How to find out if a command exists in a POSIX compliant manner?
How do you want to go about it? You can look for the command on directories in the current value of $PATH; you could look in the directories specified by default for the system PATH (getconf PATH as long as getconf exists on PATH).
Which implementation language are you going to use? (For example: I have a Perl implementation that does a decent job finding executables on $PATH — but Perl is not part of POSIX; is it remotely relevant to you?)
Why not simply try running it? If you're going to deal with Busybox-based systems, lots of the executables can't be found by searching — they're built into the shell. The major caveat is if a command does something dangerous when run with no arguments — but very few POSIX commands, if any, do that. You might also need to determine what command exit statuses indicate that the command is not found versus the command objecting to not being called with appropriate arguments. And there's little guarantee that all systems will be consistent on that. It's a fraught process, in case you hadn't gathered.
Perl implementation pathfile
#!/usr/bin/env perl## @(#)$Id: pathfile.pl,v 3.4 2015/10/16 19:39:23 jleffler Exp $## Which command is executed# Loosely based on 'which' from Kernighan & Pike "The UNIX Programming Environment"#use v5.10.0; # Uses // defined-or operator; not in Perl 5.8.xuse strict;use warnings;use Getopt::Std;use Cwd 'realpath';use File::Basename;my $arg0 = basename($0, '.pl');my $usestr = "Usage: $arg0 [-AafhqrsVwx] [-p path] command ...\n";my $hlpstr = <<EOS; -A Absolute pathname (determined by realpath) -a Print all possible matches -f Print names of files (as opposed to symlinks, directories, etc) -h Print this help message and exit -q Quiet mode (don't print messages about files not found) -r Print names of files that are readable -s Print names of files that are not empty -V Print version information and exit -w Print names of files that are writable -x Print names of files that are executable -p path Use PATHEOSsub usage{ print STDERR $usestr; exit 1;}sub help{ print $usestr; print $hlpstr; exit 0;}sub version{ my $version = 'PATHFILE Version $Revision: 3.4 $ ($Date: 2015/10/16 19:39:23 $)'; # Beware of RCS hacking at RCS keywords! # Convert date field to ISO 8601 (ISO 9075) notation $version =~ s%\$(Date:) (\d\d\d\d)/(\d\d)/(\d\d) (\d\d:\d\d:\d\d) \$%\$$1 $2-$3-$4 $5 \$%go; # Remove keywords $version =~ s/\$([A-Z][a-z]+|RCSfile): ([^\$]+) \$/$2/go; print "$version\n"; exit 0;}my %opts;usage unless getopts('AafhqrsVwxp:', \%opts);version if ($opts{V});help if ($opts{h});usage unless scalar(@ARGV);# Establish test and generate test subroutine.my $chk = 0;my $test = "-x";my $optlist = "";foreach my $opt ('f', 'r', 's', 'w', 'x'){ if ($opts{$opt}) { $chk++; $test = "-$opt"; $optlist .= " -$opt"; }}if ($chk > 1){ $optlist =~ s/^ //; $optlist =~ s/ /, /g; print STDERR "$arg0: mutually exclusive arguments ($optlist) given\n"; usage;}my $chk_ref = eval "sub { my(\$cmd) = \@_; return -f \$cmd && $test \$cmd; }";my @PATHDIRS;my %pathdirs;my $path = defined($opts{p}) ? $opts{p} : $ENV{PATH};#foreach my $element (split /:/, $opts{p} // $ENV{PATH})foreach my $element (split /:/, $path){ $element = "." if $element eq ""; push @PATHDIRS, $element if $pathdirs{$element}++ == 0;}my $estat = 0;CMD:foreach my $cmd (@ARGV){ if ($cmd =~ m%/%) { if (&$chk_ref($cmd)) { print "$cmd\n" unless $opts{q}; next CMD; } print STDERR "$arg0: $cmd: not found\n" unless $opts{q}; $estat = 1; } else { my $found = 0; foreach my $directory (@PATHDIRS) { my $file = "$directory/$cmd"; if (&$chk_ref($file)) { $file = realpath($file) if $opts{A}; print "$file\n" unless $opts{q}; next CMD unless defined($opts{a}); $found = 1; } } print STDERR "$arg0: $cmd: not found\n" unless $found || $opts{q}; $estat = 1; }}exit $estat;