How to resolve symbolic links in a shell script
https://stackoverflow.com/questions/7665
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Given an absolute or relative path (in a Unix-like system), I would like to determine the full path of the target after resolving any intermediate symlinks. Bonus points for also resolving ~username notation at the same time.
If the target is a directory, it might be possible to chdir() into the directory and then call getcwd(), but I really want to do this from a shell script rather than writing a C helper. Unfortunately, shells have a tendency to try to hide the existence of symlinks from the user (this is bash on OS X):
$ ls -ld foo bar
drwxr-xr-x 2 greg greg 68 Aug 11 22:36 bar
lrwxr-xr-x 1 greg greg 3 Aug 11 22:36 foo -> bar
$ cd foo
$ pwd
/Users/greg/tmp/foo
$
What I want is a function resolve() such that when executed from the tmp directory in the above example, resolve("foo") == "/Users/greg/tmp/bar".
OTHER TIPS
readlink -f "$path"
Editor's note: The above works with GNU readlink and FreeBSD/PC-BSD/OpenBSD readlink, but not on OS X as of 10.11.
GNU readlink offers additional, related options, such as -m for resolving a symlink whether or not the ultimate target exists.
Note since GNU coreutils 8.15 (2012-01-06), there is a realpath program available that is less obtuse and more flexible than the above. It's also compatible with the FreeBSD util of the same name. It also includes functionality to generate a relative path between two files.
realpath $path
[Admin addition below from comment by halloleo —danorton]
For Mac OS X (through at least 10.11.x), use readlink without the -f option:
readlink $path
Editor's note: This will not resolve symlinks recursively and thus won't report the ultimate target; e.g., given symlink a that points to b, which in turn points to c, this will only report b (and won't ensure that it is output as an absolute path).
Use the following perl command on OS X to fill the gap of the missing readlink -f functionality:
perl -MCwd -le 'print Cwd::abs_path(shift)' "$path"
"pwd -P" seems to work if you just want the directory, but if for some reason you want the name of the actual executable I don't think that helps. Here's my solution:
#!/bin/bash
# get the absolute path of the executable
SELF_PATH=$(cd -P -- "$(dirname -- "$0")" && pwd -P) && SELF_PATH=$SELF_PATH/$(basename -- "$0")
# resolve symlinks
while [[ -h $SELF_PATH ]]; do
# 1) cd to directory of the symlink
# 2) cd to the directory of where the symlink points
# 3) get the pwd
# 4) append the basename
DIR=$(dirname -- "$SELF_PATH")
SYM=$(readlink "$SELF_PATH")
SELF_PATH=$(cd "$DIR" && cd "$(dirname -- "$SYM")" && pwd)/$(basename -- "$SYM")
done
One of my favorites is realpath foo
realpath - return the canonicalized absolute pathname
realpath expands all symbolic links and resolves references to '/./', '/../' and extra '/' characters in the null terminated string named by path and
stores the canonicalized absolute pathname in the buffer of size PATH_MAX named by resolved_path. The resulting path will have no symbolic link, '/./' or
'/../' components.
readlink -e [filepath]
seems to be exactly what you're asking for - it accepts an arbirary path, resolves all symlinks, and returns the "real" path - and it's "standard *nix" that likely all systems already have
Another way:
# Gets the real path of a link, following all links
myreadlink() { [ ! -h "$1" ] && echo "$1" || (local link="$(expr "$(command ls -ld -- "$1")" : '.*-> \(.*\)$')"; cd $(dirname $1); myreadlink "$link" | sed "s|^\([^/].*\)\$|$(dirname $1)/\1|"); }
# Returns the absolute path to a command, maybe in $PATH (which) or not. If not found, returns the same
whereis() { echo $1 | sed "s|^\([^/].*/.*\)|$(pwd)/\1|;s|^\([^/]*\)$|$(which -- $1)|;s|^$|$1|"; }
# Returns the realpath of a called command.
whereis_realpath() { local SCRIPT_PATH=$(whereis $1); myreadlink ${SCRIPT_PATH} | sed "s|^\([^/].*\)\$|$(dirname ${SCRIPT_PATH})/\1|"; }
Putting some of the given solutions together, knowing that readlink is available on most systems, but needs different arguments, this works well for me on OSX and Debian. I'm not sure about BSD systems. Maybe the condition needs to be [[ $OSTYPE != darwin* ]] to exclude -f from OSX only.
#!/bin/bash
MY_DIR=$( cd $(dirname $(readlink `[[ $OSTYPE == linux* ]] && echo "-f"` $0)) ; pwd -P)
echo "$MY_DIR"
Note: I believe this to be a solid, portable, ready-made solution, which is invariably lengthy for that very reason.
Below is a fully POSIX-compliant script / function that is therefore cross-platform (works on macOS too, whose readlink still doesn't support -f as of 10.12 (Sierra)) - it uses only POSIX shell language features and only POSIX-compliant utility calls.
It is a portable implementation of GNU's readlink -e (the stricter version of readlink -f).
You can run the script with sh or source the function in bash, ksh, and zsh:
For instance, inside a script you can use it as follows to get the running's script true directory of origin, with symlinks resolved:
trueScriptDir=$(dirname -- "$(rreadlink "$0")")
rreadlink script / function definition:
The code was adapted with gratitude from this answer.
I've also created a bash-based stand-alone utility version here, which you can install with
npm install rreadlink -g, if you have Node.js installed.
#!/bin/sh
# SYNOPSIS
# rreadlink <fileOrDirPath>
# DESCRIPTION
# Resolves <fileOrDirPath> to its ultimate target, if it is a symlink, and
# prints its canonical path. If it is not a symlink, its own canonical path
# is printed.
# A broken symlink causes an error that reports the non-existent target.
# LIMITATIONS
# - Won't work with filenames with embedded newlines or filenames containing
# the string ' -> '.
# COMPATIBILITY
# This is a fully POSIX-compliant implementation of what GNU readlink's
# -e option does.
# EXAMPLE
# In a shell script, use the following to get that script's true directory of origin:
# trueScriptDir=$(dirname -- "$(rreadlink "$0")")
rreadlink() ( # Execute the function in a *subshell* to localize variables and the effect of `cd`.
target=$1 fname= targetDir= CDPATH=
# Try to make the execution environment as predictable as possible:
# All commands below are invoked via `command`, so we must make sure that
# `command` itself is not redefined as an alias or shell function.
# (Note that command is too inconsistent across shells, so we don't use it.)
# `command` is a *builtin* in bash, dash, ksh, zsh, and some platforms do not
# even have an external utility version of it (e.g, Ubuntu).
# `command` bypasses aliases and shell functions and also finds builtins
# in bash, dash, and ksh. In zsh, option POSIX_BUILTINS must be turned on for
# that to happen.
{ \unalias command; \unset -f command; } >/dev/null 2>&1
[ -n "$ZSH_VERSION" ] && options[POSIX_BUILTINS]=on # make zsh find *builtins* with `command` too.
while :; do # Resolve potential symlinks until the ultimate target is found.
[ -L "$target" ] || [ -e "$target" ] || { command printf '%s\n' "ERROR: '$target' does not exist." >&2; return 1; }
command cd "$(command dirname -- "$target")" # Change to target dir; necessary for correct resolution of target path.
fname=$(command basename -- "$target") # Extract filename.
[ "$fname" = '/' ] && fname='' # !! curiously, `basename /` returns '/'
if [ -L "$fname" ]; then
# Extract [next] target path, which may be defined
# *relative* to the symlink's own directory.
# Note: We parse `ls -l` output to find the symlink target
# which is the only POSIX-compliant, albeit somewhat fragile, way.
target=$(command ls -l "$fname")
target=${target#* -> }
continue # Resolve [next] symlink target.
fi
break # Ultimate target reached.
done
targetDir=$(command pwd -P) # Get canonical dir. path
# Output the ultimate target's canonical path.
# Note that we manually resolve paths ending in /. and /.. to make sure we have a normalized path.
if [ "$fname" = '.' ]; then
command printf '%s\n' "${targetDir%/}"
elif [ "$fname" = '..' ]; then
# Caveat: something like /var/.. will resolve to /private (assuming /var@ -> /private/var), i.e. the '..' is applied
# AFTER canonicalization.
command printf '%s\n' "$(command dirname -- "${targetDir}")"
else
command printf '%s\n' "${targetDir%/}/$fname"
fi
)
rreadlink "$@"
A tangent on security:
jarno, in reference to the function ensuring that builtin command is not shadowed by an alias or shell function of the same name, asks in a comment:
What if
unaliasorunsetand[are set as aliases or shell functions?
The motivation behind rreadlink ensuring that command has its original meaning is to use it to bypass (benign) convenience aliases and functions often used to shadow standard commands in interactive shells, such as redefining ls to include favorite options.
I think it's safe to say that unless you're dealing with an untrusted, malicious environment, worrying about unalias or unset - or, for that matter, while, do, ... - being redefined is not a concern.
There is something that the function must rely on to have its original meaning and behavior - there is no way around that.
That POSIX-like shells allow redefinition of builtins and even language keywords is inherently a security risk (and writing paranoid code is hard in general).
To address your concerns specifically:
The function relies on unalias and unset having their original meaning. Having them redefined as shell functions in a manner that alters their behavior would be a problem; redefinition as an alias is
not necessarily a concern, because quoting (part of) the command name (e.g., \unalias) bypasses aliases.
However, quoting is not an option for shell keywords (while, for, if, do, ...) and while shell keywords do take precedence over shell functions, in bash and zsh aliases have the highest precedence, so to guard against shell-keyword redefinitions you must run unalias with their names (although in non-interactive bash shells (such as scripts) aliases are not expanded by default - only if shopt -s expand_aliases is explicitly called first).
To ensure that unalias - as a builtin - has its original meaning, you must use \unset on it first, which requires that unset have its original meaning:
unset is a shell builtin, so to ensure that it is invoked as such, you'd have to make sure that it itself is not redefined as a function. While you can bypass an alias form with quoting, you cannot bypass a shell-function form - catch 22.
Thus, unless you can rely on unset to have its original meaning, from what I can tell, there is no guaranteed way to defend against all malicious redefinitions.
Common shell scripts often have to find their "home" directory even if they are invoked as a symlink. The script thus have to find their "real" position from just $0.
cat `mvn`
on my system prints a script containing the following, which should be a good hint at what you need.
if [ -z "$M2_HOME" ] ; then
## resolve links - $0 may be a link to maven's home
PRG="$0"
# need this for relative symlinks
while [ -h "$PRG" ] ; do
ls=`ls -ld "$PRG"`
link=`expr "$ls" : '.*-> \(.*\)$'`
if expr "$link" : '/.*' > /dev/null; then
PRG="$link"
else
PRG="`dirname "$PRG"`/$link"
fi
done
saveddir=`pwd`
M2_HOME=`dirname "$PRG"`/..
# make it fully qualified
M2_HOME=`cd "$M2_HOME" && pwd`
function realpath {
local r=$1; local t=$(readlink $r)
while [ $t ]; do
r=$(cd $(dirname $r) && cd $(dirname $t) && pwd -P)/$(basename $t)
t=$(readlink $r)
done
echo $r
}
#example usage
SCRIPT_PARENT_DIR=$(dirname $(realpath "$0"))/..
Try this:
cd $(dirname $([ -L $0 ] && readlink -f $0 || echo $0))
Since I've run into this many times over the years, and this time around I needed a pure bash portable version that I could use on OSX and linux, I went ahead and wrote one:
The living version lives here:
https://github.com/keen99/shell-functions/tree/master/resolve_path
but for the sake of SO, here's the current version (I feel it's well tested..but I'm open to feedback!)
Might not be difficult to make it work for plain bourne shell (sh), but I didn't try...I like $FUNCNAME too much. :)
#!/bin/bash
resolve_path() {
#I'm bash only, please!
# usage: resolve_path <a file or directory>
# follows symlinks and relative paths, returns a full real path
#
local owd="$PWD"
#echo "$FUNCNAME for $1" >&2
local opath="$1"
local npath=""
local obase=$(basename "$opath")
local odir=$(dirname "$opath")
if [[ -L "$opath" ]]
then
#it's a link.
#file or directory, we want to cd into it's dir
cd $odir
#then extract where the link points.
npath=$(readlink "$obase")
#have to -L BEFORE we -f, because -f includes -L :(
if [[ -L $npath ]]
then
#the link points to another symlink, so go follow that.
resolve_path "$npath"
#and finish out early, we're done.
return $?
#done
elif [[ -f $npath ]]
#the link points to a file.
then
#get the dir for the new file
nbase=$(basename $npath)
npath=$(dirname $npath)
cd "$npath"
ndir=$(pwd -P)
retval=0
#done
elif [[ -d $npath ]]
then
#the link points to a directory.
cd "$npath"
ndir=$(pwd -P)
retval=0
#done
else
echo "$FUNCNAME: ERROR: unknown condition inside link!!" >&2
echo "opath [[ $opath ]]" >&2
echo "npath [[ $npath ]]" >&2
return 1
fi
else
if ! [[ -e "$opath" ]]
then
echo "$FUNCNAME: $opath: No such file or directory" >&2
return 1
#and break early
elif [[ -d "$opath" ]]
then
cd "$opath"
ndir=$(pwd -P)
retval=0
#done
elif [[ -f "$opath" ]]
then
cd $odir
ndir=$(pwd -P)
nbase=$(basename "$opath")
retval=0
#done
else
echo "$FUNCNAME: ERROR: unknown condition outside link!!" >&2
echo "opath [[ $opath ]]" >&2
return 1
fi
fi
#now assemble our output
echo -n "$ndir"
if [[ "x${nbase:=}" != "x" ]]
then
echo "/$nbase"
else
echo
fi
#now return to where we were
cd "$owd"
return $retval
}
here's a classic example, thanks to brew:
%% ls -l `which mvn`
lrwxr-xr-x 1 draistrick 502 29 Dec 17 10:50 /usr/local/bin/mvn@ -> ../Cellar/maven/3.2.3/bin/mvn
use this function and it will return the -real- path:
%% cat test.sh
#!/bin/bash
. resolve_path.inc
echo
echo "relative symlinked path:"
which mvn
echo
echo "and the real path:"
resolve_path `which mvn`
%% test.sh
relative symlinked path:
/usr/local/bin/mvn
and the real path:
/usr/local/Cellar/maven/3.2.3/libexec/bin/mvn
To work around the Mac incompatibility, I came up with
echo `php -r "echo realpath('foo');"`
Not great but cross OS
Here's how one can get the actual path to the file in MacOS/Unix using an inline Perl script:
FILE=$(perl -e "use Cwd qw(abs_path); print abs_path('$0')")
Similarly, to get the directory of a symlinked file:
DIR=$(perl -e "use Cwd qw(abs_path); use File::Basename; print dirname(abs_path('$0'))")
Is your path a directory, or might it be a file? If it's a directory, it's simple:
(cd "$DIR"; pwd -P)
However, if it might be a file, then this won't work:
DIR=$(cd $(dirname "$FILE"); pwd -P); echo "${DIR}/$(readlink "$FILE")"
because the symlink might resolve into a relative or full path.
On scripts I need to find the real path, so that I might reference configuration or other scripts installed together with it, I use this:
SOURCE="${BASH_SOURCE[0]}"
while [ -h "$SOURCE" ]; do # resolve $SOURCE until the file is no longer a symlink
DIR="$( cd -P "$( dirname "$SOURCE" )" && pwd )"
SOURCE="$(readlink "$SOURCE")"
[[ $SOURCE != /* ]] && SOURCE="$DIR/$SOURCE" # if $SOURCE was a relative symlink, we need to resolve it relative to the path where the symlink file was located
done
You could set SOURCE to any file path. Basically, for as long as the path is a symlink, it resolves that symlink. The trick is in the last line of the loop. If the resolved symlink is absolute, it will use that as SOURCE. However, if it is relative, it will prepend the DIR for it, which was resolved into a real location by the simple trick I first described.
I believe this is the true and definitive "way to resolve a symlink" whether it is a directory or a non-directory, using Bash:
function readlinks {(
set -o errexit -o nounset
declare n=0 limit=1024 link="$1"
# If it's a directory, just skip all this.
if cd "$link" 2>/dev/null
then
pwd -P "$link"
return 0
fi
# Resolve until we are out of links (or recurse too deep).
while [[ -L $link ]] && [[ $n -lt $limit ]]
do
cd "$(dirname -- "$link")"
n=$((n + 1))
link="$(readlink -- "${link##*/}")"
done
cd "$(dirname -- "$link")"
if [[ $n -ge $limit ]]
then
echo "Recursion limit ($limit) exceeded." >&2
return 2
fi
printf '%s/%s\n' "$(pwd -P)" "${link##*/}"
)}
Note that all the cd and set stuff takes place in a subshell.
Here I present what I believe to be a cross-platform (Linux and macOS at least) solution to the answer that is working well for me currently.
crosspath()
{
local ref="$1"
if [ -x "$(which realpath)" ]; then
path="$(realpath "$ref")"
else
path="$(readlink -f "$ref" 2> /dev/null)"
if [ $? -gt 0 ]; then
if [ -x "$(which readlink)" ]; then
if [ ! -z "$(readlink "$ref")" ]; then
ref="$(readlink "$ref")"
fi
else
echo "realpath and readlink not available. The following may not be the final path." 1>&2
fi
if [ -d "$ref" ]; then
path="$(cd "$ref"; pwd -P)"
else
path="$(cd $(dirname "$ref"); pwd -P)/$(basename "$ref")"
fi
fi
fi
echo "$path"
}
Here is a macOS (only?) solution. Possibly better suited to the original question.
mac_realpath()
{
local ref="$1"
if [[ ! -z "$(readlink "$ref")" ]]; then
ref="$(readlink "$1")"
fi
if [[ -d "$ref" ]]; then
echo "$(cd "$ref"; pwd -P)"
else
echo "$(cd $(dirname "$ref"); pwd -P)/$(basename "$ref")"
fi
}
