Functional Shell: A Minimal Toolbox

I already wrote a post about adopting a functional programming style in bash scripts. Here I want to explore how to build a minimal, reusable functional toolbox for my bash scripts, avoiding redefinition of base functional bricks whenever I need them.

So, in short: I wish I could write a script (say use-functional-bricks.sh) like the following:

#!/bin/bash

double () {

  expr $1 '*' 2

}

square () {

  expr $1 '*' $1

}

input=$(seq 1 6)

square_after_double_output=$(map "square" $(map "double" $input))

echo "square_after_double_output $square_after_double_output"

sum() {

    expr $1 '+' $2

}

sum=$(reduce 0 "sum" $input)

echo "The sum is $sum"

referring to “globally” available functions map and reduce(and maybe others, too) without to re-write them everywhere they are needed and without to be bound to external scripts invocation.

The way I think we can solve the problem refers to three interesting features available in bash:

• export functions from scripts (through export -f)
• execute scripts in the current shell’s environment, through the source command
• execute scripts when bash starts

So I wrote the following script (say functional-bricks.sh):

xxxxxxxxxx

#!/bin/bash

map () {

    f=$1

    shift

    for x

    do

        $f $x

    done

}

export -f map

reduce () {

    acc=$1

    f=$2

    shift

    shift

    for curr

    do

        acc=$($f $acc $curr)

    done

    echo $acc

}

export -f reduce

and added the following line at the end of my user’s ~/.bashrc file:

xxxxxxxxxx

. ~/common/functional-bricks.sh

and… voila! now map and reduce implemented in functional-bricks.sh are available in all my bash sessions — so I can use them in all my scripts!

And because seeing is believing… if I launch the script use-functional-bricks.shdefined above, I get the following output:

xxxxxxxxxx

square_after_double_output 4

16

36

64

100

144

The sum is 21