Pietro Martinelli
functional programming

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 scripts (say use-functional-bricks.sh) like the following

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#!/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 source command
  • execute scripts when bash starts

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

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#!/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:

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. ~/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 beleiving… if I launch the script use-functional-bricks.shdefined above, I get the following output:

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square_after_double_output 4
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36
64
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144
The sum is 21