Just for fun: map as ‘higher-order function’ in bash

Defined recursively, of course:

map () {
  if [ $# -le 1 ]; then
    return
  else
    local f=$1
    local x=$2
    shift 2
    local xs=$@ 

    $f $x 

    map "$f" $xs
  fi
}

I’m not going to explain everything, but note the ‘local’ commands to keep everything in function scope (to prevent strange bugs) and the quotes around $f in the recursive call to prevent a ‘function call’ of multiple words from being split.

Now you can do some completely nonsensical things, for which you really don’t need map, such as:

$ map touch aap noot mies

But also slightly more useful things such as

$ map "echo foo" aap noot mies
foo aap
foo noot
foo mies 

$ map "echo file:" `ls`
file: aap
file: noot
file: mies

To open up some more possibilities, I also defined a ‘rota’ function, with rotates the arguments of a command such that the last comes first.

rota () {
  local f=$1
  shift
  local args=($@)
  local idx=$(($#-1))
  local last=${args[$idx]}
  args[$idx]= 

  $f $last ${args[@]}
}

I’m using a array (yes, bash has arrays) to easily get the last element, as you can see

So, how about…

$ map "rota mv /tmp" aap noot mies

(Move the aap, noot and mies file to /tmp)

There are probably better (and more useful) examples of it’s usage, but you hopefully get the gist of it.

This code is just for fun (I don’t expect I’m ever going to use it, even), so it’s nowhere near being robust. There are a lot of (possible) bugs concerning spaces and undetected grouping of words (with double quotes). Use at your own risk