How can I get the behavior of GNU's readlink -f on a Mac?

Question

On Linux, the readlink utility accepts an option -f that follows additional links. This doesn't seem to work on Mac and possibly BSD based systems. What would the equivalent be?

Here's some debug information:

$ which readlink; readlink -f
/usr/bin/readlink
readlink: illegal option -f
usage: readlink [-n] [file ...]

Answer 1

readlink -f does two things:

1. It iterates along a sequence of symlinks until it finds an actual file.
2. It returns that file's canonicalized name—i.e., its absolute pathname.

If you want to, you can just build a shell script that uses vanilla readlink behavior to achieve the same thing. Here's an example. Obviously you could insert this in your own script where you'd like to call readlink -f

#!/bin/sh

TARGET_FILE=$1

cd `dirname $TARGET_FILE`
TARGET_FILE=`basename $TARGET_FILE`

# Iterate down a (possible) chain of symlinks
while [ -L "$TARGET_FILE" ]
do
    TARGET_FILE=`readlink $TARGET_FILE`
    cd `dirname $TARGET_FILE`
    TARGET_FILE=`basename $TARGET_FILE`
done

# Compute the canonicalized name by finding the physical path 
# for the directory we're in and appending the target file.
PHYS_DIR=`pwd -P`
RESULT=$PHYS_DIR/$TARGET_FILE
echo $RESULT

Note that this doesn't include any error handling. Of particular importance, it doesn't detect symlink cycles. A simple way to do this would be to count the number of times you go around the loop and fail if you hit an improbably large number, such as 1,000.

EDITED to use pwd -P instead of $PWD.

Note that this script expects to be called like ./script_name filename, no -f, change $1 to $2 if you want to be able to use with -f filename like GNU readlink.

Answer 2

MacPorts and Homebrew provide a coreutils package containing greadlink (GNU readlink). Credit to Michael Kallweitt post in mackb.com.

brew install coreutils

greadlink -f file.txt

Answer 3

You may be interested in realpath(3), or Python's os.path.realpath. The two aren't exactly the same; the C library call requires that intermediary path components exist, while the Python version does not.

$ pwd
/tmp/foo
$ ls -l
total 16
-rw-r--r--  1 miles    wheel  0 Jul 11 21:08 a
lrwxr-xr-x  1 miles    wheel  1 Jul 11 20:49 b -> a
lrwxr-xr-x  1 miles    wheel  1 Jul 11 20:49 c -> b
$ python -c 'import os,sys;print(os.path.realpath(sys.argv[1]))' c
/private/tmp/foo/a

I know you said you'd prefer something more lightweight than another scripting language, but just in case compiling a binary is insufferable, you can use Python and ctypes (available on Mac OS X 10.5) to wrap the library call:

#!/usr/bin/python

import ctypes, sys

libc = ctypes.CDLL('libc.dylib')
libc.realpath.restype = ctypes.c_char_p
libc.__error.restype = ctypes.POINTER(ctypes.c_int)
libc.strerror.restype = ctypes.c_char_p

def realpath(path):
    buffer = ctypes.create_string_buffer(1024) # PATH_MAX
    if libc.realpath(path, buffer):
        return buffer.value
    else:
        errno = libc.__error().contents.value
        raise OSError(errno, "%s: %s" % (libc.strerror(errno), buffer.value))

if __name__ == '__main__':
    print realpath(sys.argv[1])

Ironically, the C version of this script ought to be shorter. :)

Answer 4

I hate to pile on with yet another implementation, but I needed a) a portable, pure shell implementation, and b) unit-test coverage, as the number of edge-cases for something like this are non-trivial.

See my project on Github for tests and full code. What follows is a synopsis of the implementation:

As Keith Smith astutely points out, readlink -f does two things: 1) resolves symlinks recursively, and 2) canonicalizes the result, hence:

realpath() {
    canonicalize_path "$(resolve_symlinks "$1")"
}

First, the symlink resolver implementation:

resolve_symlinks() {
    local dir_context path
    path=$(readlink -- "$1")
    if [ $? -eq 0 ]; then
        dir_context=$(dirname -- "$1")
        resolve_symlinks "$(_prepend_path_if_relative "$dir_context" "$path")"
    else
        printf '%s\n' "$1"
    fi
}

_prepend_path_if_relative() {
    case "$2" in
        /* ) printf '%s\n' "$2" ;;
         * ) printf '%s\n' "$1/$2" ;;
    esac 
}

Note that this is a slightly simplified version of the full implementation. The full implementation adds a small check for symlink cycles, as well as massages the output a bit.

Finally, the function for canonicalizing a path:

canonicalize_path() {
    if [ -d "$1" ]; then
        _canonicalize_dir_path "$1"
    else
        _canonicalize_file_path "$1"
    fi
}   

_canonicalize_dir_path() {
    (cd "$1" 2>/dev/null && pwd -P) 
}           

_canonicalize_file_path() {
    local dir file
    dir=$(dirname -- "$1")
    file=$(basename -- "$1")
    (cd "$dir" 2>/dev/null && printf '%s/%s\n' "$(pwd -P)" "$file")
}

That's it, more or less. Simple enough to paste into your script, but tricky enough that you'd be crazy to rely on any code that doesn't have unit tests for your use cases.

Answer 5

1. Install homebrew
2. Run "brew install coreutils"
3. Run "greadlink -f path"

greadlink is the gnu readlink that implements -f. You can use macports or others as well, I prefer homebrew.

Answer 6

A simple one-liner in perl that's sure to work almost everywhere without any external dependencies:

perl -MCwd -e 'print Cwd::abs_path shift' ~/non-absolute/file

Will dereference symlinks.

Usage in a script could be like this:

readlinkf(){ perl -MCwd -e 'print Cwd::abs_path shift' "$1";}
ABSPATH="$(readlinkf ./non-absolute/file)"

Answer 7

I made a script called realpath personally which looks a little something like:

#!/usr/bin/env python
import os.sys
print os.path.realpath(sys.argv[1])

Answer 8

What about this?

function readlink() {
  DIR="${1%/*}"
  (cd "$DIR" && echo "$(pwd -P)")
}

Answer 9

FreeBSD and OSX have a version of statderived from NetBSD.

You can adjust the output with format switches (see the manual pages at the links above).

%  cd  /service
%  ls -tal 
drwxr-xr-x 22 root wheel 27 Aug 25 10:41 ..
drwx------  3 root wheel  8 Jun 30 13:59 .s6-svscan
drwxr-xr-x  3 root wheel  5 Jun 30 13:34 .
lrwxr-xr-x  1 root wheel 30 Dec 13 2013 clockspeed-adjust -> /var/service/clockspeed-adjust
lrwxr-xr-x  1 root wheel 29 Dec 13 2013 clockspeed-speed -> /var/service/clockspeed-speed
% stat -f%R  clockspeed-adjust
/var/service/clockspeed-adjust
% stat -f%Y  clockspeed-adjust
/var/service/clockspeed-adjust

Some OS X versions of stat may lack the -f%R option for formats. In this case -stat -f%Y may suffice. The -f%Y option will show the target of a symlink, whereas -f%R shows the absolute pathname corresponding to the file.

EDIT:

If you're able to use Perl (Darwin/OS X comes installed with recent verions of perl) then:

perl -MCwd=abs_path -le 'print abs_path readlink(shift);' linkedfile.txt

will work.

Answer 10

Here is a portable shell function that should work in ANY Bourne comparable shell. It will resolve the relative path punctuation ".. or ." and dereference symbolic links.

If for some reason you do not have a realpath(1) command, or readlink(1) this can be aliased.

which realpath || alias realpath='real_path'

Enjoy:

real_path () {
  OIFS=$IFS
  IFS='/'
  for I in $1
  do
    # Resolve relative path punctuation.
    if [ "$I" = "." ] || [ -z "$I" ]
      then continue
    elif [ "$I" = ".." ]
      then FOO="${FOO%%/${FOO##*/}}"
           continue
      else FOO="${FOO}/${I}"
    fi

    ## Resolve symbolic links
    if [ -h "$FOO" ]
    then
    IFS=$OIFS
    set `ls -l "$FOO"`
    while shift ;
    do
      if [ "$1" = "->" ]
        then FOO=$2
             shift $#
             break
      fi
    done
    IFS='/'
    fi
  done
  IFS=$OIFS
  echo "$FOO"
}

also, just in case anybody is interested here is how to implement basename and dirname in 100% pure shell code:

## http://www.opengroup.org/onlinepubs/000095399/functions/dirname.html
# the dir name excludes the least portion behind the last slash.
dir_name () {
  echo "${1%/*}"
}

## http://www.opengroup.org/onlinepubs/000095399/functions/basename.html
# the base name excludes the greatest portion in front of the last slash.
base_name () {
  echo "${1##*/}"
}

You can find updated version of this shell code at my google site: http://sites.google.com/site/jdisnard/realpath

EDIT: This code is licensed under the terms of the 2-clause (freeBSD style) license. A copy of the license may be found by following the above hyperlink to my site.

Answer 11

The easiest way to solve this problem and enable the functionality of readlink on Mac w/ Homebrew installed or FreeBSD is to install 'coreutils' package. May also be necessary on certain Linux distributions and other POSIX OS.

For example, in FreeBSD 11, I installed by invoking:

# pkg install coreutils

On MacOS with Homebrew, the command would be:

$ brew install coreutils

Not really sure why the other answers are so complicated, that's all there is to it. The files aren't in a different place, they're just not installed yet.

Answer 12

Begin Update

This is such a frequent problem that we have put together a Bash 4 library for free use (MIT License) called realpath-lib. This is designed to emulate readlink -f by default and includes two test suites to verify (1) that it works for a given unix system and (2) against readlink -f if installed (but this is not required). Additionally, it can be used to investigate, identify and unwind deep, broken symlinks and circular references, so it can be a useful tool for diagnosing deeply-nested physical or symbolic directory and file problems. It can be found at github.com or bitbucket.org.

End Update

Another very compact and efficient solution that does not rely on anything but Bash is:

function get_realpath() {

    [[ ! -f "$1" ]] && return 1 # failure : file does not exist.
    [[ -n "$no_symlinks" ]] && local pwdp='pwd -P' || local pwdp='pwd' # do symlinks.
    echo "$( cd "$( echo "${1%/*}" )" 2>/dev/null; $pwdp )"/"${1##*/}" # echo result.
    return 0 # success

}

This also includes an environment setting no_symlinks that provides the ability to resolve symlinks to the physical system. As long as no_symlinks is set to something, ie no_symlinks='on' then symlinks will be resolved to the physical system. Otherwise they will be applied (the default setting).

This should work on any system that provides Bash, and will return a Bash compatible exit code for testing purposes.

Answer 13

There are already a lot of answers, but none worked for me... So this is what I'm using now.

readlink_f() {
  local target="$1"
  [ -f "$target" ] || return 1 #no nofile

  while [ -L "$target" ]; do
    target="$(readlink "$target")" 
  done
  echo "$(cd "$(dirname "$target")"; pwd -P)/$target"
}

Answer 14

Better late than never, I suppose. I was motivated to develop this specifically because my Fedora scripts weren't working on the Mac. The problem is dependencies and Bash. Macs don't have them, or if they do, they are often somewhere else (another path). Dependency path manipulation in a cross-platform Bash script is a headache at best and a security risk at worst - so it's best to avoid their use, if possible.

The function get_realpath() below is simple, Bash-centric, and no dependencies are required. I uses only the Bash builtins echo and cd. It is also fairly secure, as everything gets tested at each stage of the way and it returns error conditions.

If you don't want to follow symlinks, then put set -P at the front of the script, but otherwise cd should resolve the symlinks by default. It's been tested with file arguments that are {absolute | relative | symlink | local} and it returns the absolute path to the file. So far we've not had any problems with it.

function get_realpath() {

if [[ -f "$1" ]]
then
    # file *must* exist
    if cd "$(echo "${1%/*}")" &>/dev/null
    then
        # file *may* not be local
        # exception is ./file.ext
        # try 'cd .; cd -;' *works!*
        local tmppwd="$PWD"
        cd - &>/dev/null
    else
        # file *must* be local
        local tmppwd="$PWD"
    fi
else
    # file *cannot* exist
    return 1 # failure
fi

# reassemble realpath
echo "$tmppwd"/"${1##*/}"
return 0 # success

}

You can combine this with other functions get_dirname, get_filename, get_stemname and validate_path. These can be found at our GitHub repository as realpath-lib (full disclosure - this is our product but we offer it free to the community without any restrictions). It also could serve as a instructional tool - it's well documented.

We've tried our best to apply so-called 'modern Bash' practices, but Bash is a big subject and I'm certain there will always be room for improvement. It requires Bash 4+ but could be made to work with older versions if they are still around.

Answer 15

Truely platform-indpendent would be also this R-onliner

readlink(){ RScript -e "cat(normalizePath(commandArgs(T)[1]))" "$1";}

To actually mimic readlink -f <path>, $2 instead of $1 would need to be used.

Answer 16

Since my work is used by people with non-BSD Linux as well as macOS, I've opted for using these aliases in our build scripts (sed included since it has similar issues):

##
# If you're running macOS, use homebrew to install greadlink/gsed first:
#   brew install coreutils
#
# Example use:
#   # Gets the directory of the currently running script
#   dotfilesDir=$(dirname "$(globalReadlink -fm "$0")")
#   alias al='pico ${dotfilesDir}/aliases.local'
##

function globalReadlink () {
  # Use greadlink if on macOS; otherwise use normal readlink
  if [[ $OSTYPE == darwin* ]]; then
    greadlink "$@"
  else
    readlink "$@"
  fi
}

function globalSed () {
  # Use gsed if on macOS; otherwise use normal sed
  if [[ $OSTYPE == darwin* ]]; then
    gsed "$@"
  else
    sed "$@"
  fi
}

Optional check you could add to automatically install homebrew + coreutils dependencies:

if [[ "$OSTYPE" == "darwin"* ]]; then
  # Install brew if needed
  if [ -z "$(which brew)" ]; then 
    /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"; 
  fi
  # Check for coreutils
  if [ -z "$(brew ls coreutils)" ]; then
    brew install coreutils
  fi
fi

I suppose to be truly "global" it needs to check others...but that probably comes close to the 80/20 mark.

Answer 17

I wrote a realpath utility for OS X which can provide the same results as readlink -f.

Here is an example:

(jalcazar@mac tmp)$ ls -l a
lrwxrwxrwx 1 jalcazar jalcazar 11  8月 25 19:29 a -> /etc/passwd

(jalcazar@mac tmp)$ realpath a
/etc/passwd

If you are using MacPorts, you can install it with the following command: sudo port selfupdate && sudo port install realpath.

Answer 18

Explanation

coreutils is a brew package that installs GNU/Linux core utilities which correspond to the Mac OSX implementation of them so that you can use those

You may find programs or utilties on your mac osx system which seem similar to Linux coreutils ("Core Utilities") yet they differ in some ways (such as having different flags).

This is because the Mac OSX implementation of these tools are different. To get the original GNU/Linux-like behavior you can install the coreutils package via the brew package management system.

This will install corresponding core utilities, prefixed by g. E.g. for readlink, you will find a corresponding greadlink program.

In order to make readlink perform like the GNU readlink (greadlink) implementation, you can make a simple alias after you install coreutils.

Implementation

1. Install brew

Follow the instructions at https://brew.sh/

2. Install the coreutils package

brew install coreutils

3. Create an Alias

You can place your alias in ~/.bashrc, ~/.bash_profile, or wherever you are used to keeping your bash aliases. I personally keep mine in ~/.bashrc

alias readlink=greadlink

You can create similar aliases for other coreutils such as gmv, gdu, gdf, and so on. But beware that the GNU behavior on a mac machine may be confusing to others used to working with native coreutils, or may behave in unexpected ways on your mac system.

Answer 19

This is what I use:

stat -f %N $your_path

Answer 20

The paths to readlink are different between my system and yours. Please try specifying the full path:

/sw/sbin/readlink -f

Answer 21

Perl has a readlink function (e.g. How do I copy symbolic links in Perl?). This works across most platforms, including OS X:

perl -e "print readlink '/path/to/link'"

For example:

$ mkdir -p a/b/c
$ ln -s a/b/c x
$ perl -e "print readlink 'x'"
a/b/c

Answer 22

The answer from @Keith Smith gives an infinite loop.

Here is my answer, which i use only on SunOS (SunOS miss so much POSIX and GNU commands).

It's a script file you have to put in one of your $PATH directories:

#!/bin/sh
! (($#)) && echo -e "ERROR: readlink <link to analyze>" 1>&2 && exit 99

link="$1"
while [ -L "$link" ]; do
  lastLink="$link"
  link=$(/bin/ls -ldq "$link")
  link="${link##* -> }"
  link=$(realpath "$link")
  [ "$link" == "$lastlink" ] && echo -e "ERROR: link loop detected on $link" 1>&2 && break
done

echo "$link"