Re: mapfile -C issue

[Emanuele Torre]

Note that when you use mapfile with -C it will still store the values
that are read in the array specified by the argument or MAPFILE, so if
you want to use mapfile -C to read data, and run a function for every
line, it is not ideal, it makes more sense to use a while read loop.

Anyway,  -C C -c 3  will call C every 3 lines/records read with the
current index of the array that will be set, and the current record.
Note that the callback is called before that record is stored in the
array.

  C () {
    declare -p MAPFILE
    printf '$1 = %s\t$2 = %s\n' "${1@Q}" "${2@Q}"
  }

  mapfile -td '' -c 3 -C C < <(printf %s\\0 11 22 33 44 55 66 77)
  # declare -a MAPFILE=([0]="11" [1]="22")
  # $1 = '2'  $2 = '33'
  # declare -a MAPFILE=([0]="11" [1]="22" [2]="33" [3]="44" [4]="55")
  # $1 = '5'  $2 = '66'

  declare -p MAPFILE
  # declare -a MAPFILE=([0]="11" [1]="22" [2]="33" [3]="44" [4]="55" [5]="66" 
[6]="77")

If you want C to be called for every element, you use -c1, not -c3.

  mapfile -td '' -c 1 -C C < <(printf %s\\0 11 22 33 44 55 66 77)
  # declare -a MAPFILE=()
  # $1 = '0'  $2 = '11'
  # declare -a MAPFILE=([0]="11")
  # $1 = '1'  $2 = '22'
  # declare -a MAPFILE=([0]="11" [1]="22")
  # $1 = '2'  $2 = '33'
  # declare -a MAPFILE=([0]="11" [1]="22" [2]="33")
  # $1 = '3'  $2 = '44'
  # declare -a MAPFILE=([0]="11" [1]="22" [2]="33" [3]="44")
  # $1 = '4'  $2 = '55'
  # declare -a MAPFILE=([0]="11" [1]="22" [2]="33" [3]="44" [4]="55")
  # $1 = '5'  $2 = '66'
  # declare -a MAPFILE=([0]="11" [1]="22" [2]="33" [3]="44" [4]="55" [5]="66")
  # $1 = '6'  $2 = '77'

  declare -p MAPFILE
  # declare -a MAPFILE=([0]="11" [1]="22" [2]="33" [3]="44" [4]="55" [5]="66" 
[6]="77")

And -c3 WILL NOT call C every 3 records, before assigning those three
records passing   key1 value1 key2 value2 key3 value3   to C.

That is not how it is documented to work.

o/
 emanuele6