例子:
absolute="/foo/bar"
current="/foo/baz/foo"
# Magic
relative="../../bar"
我如何创造魔法(希望不是太复杂的代码…)?
当前回答
这个答案并没有解决问题的Bash部分，但是因为我试图使用这个问题中的答案在Emacs中实现这个功能，所以我就把它扔到那里。
Emacs实际上有一个开箱即用的函数:
ELISP> (file-relative-name "/a/b/c" "/a/b/c")
"."
ELISP> (file-relative-name "/a/b/c" "/a/b")
"c"
ELISP> (file-relative-name "/a/b/c" "/c/b")
"../../a/b/c"
其他回答
可悲的是，Mark Rushakoff的答案(现在删除了-它引用了这里的代码)在适应时似乎不能正确工作:
source=/home/part2/part3/part4
target=/work/proj1/proj2
注释中概述的思想可以经过改进，使其在大多数情况下正确工作。我假设脚本有一个源参数(您所在的位置)和一个目标参数(您想要到达的位置)，这两个参数要么是绝对路径名，要么是相对路径名。如果一个是绝对的，另一个是相对的，最简单的方法是用当前工作目录作为相对名称的前缀——但是下面的代码没有这样做。
当心
下面的代码接近正确工作，但不是很正确。
丹尼斯·威廉姆森(Dennis Williamson)的评论中提到了一个问题。 还有一个问题，这种纯文本的路径名处理，你可能会被奇怪的符号链接搞得一团糟。 该代码不处理'xyz/./pqr'等路径中的零散'点'。 该代码不处理'xyz/../pqr'等路径中的流浪'双点'。 简单地说:代码没有删除前导'。/'从路径。
Dennis的代码更好，因为它修复了1和5，但也有相同的问题2,3,4。 因此，请使用Dennis的代码(并在此之前对其进行投票)。
注意:POSIX提供了一个系统调用realpath()来解析路径名，这样路径名中就没有符号链接了。将其应用于输入名称，然后使用Dennis的代码每次都会给出正确的答案。编写包装realpath()的C代码很简单——我已经做过了——但我不知道有什么标准实用程序可以这样做。)
为此，我发现Perl比shell更容易使用，尽管bash对数组有很好的支持，并且可能也可以做到这一点——这是读者的练习。因此，给定两个兼容的名称，将它们分别分解为组件:
Set the relative path to empty. While the components are the same, skip to the next. When corresponding components are different or there are no more components for one path: If there are no remaining source components and the relative path is empty, add "." to the start. For each remaining source component, prefix the relative path with "../". If there are no remaining target components and the relative path is empty, add "." to the start. For each remaining target component, add the component to the end of the path after a slash.
因此:
#!/bin/perl -w
use strict;
# Should fettle the arguments if one is absolute and one relative:
# Oops - missing functionality!
# Split!
my(@source) = split '/', $ARGV[0];
my(@target) = split '/', $ARGV[1];
my $count = scalar(@source);
$count = scalar(@target) if (scalar(@target) < $count);
my $relpath = "";
my $i;
for ($i = 0; $i < $count; $i++)
{
last if $source[$i] ne $target[$i];
}
$relpath = "." if ($i >= scalar(@source) && $relpath eq "");
for (my $s = $i; $s < scalar(@source); $s++)
{
$relpath = "../$relpath";
}
$relpath = "." if ($i >= scalar(@target) && $relpath eq "");
for (my $t = $i; $t < scalar(@target); $t++)
{
$relpath .= "/$target[$t]";
}
# Clean up result (remove double slash, trailing slash, trailing slash-dot).
$relpath =~ s%//%/%;
$relpath =~ s%/$%%;
$relpath =~ s%/\.$%%;
print "source = $ARGV[0]\n";
print "target = $ARGV[1]\n";
print "relpath = $relpath\n";
测试脚本(方括号包含一个空格和一个制表符):
sed 's/#.*//;/^[ ]*$/d' <<! |
/home/part1/part2 /home/part1/part3
/home/part1/part2 /home/part4/part5
/home/part1/part2 /work/part6/part7
/home/part1 /work/part1/part2/part3/part4
/home /work/part2/part3
/ /work/part2/part3/part4
/home/part1/part2 /home/part1/part2/part3/part4
/home/part1/part2 /home/part1/part2/part3
/home/part1/part2 /home/part1/part2
/home/part1/part2 /home/part1
/home/part1/part2 /home
/home/part1/part2 /
/home/part1/part2 /work
/home/part1/part2 /work/part1
/home/part1/part2 /work/part1/part2
/home/part1/part2 /work/part1/part2/part3
/home/part1/part2 /work/part1/part2/part3/part4
home/part1/part2 home/part1/part3
home/part1/part2 home/part4/part5
home/part1/part2 work/part6/part7
home/part1 work/part1/part2/part3/part4
home work/part2/part3
. work/part2/part3
home/part1/part2 home/part1/part2/part3/part4
home/part1/part2 home/part1/part2/part3
home/part1/part2 home/part1/part2
home/part1/part2 home/part1
home/part1/part2 home
home/part1/part2 .
home/part1/part2 work
home/part1/part2 work/part1
home/part1/part2 work/part1/part2
home/part1/part2 work/part1/part2/part3
home/part1/part2 work/part1/part2/part3/part4
!
while read source target
do
perl relpath.pl $source $target
echo
done
测试脚本的输出:
source = /home/part1/part2
target = /home/part1/part3
relpath = ../part3
source = /home/part1/part2
target = /home/part4/part5
relpath = ../../part4/part5
source = /home/part1/part2
target = /work/part6/part7
relpath = ../../../work/part6/part7
source = /home/part1
target = /work/part1/part2/part3/part4
relpath = ../../work/part1/part2/part3/part4
source = /home
target = /work/part2/part3
relpath = ../work/part2/part3
source = /
target = /work/part2/part3/part4
relpath = ./work/part2/part3/part4
source = /home/part1/part2
target = /home/part1/part2/part3/part4
relpath = ./part3/part4
source = /home/part1/part2
target = /home/part1/part2/part3
relpath = ./part3
source = /home/part1/part2
target = /home/part1/part2
relpath = .
source = /home/part1/part2
target = /home/part1
relpath = ..
source = /home/part1/part2
target = /home
relpath = ../..
source = /home/part1/part2
target = /
relpath = ../../../..
source = /home/part1/part2
target = /work
relpath = ../../../work
source = /home/part1/part2
target = /work/part1
relpath = ../../../work/part1
source = /home/part1/part2
target = /work/part1/part2
relpath = ../../../work/part1/part2
source = /home/part1/part2
target = /work/part1/part2/part3
relpath = ../../../work/part1/part2/part3
source = /home/part1/part2
target = /work/part1/part2/part3/part4
relpath = ../../../work/part1/part2/part3/part4
source = home/part1/part2
target = home/part1/part3
relpath = ../part3
source = home/part1/part2
target = home/part4/part5
relpath = ../../part4/part5
source = home/part1/part2
target = work/part6/part7
relpath = ../../../work/part6/part7
source = home/part1
target = work/part1/part2/part3/part4
relpath = ../../work/part1/part2/part3/part4
source = home
target = work/part2/part3
relpath = ../work/part2/part3
source = .
target = work/part2/part3
relpath = ../work/part2/part3
source = home/part1/part2
target = home/part1/part2/part3/part4
relpath = ./part3/part4
source = home/part1/part2
target = home/part1/part2/part3
relpath = ./part3
source = home/part1/part2
target = home/part1/part2
relpath = .
source = home/part1/part2
target = home/part1
relpath = ..
source = home/part1/part2
target = home
relpath = ../..
source = home/part1/part2
target = .
relpath = ../../..
source = home/part1/part2
target = work
relpath = ../../../work
source = home/part1/part2
target = work/part1
relpath = ../../../work/part1
source = home/part1/part2
target = work/part1/part2
relpath = ../../../work/part1/part2
source = home/part1/part2
target = work/part1/part2/part3
relpath = ../../../work/part1/part2/part3
source = home/part1/part2
target = work/part1/part2/part3/part4
relpath = ../../../work/part1/part2/part3/part4
面对奇怪的输入，这个Perl脚本在Unix上运行得相当彻底(它没有考虑Windows路径名的所有复杂性)。它使用模块Cwd及其函数realpath解析存在的名称的真实路径，并对不存在的路径进行文本分析。在所有情况下，除了一种情况，它产生的输出都与Dennis的脚本相同。越轨的情况是:
source = home/part1/part2
target = .
relpath1 = ../../..
relpath2 = ../../../.
这两个结果是等价的，只是不完全相同。(输出来自测试脚本的一个轻微修改版本——下面的Perl脚本只是输出答案，而不是像上面的脚本那样输出输入和答案。)现在，我应该排除无效的答案吗?也许……
#!/bin/perl -w
# Based loosely on code from: http://unix.derkeiler.com/Newsgroups/comp.unix.shell/2005-10/1256.html
# Via: http://stackoverflow.com/questions/2564634
use strict;
die "Usage: $0 from to\n" if scalar @ARGV != 2;
use Cwd qw(realpath getcwd);
my $pwd;
my $verbose = 0;
# Fettle filename so it is absolute.
# Deals with '//', '/./' and '/../' notations, plus symlinks.
# The realpath() function does the hard work if the path exists.
# For non-existent paths, the code does a purely textual hack.
sub resolve
{
my($name) = @_;
my($path) = realpath($name);
if (!defined $path)
{
# Path does not exist - do the best we can with lexical analysis
# Assume Unix - not dealing with Windows.
$path = $name;
if ($name !~ m%^/%)
{
$pwd = getcwd if !defined $pwd;
$path = "$pwd/$path";
}
$path =~ s%//+%/%g; # Not UNC paths.
$path =~ s%/$%%; # No trailing /
$path =~ s%/\./%/%g; # No embedded /./
# Try to eliminate /../abc/
$path =~ s%/\.\./(?:[^/]+)(/|$)%$1%g;
$path =~ s%/\.$%%; # No trailing /.
$path =~ s%^\./%%; # No leading ./
# What happens with . and / as inputs?
}
return($path);
}
sub print_result
{
my($source, $target, $relpath) = @_;
if ($verbose)
{
print "source = $ARGV[0]\n";
print "target = $ARGV[1]\n";
print "relpath = $relpath\n";
}
else
{
print "$relpath\n";
}
exit 0;
}
my($source) = resolve($ARGV[0]);
my($target) = resolve($ARGV[1]);
print_result($source, $target, ".") if ($source eq $target);
# Split!
my(@source) = split '/', $source;
my(@target) = split '/', $target;
my $count = scalar(@source);
$count = scalar(@target) if (scalar(@target) < $count);
my $relpath = "";
my $i;
# Both paths are absolute; Perl splits an empty field 0.
for ($i = 1; $i < $count; $i++)
{
last if $source[$i] ne $target[$i];
}
for (my $s = $i; $s < scalar(@source); $s++)
{
$relpath = "$relpath/" if ($s > $i);
$relpath = "$relpath..";
}
for (my $t = $i; $t < scalar(@target); $t++)
{
$relpath = "$relpath/" if ($relpath ne "");
$relpath = "$relpath$target[$t]";
}
print_result($source, $target, $relpath);
Python的os.path.relpath作为shell函数
这个relpath练习的目标是模仿xni提出的Python 2.7的os.path.relpath函数(从Python 2.6版可用，但只能在2.7中正常工作)。因此，一些结果可能与其他答案中提供的函数不同。
(我没有在路径中测试换行符，因为它破坏了基于从ZSH调用python -c的验证。经过一些努力，这当然是可能的。)
关于Bash中的“魔法”，我很久以前就放弃了在Bash中寻找魔法，但我已经在ZSH中找到了我需要的所有魔法，然后是一些。
因此，我提出了两种实现。
第一个实现的目标是完全兼容posix。我已经在Debian 6.0.6的“挤压”上用/bin/dash测试了它。它还可以在OS X 10.8.3上完美地与/bin/sh一起工作，这实际上是伪装成POSIX shell的Bash版本3.2。
第二个实现是一个ZSH shell函数，它对路径中的多个斜杠和其他麻烦具有健壮性。如果您有可用的ZSH，这是推荐的版本，即使您是在下面给出的脚本形式中调用它(即使用#!/usr/bin/env zsh)。
最后，我编写了一个ZSH脚本，根据其他答案中提供的测试用例，验证$PATH中relpath命令的输出。我通过添加一些空格、制表符和标点符号，例如!？ *在这里和那里，还抛出了另一个测试与vim-powerline中发现的奇异UTF-8字符。
POSIX外壳函数
首先，posix兼容的shell函数。它适用于各种路径，但不清除多个斜杠或解析符号链接。
#!/bin/sh
relpath () {
[ $# -ge 1 ] && [ $# -le 2 ] || return 1
current="${2:+"$1"}"
target="${2:-"$1"}"
[ "$target" != . ] || target=/
target="/${target##/}"
[ "$current" != . ] || current=/
current="${current:="/"}"
current="/${current##/}"
appendix="${target##/}"
relative=''
while appendix="${target#"$current"/}"
[ "$current" != '/' ] && [ "$appendix" = "$target" ]; do
if [ "$current" = "$appendix" ]; then
relative="${relative:-.}"
echo "${relative#/}"
return 0
fi
current="${current%/*}"
relative="$relative${relative:+/}.."
done
relative="$relative${relative:+${appendix:+/}}${appendix#/}"
echo "$relative"
}
relpath "$@"
ZSH壳函数
现在是更加健壮的zsh版本。如果您希望它将参数解析为真实路径à la realpath -f(在Linux coreutils包中可用)，请将第3行和第4行上的:a替换为:a。
要在zsh中使用它，请删除第一行和最后一行，并将其放在$FPATH变量中的目录中。
#!/usr/bin/env zsh
relpath () {
[[ $# -ge 1 ]] && [[ $# -le 2 ]] || return 1
local target=${${2:-$1}:a} # replace `:a' by `:A` to resolve symlinks
local current=${${${2:+$1}:-$PWD}:a} # replace `:a' by `:A` to resolve symlinks
local appendix=${target#/}
local relative=''
while appendix=${target#$current/}
[[ $current != '/' ]] && [[ $appendix = $target ]]; do
if [[ $current = $appendix ]]; then
relative=${relative:-.}
print ${relative#/}
return 0
fi
current=${current%/*}
relative="$relative${relative:+/}.."
done
relative+=${relative:+${appendix:+/}}${appendix#/}
print $relative
}
relpath "$@"
测试脚本
最后是测试脚本。它接受一个选项，即-v来启用详细输出。
#!/usr/bin/env zsh
set -eu
VERBOSE=false
script_name=$(basename $0)
usage () {
print "\n Usage: $script_name SRC_PATH DESTINATION_PATH\n" >&2
exit ${1:=1}
}
vrb () { $VERBOSE && print -P ${(%)@} || return 0; }
relpath_check () {
[[ $# -ge 1 ]] && [[ $# -le 2 ]] || return 1
target=${${2:-$1}}
prefix=${${${2:+$1}:-$PWD}}
result=$(relpath $prefix $target)
# Compare with python's os.path.relpath function
py_result=$(python -c "import os.path; print os.path.relpath('$target', '$prefix')")
col='%F{green}'
if [[ $result != $py_result ]] && col='%F{red}' || $VERBOSE; then
print -P "${col}Source: '$prefix'\nDestination: '$target'%f"
print -P "${col}relpath: ${(qq)result}%f"
print -P "${col}python: ${(qq)py_result}%f\n"
fi
}
run_checks () {
print "Running checks..."
relpath_check '/ a b/å/⮀*/!' '/ a b/å/⮀/xäå/?'
relpath_check '/' '/A'
relpath_check '/A' '/'
relpath_check '/ & / !/*/\\/E' '/'
relpath_check '/' '/ & / !/*/\\/E'
relpath_check '/ & / !/*/\\/E' '/ & / !/?/\\/E/F'
relpath_check '/X/Y' '/ & / !/C/\\/E/F'
relpath_check '/ & / !/C' '/A'
relpath_check '/A / !/C' '/A /B'
relpath_check '/Â/ !/C' '/Â/ !/C'
relpath_check '/ & /B / C' '/ & /B / C/D'
relpath_check '/ & / !/C' '/ & / !/C/\\/Ê'
relpath_check '/Å/ !/C' '/Å/ !/D'
relpath_check '/.A /*B/C' '/.A /*B/\\/E'
relpath_check '/ & / !/C' '/ & /D'
relpath_check '/ & / !/C' '/ & /\\/E'
relpath_check '/ & / !/C' '/\\/E/F'
relpath_check /home/part1/part2 /home/part1/part3
relpath_check /home/part1/part2 /home/part4/part5
relpath_check /home/part1/part2 /work/part6/part7
relpath_check /home/part1 /work/part1/part2/part3/part4
relpath_check /home /work/part2/part3
relpath_check / /work/part2/part3/part4
relpath_check /home/part1/part2 /home/part1/part2/part3/part4
relpath_check /home/part1/part2 /home/part1/part2/part3
relpath_check /home/part1/part2 /home/part1/part2
relpath_check /home/part1/part2 /home/part1
relpath_check /home/part1/part2 /home
relpath_check /home/part1/part2 /
relpath_check /home/part1/part2 /work
relpath_check /home/part1/part2 /work/part1
relpath_check /home/part1/part2 /work/part1/part2
relpath_check /home/part1/part2 /work/part1/part2/part3
relpath_check /home/part1/part2 /work/part1/part2/part3/part4
relpath_check home/part1/part2 home/part1/part3
relpath_check home/part1/part2 home/part4/part5
relpath_check home/part1/part2 work/part6/part7
relpath_check home/part1 work/part1/part2/part3/part4
relpath_check home work/part2/part3
relpath_check . work/part2/part3
relpath_check home/part1/part2 home/part1/part2/part3/part4
relpath_check home/part1/part2 home/part1/part2/part3
relpath_check home/part1/part2 home/part1/part2
relpath_check home/part1/part2 home/part1
relpath_check home/part1/part2 home
relpath_check home/part1/part2 .
relpath_check home/part1/part2 work
relpath_check home/part1/part2 work/part1
relpath_check home/part1/part2 work/part1/part2
relpath_check home/part1/part2 work/part1/part2/part3
relpath_check home/part1/part2 work/part1/part2/part3/part4
print "Done with checks."
}
if [[ $# -gt 0 ]] && [[ $1 = "-v" ]]; then
VERBOSE=true
shift
fi
if [[ $# -eq 0 ]]; then
run_checks
else
VERBOSE=true
relpath_check "$@"
fi
自2001年以来，它被内置到Perl中，因此它几乎可以在您能想象到的所有系统上工作，甚至VMS。
perl -le 'use File::Spec; print File::Spec->abs2rel(@ARGV)' FILE BASE
而且，解决方案很容易理解。
举个例子:
perl -le 'use File::Spec; print File::Spec->abs2rel(@ARGV)' $absolute $current
.．.会很好。
另一个解决方案，纯bash + GNU readlink，在以下上下文中易于使用:
ln -s "$(relpath "$A" "$B")" "$B"
编辑:确保“$B”是不存在或没有软链接在这种情况下，否则relpath遵循这个链接，这不是你想要的!
这几乎适用于当前所有的Linux。如果readlink -m在您这边不起作用，请尝试readlink -f。请参见https://gist.github.com/hilbix/1ec361d00a8178ae8ea0查看可能的更新:
: relpath A B
# Calculate relative path from A to B, returns true on success
# Example: ln -s "$(relpath "$A" "$B")" "$B"
relpath()
{
local X Y A
# We can create dangling softlinks
X="$(readlink -m -- "$1")" || return
Y="$(readlink -m -- "$2")" || return
X="${X%/}/"
A=""
while Y="${Y%/*}"
[ ".${X#"$Y"/}" = ".$X" ]
do
A="../$A"
done
X="$A${X#"$Y"/}"
X="${X%/}"
echo "${X:-.}"
}
注:
Care was taken that it is safe against unwanted shell meta character expansion, in case filenames contain * or ?. The output is meant to be usable as the first argument to ln -s: relpath / / gives . and not the empty string relpath a a gives a, even if a happens to be a directory Most common cases were tested to give reasonable results, too. This solution uses string prefix matching, hence readlink is required to canonicalize paths. Thanks to readlink -m it works for not yet existing paths, too.
在旧系统上，readlink -m不可用，如果文件不存在，readlink -f将失败。所以你可能需要一些像这样的解决方法(未经测试!):
readlink_missing()
{
readlink -m -- "$1" && return
readlink -f -- "$1" && return
[ -e . ] && echo "$(readlink_missing "$(dirname "$1")")/$(basename "$1")"
}
这在$1包含的情况下是不正确的。或. .对于不存在的路径(如/doesnotexist/./a)，但它应该涵盖大多数情况。
(用readlink_missing替换上面的readlink -m——)
编辑，因为下面是反对票
下面是一个测试，这个函数确实是正确的:
check()
{
res="$(relpath "$2" "$1")"
[ ".$res" = ".$3" ] && return
printf ':WRONG: %-10q %-10q gives %q\nCORRECT %-10q %-10q gives %q\n' "$1" "$2" "$res" "$@"
}
# TARGET SOURCE RESULT
check "/A/B/C" "/A" ".."
check "/A/B/C" "/A.x" "../../A.x"
check "/A/B/C" "/A/B" "."
check "/A/B/C" "/A/B/C" "C"
check "/A/B/C" "/A/B/C/D" "C/D"
check "/A/B/C" "/A/B/C/D/E" "C/D/E"
check "/A/B/C" "/A/B/D" "D"
check "/A/B/C" "/A/B/D/E" "D/E"
check "/A/B/C" "/A/D" "../D"
check "/A/B/C" "/A/D/E" "../D/E"
check "/A/B/C" "/D/E/F" "../../D/E/F"
check "/foo/baz/moo" "/foo/bar" "../bar"
困惑吗?好吧，这是正确的结果!即使你认为它不符合问题，以下是正确的证明:
check "http://example.com/foo/baz/moo" "http://example.com/foo/bar" "../bar"
毫无疑问，……/bar是从页面moo中看到的页面栏的准确且唯一正确的相对路径。其他一切都是完全错误的。
采用问题的输出很简单，显然假设current是一个目录:
absolute="/foo/bar"
current="/foo/baz/foo"
relative="../$(relpath "$absolute" "$current")"
这将返回所请求的内容。
在你感到惊讶之前，这里有一个稍微复杂一点的relpath变体(注意细微的区别)，它也应该适用于url语法(因此，由于一些bash魔法，末尾/幸存下来):
# Calculate relative PATH to the given DEST from the given BASE
# In the URL case, both URLs must be absolute and have the same Scheme.
# The `SCHEME:` must not be present in the FS either.
# This way this routine works for file paths an
: relpathurl DEST BASE
relpathurl()
{
local X Y A
# We can create dangling softlinks
X="$(readlink -m -- "$1")" || return
Y="$(readlink -m -- "$2")" || return
X="${X%/}/${1#"${1%/}"}"
Y="${Y%/}${2#"${2%/}"}"
A=""
while Y="${Y%/*}"
[ ".${X#"$Y"/}" = ".$X" ]
do
A="../$A"
done
X="$A${X#"$Y"/}"
X="${X%/}"
echo "${X:-.}"
}
这里有一些检查，只是为了弄清楚:它确实像所说的那样工作。
check()
{
res="$(relpathurl "$2" "$1")"
[ ".$res" = ".$3" ] && return
printf ':WRONG: %-10q %-10q gives %q\nCORRECT %-10q %-10q gives %q\n' "$1" "$2" "$res" "$@"
}
# TARGET SOURCE RESULT
check "/A/B/C" "/A" ".."
check "/A/B/C" "/A.x" "../../A.x"
check "/A/B/C" "/A/B" "."
check "/A/B/C" "/A/B/C" "C"
check "/A/B/C" "/A/B/C/D" "C/D"
check "/A/B/C" "/A/B/C/D/E" "C/D/E"
check "/A/B/C" "/A/B/D" "D"
check "/A/B/C" "/A/B/D/E" "D/E"
check "/A/B/C" "/A/D" "../D"
check "/A/B/C" "/A/D/E" "../D/E"
check "/A/B/C" "/D/E/F" "../../D/E/F"
check "/foo/baz/moo" "/foo/bar" "../bar"
check "http://example.com/foo/baz/moo" "http://example.com/foo/bar" "../bar"
check "http://example.com/foo/baz/moo/" "http://example.com/foo/bar" "../../bar"
check "http://example.com/foo/baz/moo" "http://example.com/foo/bar/" "../bar/"
check "http://example.com/foo/baz/moo/" "http://example.com/foo/bar/" "../../bar/"
下面是如何用它从问题中得到想要的结果:
absolute="/foo/bar"
current="/foo/baz/foo"
relative="$(relpathurl "$absolute" "$current/")"
echo "$relative"
如果你发现什么东西不起作用，请在下面的评论中告诉我。谢谢。
PS:
为什么relpath的论点与这里的所有其他答案相反?
如果你改变
Y="$(readlink -m -- "$2")" || return
to
Y="$(readlink -m -- "${2:-"$PWD"}")" || return
然后你可以去掉第二个参数，这样BASE就是当前目录/URL/任何东西。这只是Unix的原则。
$ python -c "import os.path; print os.path.relpath('/foo/bar', '/foo/baz/foo')"
给:
../../bar
