Unix-portable way to get script's absolute path in zsh?

Question

What is a portable¹ way for a (zsh) script to determine its absolute path?

On Linux I use something like

mypath=$(readlink -f $0)

...but this is not portable. (E.g., readlink on darwin does not recognize the -f flag, nor has any equivalent.) (Also, using readlink for this is, admittedly, a pretty obscure-looking hack.)

What's a more portable way?

---

^{1 Portable across OSs in the Unix family, that is.}

Answer 1

In zsh you can do the following:

mypath=${0:a}

Or, to get the directory in which the script resides:

mydir=${0:a:h}

See the Zsh documentation on history expansion modifiers, visible locally in man zshexpn or with info -f zsh -n Modifiers if the Info documentation is installed.

Answer 2

With zsh, it's just:

mypath=$0:A

Or

mypath=$0:P

In newer versions (see the manual for the details on how they differ).

Now for other shells, though realpath() and readlink() are standard functions (the latter being a system call), realpath and readlink are not standard command, though some systems have one or the other or both with various behaviour and feature set.

As often, for portability, you may want to resort to perl:

abs_path() {
  perl -MCwd -le '
    for (@ARGV) {
      if ($p = Cwd::abs_path $_) {
        print $p;
      } else {
        warn "abs_path: $_: $!\n";
        $ret = 1;
      }
    }
    exit $ret' "$@"
}

That would behave more like GNU's readlink -f than realpath() (GNU readlink -e) in that it will not complain if the file doesn't exist as long as its dirname does.

Answer 3

I've been using this for several years now:

# The absolute, canonical ( no ".." ) path to this script
canonical=$(cd -P -- "$(dirname -- "$0")" && printf '%s\n' "$(pwd -P)/$(basename -- "$0")")

Answer 4

This syntax should be portable to any Bourne shell style interpreter (tested with bash, ksh88, ksh93, zsh, mksh, dash and busybox sh):

mypath=$(exec 2>/dev/null;cd -- $(dirname "$0"); unset PWD; /usr/bin/pwd || /bin/pwd || pwd)
echo mypath=$mypath

This version adds compatibility to the legacy AT&T Bourne shell (non POSIX):

mypath=`dirname "$0"`
mypath=`exec 2>/dev/null;(cd -- "$mypath") && cd -- "$mypath"|| cd "$mypath"; unset PWD; /usr/bin/pwd || /bin/pwd || pwd`
echo mypath=$mypath

Answer 5

Assuming you really meant the absolute path, i.e. a path from the root directory:

case $0 in
  /*) mypath=$0;;
  *) mypath=$PWD/$0;;
esac

This works in any Bourne-style shell, by the way.

If you meant a path with all symbolic links resolved, that's a different matter. readlink -f works on Linux (excluding some stripped-down BusyBox systems), FreeBSD, NetBSD, OpenBSD and Cygwin, but not on OS/X, AIX, HP/UX or Solaris. If you have readlink, you can call it in a loop:

realpath () {
  [ -e "$1" ] || return
  case $1 in
    /*) :;;
    *) set "$PWD/$1";;
  esac
  while [ -L "$1" ]; do
    set "${1%/*}" "$(readlink "$1")"
    case $2 in
      /*) set "$2";;
      *) if [ -z "$1" ]; then set "/$2"; else set "$(cd "$1" && pwd -P)/$2"; fi;;
    esac
  done
  case $1 in
    */.|*/..) set "$(cd "$1" && pwd -P)";;
    */./*|*/../*) set "$(cd "${1%/*}" && pwd -P)/${1##*/}"
  esac
  realpath=$1
}

If you don't have readlink, you can approximate it with ls -n, but this only works if ls doesn't mangle any non-printable character in the file name.

poor_mans_readlink () {
  if [ -L "$1" ]; then
    set -- "$1" "$(LC_ALL=C command ls -n -- "$2"; echo z)"
    set -- "${2%??}"
    set -- "${2#*"$1 -> "}"
  fi
  printf '%s\n' "$1"
}

(The extra z is in case the link target ends in a newline, which command substitution would otherwise eat up. The realpath function doesn't handle that case for directory names, by the way.)

Answer 6

Provided you have execute permissions on the current directory - or on the directory from which you executed your shell script - if you want an absolute path to a directory all you need is cd.

Step 10 of cd's spec

If the -P option is in effect, the $PWD environment variable shall be set to the string that would be output by pwd -P. If there is insufficient permission on the new directory, or on any parent of that directory, to determine the current working directory, the value of the $PWD environment variable is unspecified.

And on pwd -P

The pathname written to standard output shall not contain any components that refer to files of type symbolic link. If there are multiple pathnames that the pwd utility could write to standard output, one beginning with a single /slash character and one or more beginning with two /slash characters, then it shall write the pathname beginning with a single /slash character. The pathname shall not contain any unnecessary /slash characters after the leading one or two /slash characters.

It's because cd -P has to set the current working directory to what pwd -P should otherwise print and that cd - has to print the $OLDPWD that the following works:

mkdir ./dir
ln -s ./dir ./ln
cd ./ln ; cd . ; cd -

OUTPUT

/home/mikeserv/test/ln

wait for it...

cd -P . ; cd . ; cd -

OUTPUT

/home/mikeserv/test/dir

And when I print with cd - I'm printing $OLDPWD. cd sets $PWD as soon as I cd -P . $PWD is now an absolute path to / - so I don't need any other variables. And actually, I shouldn't even need the trailing . but there is a specified behavior of resetting $PWD to $HOME in an interactive shell when cd is unadorned. So it's just a good habit to develop.

So just doing the above on the path in ${0%/*} should be more than enough to verify $0's path, but in the case that $0 is itself a soft-link, you probably cannot change directory into it, unfortunately.

Here is a function that will handle that:

zpath() { cd -P . || return
    _out() { printf "%s$_zdlm\n" "$PWD/${1##*/}"; }
    _cd()  { cd -P "$1" ; } >/dev/null 2>&1
    while [ $# -gt 0 ] && _cd .
    do  if     _cd "$1" 
        then   _out
        elif ! [ -L "$1" ] && [ -e "$1" ] 
        then   _cd "${1%/*}"; _out "$1"
        elif   [ -L "$1" ]
        then   ( while set -- "${1%?/}"; _cd "${1%/*}"; [ -L "${1##*/}" ]
                 do    set " $1" "$(_cd -; ls -nd -- "$1"; echo /)"
                       set -- "${2#*"$1" -> }"
                 done; _out "$1" 
    );  else   ( PS4=ERR:\ NO_SUCH_PATH; set -x; : "$1" )
    fi; _cd -; shift; done
    unset -f _out _cd; unset -v _zdlm                                    
}

It strives to do as much as it might in the current shell - without invoking a subshell - though there are subshells invoked for errors and soft links which do not point to directories. It depends on a POSIX-compatible shell and a POSIX-compatible ls as well as a clean _function() namespace. It will still function just fine without the latter, though it may overwrite then unset some current shell functions in that case. In general all of these dependencies should be pretty reliably available on a Unix machine.

Called with or without arguments the first thing it does is reset $PWD to its canonical value - it resolves any links therein to their targets as necessary. Called without arguments and that's about it; but called with them and it will resolve and canonicalize the path for each or else print a message to stderr why not.

Because it mostly operates in the current shell it should be able to handle an argument list of any length. It also looks for the $_zdlm variable (which it also unsets when it is through) and prints its C-escaped value immediately to the right of each of its arguments, each of which is always followed also by a single \newline character.

It does a lot of directory changing, but, other than setting it to its canonical value, it does not affect $PWD, though $OLDPWD cannot by any means be counted upon when it is through.

It tries to quit each of its arguments as soon as it might. It first tries to cd into $1. If it can it prints the argument's canonical path to stdout. If it cannot it checks that $1 exists and is not a soft link. If true, it prints.

In this way it handles any file type argument which the shell has permissions to address unless $1 is a symbolic link that does not point to a directory. In that case it calls while loop in a subshell.

It calls ls to read the link. The current directory must be changed to its initial value first in order to reliably handle any referent paths and so, in the command substitution subshell the function does:

cd -...ls...echo /

It strips from the left of ls's output as little as it must to fully contain the link's name and the string ->. While I at first tried to avoid do this with shift and $IFS it turns out this is the most reliable method as near as I can figure. This is the same thing Gilles's poor_mans_readlink does - and it is well done.

It will repeat this process in a loop until the filename returned from ls is definitely not a soft link. At that point it canonicalizes that path as before with cd then prints.

Example usage:

zpath \
    /tmp/script \   #symlink to $HOME/test/dir/script.sh
    ln \            #symlink to ./dir/
    ln/nl \         #symlink to ../..
    /dev/fd/0 \     #currently a here-document like : dash <<\HD
    /dev/fd/1 \     #(zlink) | dash
    file \          #regular file                                             
    doesntexist \   #doesnt exist
    /dev/disk/by-path/pci-0000:00:16.2-usb-0:3:1.0-scsi-0:0:0:0 \
    /dev/./././././././null \
    . ..      

OUTPUT

/home/mikeserv/test/dir/script.sh
/home/mikeserv/test/dir/
/home/mikeserv/test/
/tmp/zshtpKRVx (deleted)
/proc/17420/fd/pipe:[1782312]
/home/mikeserv/test/file
ERR: NO_SUCH_PATH: doesntexist
/dev/sdd
/dev/null
/home/mikeserv/test/
/home/mikeserv/

Or possibly...

ls
dir/  file  file?  folder/  link@  ln@  script*  script3@  script4@

zdlm=\\0 zpath * | cat -A

OUTPUT

/home/mikeserv/test/dir/^@$               
/home/mikeserv/test/file^@$
/home/mikeserv/test/file$
^@$
/home/mikeserv/test/folder/^@$
/home/mikeserv/test/file$               #'link' -> 'file\n'
^@$
/home/mikeserv/test/dir/^@$             #'ln' -> './dir'
/home/mikeserv/test/script^@$
/home/mikeserv/test/dir/script.sh^@$    #'script3' -> './dir/script.sh'
/home/mikeserv/test/dir/script.sh^@$    #'script4' -> '/tmp/script' -> ...

Answer 7

what about a sympathetic one-liner when theres python available to prevent from redefining an algorithm?

function readlink { python -c "import os.path; print os.path.realpath('$1')"; }

same as https://stackoverflow.com/a/7305217