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

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