

add 2 4

Run file:

$ ./example.sh
2 + 4 = 6

Function with Returning Values

Functions can return a value in one of four different ways:

  • Change the value of variables
  • Use the exit command to end the shell script
  • Use the return command to end the function, and return the value
  • echo output to stdout, which will be caught by the caller

Change Variables

Like in Javascript (non-strict mode) there is no scope of variables, except the parameters ($1, $2, $@, etc). This allows us to change and define variables inside the function which can be useful sometimes.

#!/bin/bash

hello () {
  msg='Hello World!'
}

hello
echo $msg

Run file:

$ ./example.sh
Hello World

Return command

The return command can return a code (number)

#!/bin/bash

hello() {
  if [ $1 == "world" ]
  then
    return 1
  elif [ $1 == "there" ]
  then
    return 2
  fi
  return 0
}

hello there
HELLO_RETURN=$?
if [ $HELLO_RETURN == 1 ]
then
  echo hi
elif [ $HELLO_RETURN == 2 ]
then
  echo General Kenoby
else
  echo ...
fi

Run file:

$ ./example.sh
General Kenobi

Useful Links:

edit-iconEdit this page