posix shell: understanding command search and execution

x.sh

#!/bin/sh

get_path () {
    command -v "$1";
};

has_operator () {
    case "$1" in
        (*";"*|*"&"*|*"&&"*|*"||"*|*";;"*|*"("*|*")"*)
            command \return 1;;
        (*"<"*|*">"*|*"<<"*|*">>"*|*"<&"*|*">&"*|*"<>"*|*"<<-"*|*">|"*)
            command \return 2;;
    esac;
};

has_slash () {
    [ "${1##*/*}" = "$1" ];
};

is_builtin () {
    [ "$1" = "built-in" ];
};

is_function () {
    [ "$1" = "function" ];
};

is_reserved_word () {
    case "$1" in
        (if|then|else|elif|fi|do|done| \
        case|esac|while|until|for| \
        \{|\}|\!)
            command \return 1;;
        (\[\[|\]\]|function|select|?*:)
            command \return 2;;
    esac;
};

is_special_builtin () {
    case "$1" in
        (break|:|continue|.|eval|exec|exit|export|readonly|return|set|shift| \
        times|trap|unset)
            command \return 1;;
    esac;
};

is_unspecified_utility () {
    case "$1" in
        ( alloc|autoload|bind|bindkey|builtin|bye|caller|cap|chdir|clone| \
        comparguments|compcall|compctl|compdescribe|compfiles|compgen| \
        compgroups|complete|compquote|comptags|comptry|compvalues| \
        declare|dirs|disable|disown|dosh|echotc|echoti|help|history| \
        hist|let|local|login|logout|map|mapfile|popd|print|pushd| \
        readarray|repeat|savehistory|source|shopt|stop|suspend|typeset|whence)
            command \return 1;;
    esac;
};

set_type () {
    case "$(command \type "${1:-_}" 2>/dev/null)" in
        (*\ function)
            command \echo function;;
        (*\ builtin)
            command \echo built-in;;
        (*\ keyword)
            command \echo keyword;;
        (*\ not\ found|"")
            command \echo unkown;;
        (*)
            command \echo file;;
    esac;
};

test_name ()
case "$1" in
   (admin|ar|asa|at|awk|basename|batch|bc|c99|cal|cat|cflow|chgrp| \
    chmod|chown|cksum|cmp|comm|compress|cp|crontab|csplit|ctags|cut| \
    cxref|date|dd|delta|df|diff|dirname|du|echo|ed|env|ex|expand| \
    expr|file|find|fold|fort77|fuser|gencat|get|getconf|grep|head| \
    iconv|id|ipcrm|ipcs|join|lex|link|ln|locale|localedef|logger| \
    logname|lp|ls|m4|mailx|make|man|mesg|mkdir|mkfifo|more|mv|nice| \
    nl|nm|nohup|od|paste|patch|pathchk|pax|pr|printf|prs|ps|qalter| \
    qdel|qhold|qmove|qmsg|qrerun|qrls|qselect|qsig|qstat|qsub|renice| \
    rm| rmdel|rmdir|sact|sccs|sed|sh|sleep|sort|split|strings|strip| \
    stty|tabs|tail|talk|tee|test|time|touch|tput|tr|tsort|tty|uname| \
    uncompress|unexpand|unget|uniq|unlink|uucp|uudecode|uuencode|uustat| \
    uux|val|vi|wc|what|who|write|xargs|yacc|zcat)
        command \echo 1;;
    (alias|bg|cd|command|false|fc|fg|getopts|hash|jobs|kill|newgrp|pwd|read| \
    true|type|ulimit|umask|unalias|wait)
        command \echo 2;;
esac;

iam () {
    command \local -;
    command \set -- ${1};
    command \unalias "$1" 2>/dev/null;
    command \local cmd=;
    cmd="$1";

    command \printf '%s\n' "Using token '${cmd}'";

    command \printf '%s' "Testing if token matches operator ... ";
    'has_operator' "$cmd";
    case "$?" in
        (0)
            command \printf '%s\n' "False";;
        (1)
            command \printf '%s\n' "True";
            command \printf '\t%s\n' "Token matches control operator";
            command \printf '\t%s\n' "Skipping";
            command \return 0;;
        (2)
            command \printf '%s\n' "True";
            command \printf '\t%s\n' "Token matches redirection operator";
            command \printf '\t%s\n' "Skipping";
            command \return 0;;
    esac;

    command \printf '%s' "Testing if token is reserved word ... ";
    'is_reserved_word' "$cmd";
    case "$?" in
        (0)
            command \printf '%s\n' "False";;
        (1)
            command \printf '%s\n' "True";
            command \printf '\t%s\n' "Skipping";
            command \return 0;;
        (2)
            command \printf '%s\n' "True";
            command \printf '\t%s\n' "Token is unspecific reserved word";
            command \printf '\t%s\n' "Skipping";
            command \return 0;;
    esac;

    command \printf '%s\n' "Starting command search and execution";

    command \printf '%s' "Testing if name has slashes ... ";
    'has_slash' "$cmd" || {
        command \printf '%s\n' "True";
        command \printf '\t%s' "Searching name using the PATH environment variable ... ";
        command \local p=;
        p="$('get_path' "$cmd")";
        if
            command \test -n "$p";
        then
            command \printf '%s\n' "Match";
            case "$('test_name' "$(command \basename "$p")")" in
                (1)
                    command \printf '\t\t%s\n' "Name is a POSIX standard utility";;
            esac;
            command \printf '\t\t%s\n' "Name points to file '${p}'";
            command \printf '\t\t%s\n' "Executing file '${p}'";
            command \return 0;
        else
            command \printf '%s\n' "Unmatch";
            command \printf '\t%s\n' "Skipping command search and execution of '${cmd}'";
            command \return 127;
        fi;
    };

    command \printf '%s\n' "False";

    command \printf '%s' "Testing if name is POSIX special built-in ... ";
    if
        'is_special_builtin' "$cmd";
    then
        command \printf '%s\n' "False";
    else
        command \printf '%s\n' "True";
        command \printf '\t%s\n' "Executing special built-in '${cmd}'";
        command \return 0;
    fi;

    command \printf '%s' "Testing if name is POSIX-unspecific utility ... ";
    if
        'is_unspecified_utility' "$cmd";
    then
        command \printf '%s\n' "False";
    else
        command \printf '%s\n' "True";
        command \printf '\t%s\n' "Skipping command search and execution of '${cmd}'";
        command \return 0;
    fi;

    command \local t=;
    t="$('set_type' "$cmd")";

    command \printf '%s' "Testing if name is definied function ... ";
    if
        'is_function' "$t";
    then
        command \printf '%s\n' "True";
        command \printf '\t%s' "Testing if function is named as POSIX standard utility ... ";
        case "$('test_name' "$cmd")" in
            (1)
                command \printf '%s\n' "True";
                command \printf '\t\t%s\n' "Postponing execution of '${cmd}'";;
            (2)
                command \printf '%s\n' "True";
                command \printf '\t\t%s\n' \
                    "Function is named as POSIX regular built-in" \
                    "Executing function '${cmd}'";
                command \return 0;;
            (*)
                command \printf '%s\n' "False";
                command \printf '\t%s\n' "Executing function '${cmd}'";
                command \return 0;;
        esac;
    else
        command \printf '%s\n' "False";
    fi;

    command \printf '%s' "Testing if name is built-in ... ";
    if
        'is_builtin' "$t";
    then
        command \printf '%s\n' "True";
        command \printf '\t%s' "Testing if built-in is a POSIX regular built-in ... ";
        case "$('test_name' "$cmd")" in
            (1)
                command \printf '%s\n' "False";
                command \printf '\t%s\n' "Built-in is regular for this shell, but only a POSIX standard utility";
                command \printf '\t%s\n' "Postponing execution of ${cmd}";;
            (2)
                command \printf '%s\n' "True";
                command \printf '\t\t%s\n' \
                    "Executing POSIX regluar built-in '${cmd}'";
                command \return 0;;
            (*)
                command \printf '%s\n' "False";
                command \printf '\t%s\n' "Unknown POSIX non-standard built-in";
                command \printf '\t%s\n' "Skipping command search and execution of '${cmd}'";
                command \return 0;;
        esac;
    else
        command \printf '%s\n' "False";
    fi;

    command \printf '%s' "Searching name using the PATH environment variable ... ";
    command \local p=;
    p="$('get_path' "$cmd")";
    if
        command \test -n "$p";
    then
        command \printf '%s\n' "Match";
        case "$t" in
            (built-in|function)
                command \printf '\t%s\n' "Executing postponed ${t} '${cmd}'";
                command \return 0;;
            (file)
                case "$('test_name' "$cmd")" in
                    (1)
                        command \printf '\t%s\n' "Name is a POSIX standard utility";
                esac;
                command \printf '\t%s\n' "Name points to file '${p}'";
                command \printf '\t%s\n' "Executing file '${p}'";;
        esac;
    else
        command \printf '%s\n' "Unmatch";
        command \printf '%s\n' "Skipping command search and execution of '${cmd}'";
        command \return 127;
    fi;
};

# -- MAIN.

'iam' "${1:?}" 1>&2;

# vim: set ts=4 sw=4 tw=0 et :

# no newline in token please

$ dash -c '. x.sh token'
$ dash x.sh token