Converting relative path into absolute path?

The most reliable method I've come across in unix is readlink -f:

$ readlink -f /x/y/../../a/b/z/../c/d
/a/b/c/d

A couple caveats:

1. This also has the side-effect of resolving all symlinks. This may or may not be desirable, but usually is.
2. readlink will give a blank result if you reference a non-existant directory. If you want to support non-existant paths, use readlink -m instead. Unfortunately this option doesn't exist on versions of readlink released before ~2005.

From this source comes:

#!/bin/bash

# Assume parameter passed in is a relative path to a directory.
# For brevity, we won't do argument type or length checking.

ABS_PATH=`cd "$1"; pwd` # double quotes for paths that contain spaces etc...
echo "Absolute path: $ABS_PATH"

You can also do a Perl one-liner, e.g. using Cwd::abs_path

Using bash

# Directory
relative_dir="folder/subfolder/"
absolute_dir="$( cd "$relative_dir" && pwd )"

# File
relative_file="folder/subfolder/file"
absolute_file="$( cd "${relative_file%/*}" && pwd )"/"${relative_file##*/}"

• ${relative_file%/*} is same result as dirname "$relative_file"
• ${relative_file##*/} is same result as basename "$relative_file"

Caveats: Does not resolve symbolic links (i.e. does not canonicalize path ) => May not differentiate all duplicates if you use symbolic links.

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Using realpath

Command realpath does the job. An alternative is to use readlink -e (or readlink -f). However realpath is not often installed by default. If you cannot be sure realpath or readlink is present, you can substitute it using perl (see below).

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Using perl

Steven Kramer proposes a shell alias if realpath is not available in your system:

$ alias realpath="perl -MCwd -e 'print Cwd::realpath(\$ARGV[0]),qq<\n>'"
$ realpath path/folder/file
/home/user/absolute/path/folder/file

or if you prefer using directly perl:

$ perl -MCwd -e 'print Cwd::realpath($ARGV[0]),qq<\n>' path/folder/file
/home/user/absolute/path/folder/file

This one-line perl command uses Cwd::realpath. There are in fact three perl functions. They take a single argument and return the absolute pathname. Below details are from documentation Perl5 > Core modules > Cwd.

• abs_path() uses the same algorithm as getcwd(). Symbolic links and relative-path components (. and ..) are resolved to return the canonical pathname, just like realpath.

  use Cwd 'abs_path';
  my $abs_path = abs_path($file);
  

• realpath() is a synonym for abs_path()

  use Cwd 'realpath';
  my $abs_path = realpath($file);
  

• fast_abs_path() is a more dangerous, but potentially faster version of abs_path()

  use Cwd 'fast_abs_path';
  my $abs_path = fast_abs_path($file);
  

These functions are exported only on request => therefore use Cwd to avoid the "Undefined subroutine" error as pointed out by arielf. If you want to import all these three functions, you can use a single use Cwd line:

use Cwd qw(abs_path realpath fast_abs_path);