How to recursively resolve symlinks without readlink or realpath?

问题

What's the best portable (POSIX?) way to script finding the target for a link if readlink and realpath are not available?

Would you ls -l, and if it starts with l take the text after the -> with sed and repeat until it no longer starts with l ?

回答1:

Per BashFAQ #29 (which also endorses the GNU find approach suggested by @EugeniuRosca):

One widely available (though not pure-POSIX) option is to use perl:

target=/path/to/symlink-name perl -le 'print readlink $ENV{target}'

If your symlink's name is guaranteed not to contain ->, you can parse the output of ls.

---

The below code combines both approaches:

# define the best readlink function available for this platform
if command -v readlink >/dev/null 2>/dev/null; then
  # first choice: Use the real readlink command
  readlink() {
    command readlink -- "$@"
  }
elif find . -maxdepth 0 -printf '%l' >/dev/null 2>/dev/null; then
  # second choice: use GNU find
  readlink() {
    local ll candidate >/dev/null 2>&1 ||:
    if candidate=$(find "$1" -maxdepth 0 -printf '%l') && [ "$candidate" ]; then
      printf '%s\n' "$candidate"
    else
      printf '%s\n' "$1"
    fi
  }
elif command -v perl >/dev/null 2>/dev/null; then
  # third choice: use perl
  readlink() {
    local candidate ||:
    candidate=$(target=$1 perl -le 'print readlink $ENV{target}')
    if [ "$candidate" ]; then
      printf '%s\n' "$candidate"
    else
      printf '%s\n' "$1"
    fi
  }
else
  # fourth choice: parse ls -ld
  readlink() {
    local ll candidate >/dev/null 2>&1 ||:
    ll=$(LC_ALL=C ls -ld -- "$1" 2>/dev/null)
    candidate=${ll#* -> }
    if [ "$candidate" = "$ll" ]; then
      printf '%s\n' "$1"
    else
      printf '%s\n' "$candidate"
    fi
  }
fi

readlink_recursive() {
    local path prev_path oldwd found_recursion >/dev/null 2>&1 ||:
    oldwd=$PWD; path=$1; found_recursion=0

    while [ -L "$path" ] && [ "$found_recursion" = 0 ]; do
        if [ "$path" != "${path%/*}" ]; then
          cd -- "${path%/*}" || {
            cd -- "$oldwd" ||:
            echo "ERROR: Directory '${path%/*}' does not exist in '$PWD'" >&2
            return 1
          }
          path=${PWD}/${path##*/}
        fi
        path=$(readlink "$path")
        if [ -d "$path" ]; then
          cd -- "$path"
          path=$PWD
          break
        fi
        if [ "$path" != "${path%/*}" ]; then
          cd -- "${path%/*}" || {
            echo "ERROR: Could not traverse from $PWD to ${path%/*}" >&2
            return 1
          }
          path=${PWD}/${path##*/}
        elif [ "$PWD" != "$oldwd" ]; then
          path=${PWD}/$path
        fi
        for prev_path; do
          if [ "$path" = "$prev_path" ]; then
            found_recursion=1
            break
          fi
        done
        set -- "$path" "$@" # record path for recursion check
    done

    if [ "$path" != "${path%/../*}" ]; then
      cd "${path%/*}" || {
        echo "ERROR: Directory '${path%/*}' does not exist in $PWD" >&2
        return 1
      }
      printf '%s\n' "$PWD/${path##*/}"
    else
      printf '%s\n' "$path"
    fi
    cd -- "$oldwd" ||:
}

回答2:

LIMITATION: -printf is not a POSIX-specified option

#!/bin/bash

tmp=<symlink-name>
tmp1=''
while tmp=$(find "$tmp" -prune -printf "%l" 2>/dev/null); do
    target="$tmp1" && tmp1="$tmp"
done;
echo "$target"

回答3:

Here is another solution that is very similar to Charles Duffy's. I'm not that experienced so there may be some non-POSIX or performance issues. I arrived at this after looking at Charles' solution and replacing anything I didn't understand :-P It's very possible that after any issues here are fixed, you end up with Charles' solution all over again.

resolve() {
    local arg path absolute ll dir prev_path oldwd found_recursion base >/dev/null 2>&1 ||:
    arg="$1"; path="$1"; oldwd=$PWD; found_recursion=0
    dir=$(dirname "$path")
    cd -- "$dir" || {
        cd -- "$oldwd" ||:
        echo "While resolving '$arg' could not go to '$dir'" >&2
        return 1
    }
    if [ $PWD = "/" ]; then
        absolute="/$(basename $path)"
    else
        absolute="$PWD/$(basename $path)"
    fi
    [ "$path" != "$absolute" ] && set -- "$absolute"
    while [ -L "$absolute" ] && [ "$found_recursion" = 0 ]; do
        ll=$(LC_ALL=C \ls -ld -- "$absolute" 2>/dev/null)
        path=${ll#* -> }
        dir=$(dirname "$path")
        cd -- "$dir" || {
            cd -- "$oldwd" ||:
            echo "While resolving '$arg' could not go to '$dir'" >&2
            return 1
        }
        base=$(basename "$path")
        absolute="$PWD/$base"
        for prev_path; do
            if [ "$absolute" = "$prev_path" ]; then
                found_recursion=1
                break
            fi
        done
        set -- "$absolute" "$@"
    done
    if [ -d "$absolute" ]; then
        cd -- "$absolute" || {
            cd -- "$oldwd" ||:
            echo "While resolving '$arg' could not go to '$absolute'" >&2
            return 1
        }
        printf '%s\n' "$PWD"
    else
        printf '%s\n' "$absolute"
    fi
}

Edit: now using $PWD and printf, canonicalize result if directory.

来源：https://stackoverflow.com/questions/31596363/how-to-recursively-resolve-symlinks-without-readlink-or-realpath