如何在没有readlink或realpath的情况下递归解析符号链接？

Question

什么是最好的便携式(POSIX？)方法来脚本找到链接的目标,如果readlink和realpath不可用？

你会ls -l,如果它开始时带有l文本->,sed并重复,直到它不再开始l？

Answer 1

Per BashFAQ#29(也赞同@EugeniuRosca建议的GNU查找方法):

一种广泛可用(但不是纯POSIX)选项是使用perl:

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

如果保证符号链接的名称不包含->,则可以解析输出ls.

以下代码结合了两种方法:

# 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" ||:
}