如何确定命令是否以符合POSIX的方式存在？

Question

请参阅Is `command -v` option required in a POSIX shell? Is posh compliant with POSIX?上的讨论。它描述了type中的command -v和type选项是可选的。

在POSIX.1-2004标记为正确的答案也无济于事。与hash一样，Sound.__init__在POSIX.1-2004中也标记为XSI。请参阅Check if a program exists from a Bash script。

那么编写shell脚本以查找系统上是否存在命令的POSIX兼容方法是什么？

Answer 1

你想怎么做？您可以在当前值$PATH中查找目录上的命令;您可以查看系统路径默认指定的目录（getconf PATH，只要getconf  在PATH上存在。

您打算使用哪种实现语言？ （例如：我有一个Perl实现，可以在$PATH上找到可执行文件，但Perl不是POSIX的一部分;它与您远程相关吗？）

为什么不尝试运行呢？如果您打算处理基于Busybox的系统，则无法通过搜索找到大量可执行文件 - 它们内置于shell中。主要的警告是，如果命令在没有参数的情况下运行时会发生危险 - 但很少有POSIX命令（如果有的话）这样做。您可能还需要确定哪些命令退出状态表明找不到命令，而不是使用适当的参数反对命令。并且几乎不能保证所有系统都能保持一致。如果你还没有聚集，这是一个充满挑战的过程。

Perl实现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.x
use 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 PATH
EOS

sub 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;