sh: mimics the core routine of GNU `which` command

which.sh

#!/usr/bin/env sh
#
# Mimics the core routine of UNIX `which` command[1]
# 
# Implemented on the top of shell builtins to provide the way
# to resolve external dependencies (commands) for shell scripts
# whether external `which` command available or not
#
# Designed to be fully `sh` compatible, strictly honors POSIX standards
# and completely meets Shell Command Language specification[2], thus
# can be safely utilized with any Bourne Family shell
#
# Developed by https://juliyvchirkov.github.io 2018-2022 under the MIT license
#
# Tested with `bash -r`, `dash -r` and `posh`[3]
#
# Get the last revision at https://gist.github.com/juliyvchirkov/d2c7ff01846157f58b1fc1f3a3b1e36c
# 
# [1] https://www.freebsd.org/cgi/man.cgi?query=which
# [2] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
# [3] https://www.commandlinux.com/man-page/man1/posh.1.html

which() (
    [ "${1}" = "--" ] && shift

    if [ $# -gt 0 ]; then
        for path in $(IFS=: && printf "%s " ${PATH}); do
            [ -x "${path}/${1}" ] && printf "%s/%s" "${path}" "${1}" && return 0
        done

        return 1
    fi

    printf >&2 "%s: %s [--] %s\n%s\n" "Usage" "which" "COMMAND" "Writes the full path of COMMAND to standard output."

    return 1
)