The Wiert Corner – irregular stream of stuff

Cool: you can return strings both as a function result, and by reference: they are explained in the question, second and fourth answer of [WayBack] How to return a string value from a Bash function – Stack Overflow.

Returning them by reference has two important benefits:

1. it is much faster (especially useful in tight loop)
2. you can use echo (normally used to return a result) for debugging purposes

I also needed a bit of switch magic which I found at [WayBack] bash – Switch case with fallthrough? – Stack Overflow and array magic (from [WayBack] Array variables) as arrays are far more readable than indirection (on the why not, see [WayBack] BashFAQ/006 – Greg’s Wiki: How can I use variable variables (indirect variables, pointers, references) or associative arrays?).

So here is a function that returns a specific IPv4 octet.

function getIpv4Octet() {
  IPv4=$1
  octetIndex=$2
  outputVariable=$3

  slice="${IPv4}"
  count=1
  while [ "${count}" -le 4 ]
  do
    octet[${count}]="${slice%%.*}"
    slice="${slice#*.}"
    count=$((count+1))
  done
   
  case "${octetIndex}" in
    "1" | "2" | "3" | "4")
      ;;
    *)
      octetIndex="4"
      ;;
  esac
  eval $outputVariable="${octet[$octetIndex]}"
}

You call it like this:

$ getIpv4Octet "192.168.178.32" 3 result && echo ${result}
178

–jeroen

I needed to enumerate all the parameters to a function and access many of them by index in the same function, so thanks to both these:

• [WayBack] how to count the length of an array defined in bash? – Unix & Linux Stack Exchange
• [WayBack] bash – access element of $* (or $@) by index – Stack Overflow

I got at this:

  declare -a arguments=("$@")
  for index in ${!arguments[@]}; do
    echo $index/${#arguments[@]}:${arguments[$index]}
  done

These are all forms of Array handling or Shell Parameter Expansion with special cases for array variables:

• ! does indirection, in this case from the array to the index of the array
• # gets the lengt of the parameter (for arrays: the number of elements)
• [] acccesse an array variable using an index

Via:

• [WayBack] Bash Reference Manual: Shell Parameter Expansion
• [WayBack] Bash Reference Manual: Arrays
• [WayBack] bash – variable expansion in curly braces – Stack Overflow
• [WayBack] Variable substitution with an exclamation mark in bash – Unix & Linux Stack Exchange
• [WayBack] shell – BASH scripting: n-th parameter of $@ when the index is a variable? – Stack Overflow
• [WayBack] BashFAQ/006 – Greg’s Wiki: How can I use variable variables (indirect variables, pointers, references) or associative arrays?

–jeroen