kugarocks

Function

Creating a Function

function name {
    commands
}

or

name() {
    commands
}

Function Return Value

There are multiple forms of return values.

Using $?

Represents the exit status code of the last command in the function.

echo $?

Using Return

#!/usr/bin/env bash

function double {
    echo "Double value"
    read -p "Enter a value:" value
    return $[ $value * 2 ]
}

double
echo "Result: $?"

Essentially, it’s the exit status code, ranging from [0, 255], and exceeding will be modulo 256.

Using Standard Output

#!/usr/bin/env bash

function double {
    read -p "Enter a value:" value
    echo $[ $value * 2 ]
}

result=$(double)
echo "Result: $result"

result will save all the standard output from the function.

Function Parameters

#!/usr/bin/env bash

function add {
    if [ $# -lt 2 ]; then
        echo "invalid params"
    else
        echo $[ $1 + $2 ]
    fi
}

result=$(add 1 2)
echo "Result: $result"

The $#, $1, and parameters within the function are independent of the outer parameters.

Variable Scope

Outside the Function

Accessible anywhere.

#!/usr/bin/env bash

soda=green
function foo {
    echo $soda
    soda=yellow
}
foo
echo $soda
green
yellow

Inside the Function

#!/usr/bin/env bash

function foo {
    soda=green
}
# Cannot access before the function is executed
echo $soda
foo
echo $soda
soda=yellow
echo $soda

green
yellow

Using Local

local variables only take effect within the function and are independent of external variables with the same name.

#!/usr/bin/env bash

soda=green
function foo {
    echo $soda
    local soda=yellow
    echo $soda
}
foo
echo $soda
green
yellow
green

Is a Variable Defined

Can use parameter expansion’s + marker.

Is a Function Defined

Later-defined functions with the same name will override previously defined functions with the same name, so it’s a good idea to check before defining a function.

declare -f FUNC_NAME

For example, writing the function in the .bashrc file.

function sayhello() {
    echo "hello"
}
declare -f sayhello
echo $?

The function is defined, exit code is 0, not defined, exit code is 1.

if declare -f sayhello > /dev/null; then
  echo "sayhello is defined"
else
  echo "sayhello not defined"
fi
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