How to split a string by ':' character in bash/zsh?

The : there is any arbitrary character:

You can use:

parts=(${(s/:/)str})

Some common character pairs are also supported like:

parts=(${(s[:])str})

If you're going to use the @ flag to preserve empty elements, then you need to quote:

parts=("${(@s[:])str}")

Otherwise @ makes no difference.

If it's to process variables like $PATH/$LD_LIBRARY_PATH... see also typeset -T which ties an array variable to a scalar variable:

$ typeset -T str str_array
$ str='a::b'
$ typeset -p str
typeset -T str str_array=( a '' b )

zsh does tie $path to $PATH by default (like in csh/tcsh).

bash's

parts=(${str//:/ })

Is wrong as it applies split+glob after having replaced : with SPC.

You'd want:

IFS=:            # split on : instead of default SPC TAB NL
set -o noglob    # disable glob
parts=( $str"" ) # split+glob (leave expansion unquoted), preserve trailing
                 # empty part.

That code would also work in zsh, if it was in sh or ksh emulation mode. If your goal is to write code compatible to both bash and zsh, you may want to write it using ksh syntax and make sure that zsh is put in ksh emulation (possibly only locally to some function) when interpreting it.

To test whether the shell is bash or zsh, you'd test for the presence of the $BASH_VERSION/$BASH_VERSINFO or $ZSH_VERSION variables.

split() { # args: string delimiter result_var
  if
    [ -n "$ZSH_VERSION" ] &&
      autoload is-at-least &&
      is-at-least 5.0.8 # for ps:$var:
  then
    eval $3'=("${(@ps:$2:)1}")'
  elif
    [ "$BASH_VERSINFO" -gt 4 ] || {
      [ "$BASH_VERSINFO" -eq 4 ] && [ "${BASH_VERSINFO[1]}" -ge 4 ]
      # 4.4+ required for "local -"
    }
  then
    local - IFS="$2"
    set -o noglob
    eval "$3"'=( $1"" )'
  else
    echo >&2 "Your shell is not supported"
    exit 1
  fi
}

split "$str" : parts