scriptdir

if [ -n "$BASH_VERSION" ]; then
    # http://stackoverflow.com/questions/59895
    scriptdir="$(unset CDPATH && cd -- "$(dirname "${BASH_SOURCE[0]}")" && pwd -P)"
elif [ -n "$ZSH_VERSION" ]; then
    # https://stackoverflow.com/questions/9901210
    scriptdir="${${(%):-%x}:A:h}"
else
    # https://stackoverflow.com/questions/29832037
    scriptdir=$(unset CDPATH && cd -- "$(dirname -- "$0")" && pwd -P)
fi

The exact function for POSIX is too complex. Refer the answer in https://stackoverflow.com/questions/29832037.

Note

I collected ideas from various answers and modified the original snippets suggested in the Stackoverflow threads.

• scriptdir: it shoud be in lower case letters
• CDPATH=: to not use CDPATH [1], [3]
• unset CDPATH: similar to CDPATH= but it seems more reliable in some implementations
• \cd(?): to ignore the alias even if cd is aliased
  • (you may use unalias -a to remove all alias definitions)
  • but protecting the shell script form alias definitions is another issue
• >/dev/null 2>&1: here 2>&1 is not necessary as $() does not capture the standard error
• >/dev/null: not necessary as cd will not echo when CDPATH is empty and "$1" (of cd) is not "-". [3][4]

Referencees

1. http://stackoverflow.com/questions/59895
2. https://stackoverflow.com/questions/9901210
3. https://stackoverflow.com/questions/29832037
4. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/cd.html