这是对@pini目前评分最高的解决方案(遗憾的是，它只处理少数情况)的更正，全功能改进

提醒:'-z'测试如果字符串是零长度(=空)，'-n'测试如果字符串不是空。

# both $1 and $2 are absolute paths beginning with /
# returns relative path to $2/$target from $1/$source
source=$1
target=$2

common_part=$source # for now
result="" # for now

while [[ "${target#$common_part}" == "${target}" ]]; do
    # no match, means that candidate common part is not correct
    # go up one level (reduce common part)
    common_part="$(dirname $common_part)"
    # and record that we went back, with correct / handling
    if [[ -z $result ]]; then
        result=".."
    else
        result="../$result"
    fi
done

if [[ $common_part == "/" ]]; then
    # special case for root (no common path)
    result="$result/"
fi

# since we now have identified the common part,
# compute the non-common part
forward_part="${target#$common_part}"

# and now stick all parts together
if [[ -n $result ]] && [[ -n $forward_part ]]; then
    result="$result$forward_part"
elif [[ -n $forward_part ]]; then
    # extra slash removal
    result="${forward_part:1}"
fi

echo $result

测试用例:

compute_relative.sh "/A/B/C" "/A"           -->  "../.."
compute_relative.sh "/A/B/C" "/A/B"         -->  ".."
compute_relative.sh "/A/B/C" "/A/B/C"       -->  ""
compute_relative.sh "/A/B/C" "/A/B/C/D"     -->  "D"
compute_relative.sh "/A/B/C" "/A/B/C/D/E"   -->  "D/E"
compute_relative.sh "/A/B/C" "/A/B/D"       -->  "../D"
compute_relative.sh "/A/B/C" "/A/B/D/E"     -->  "../D/E"
compute_relative.sh "/A/B/C" "/A/D"         -->  "../../D"
compute_relative.sh "/A/B/C" "/A/D/E"       -->  "../../D/E"
compute_relative.sh "/A/B/C" "/D/E/F"       -->  "../../../D/E/F"

$ python -c "import os.path; print os.path.relpath('/foo/bar', '/foo/baz/foo')"

给:

../../bar

我将只使用Perl来完成这个不那么简单的任务:

absolute="/foo/bar"
current="/foo/baz/foo"

# Perl is magic
relative=$(perl -MFile::Spec -e 'print File::Spec->abs2rel("'$absolute'","'$current'")')

我使用的macOS默认情况下没有realpath命令，所以我做了一个纯bash函数来计算它。

#!/bin/bash

##
# print a relative path from "source folder" to "target file"
#
# params:
#  $1 - target file, can be a relative path or an absolute path.
#  $2 - source folder, can be a relative path or an absolute path.
#
# test:
#  $ mkdir -p ~/A/B/C/D; touch ~/A/B/C/D/testfile.txt; touch ~/A/B/testfile.txt
#
#  $ getRelativePath ~/A/B/C/D/testfile.txt  ~/A/B
#  $ C/D/testfile.txt
#  
#  $ getRelativePath ~/A/B/testfile.txt  ~/A/B/C
#  $ ../testfile.txt
#
#  $ getRelativePath ~/A/B/testfile.txt  /
#  $ home/bunnier/A/B/testfile.txt 
#
function getRelativePath(){
    local targetFilename=$(basename $1)
    local targetFolder=$(cd $(dirname $1);pwd) # absolute target folder path
    local currentFolder=$(cd $2;pwd) # absulute source folder
    local result=.

    while [ "$currentFolder" != "$targetFolder" ];do
      if [[ "$targetFolder" =~ "$currentFolder"* ]];then
          pointSegment=${targetFolder#$currentFolder}
          result=$result/${pointSegment#/}
          break
      fi  
      result="$result"/..
      currentFolder=$(dirname $currentFolder)
    done

    result=$result/$targetFilename
    echo ${result#./}
}

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"

在bash中:

realDir=''
cd $(dirname $0) || exit
realDir=$(pwd)
cd -
echo $realDir