Traverse all symlinks until the file is reached

jlinkels · 10-12-2014, 07:30 PM

I know there is a command which enables me to find all symlinks until I found the real file or program.

For example, vi on my system is found like this:

Code:

which vi
/usr/bin/vi
file /usr/bin/vi
/usr/bin/vi: symbolic link to `/etc/alternatives/vi'
file /etc/alternatives/vi
/etc/alternatives/vi: symbolic link to `/usr/bin/vim.basic'
file /usr/bin/vim.basic
/usr/bin/vim.basic: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.26, BuildID[sha1]=0x34857ec79e4fc4cece4ed3a82cae1dff71dc63fe, stripped

But there must be a command which shows me at once all the links until I end up at /usr/bin/vim.basic. I think I even saw it here but for the life of it I can't find it anymore. Don't try to google for "file symlink follow"

jlinkels

rknichols · 10-12-2014, 08:17 PM

I vaguely recall seeing something like that before, but it's pretty easy to write a script:

Code:

#!/bin/sh
# Show the chain of symlinks to a target
X="$1"
while [ -L "$X" ]; do
    echo -n "$X => "
    X="$(readlink "$X")"
done
echo "$X"

Sample output:

Code:

$ showlink /sbin/iptables
/sbin/iptables => /etc/alternatives/iptables.x86_64 => /sbin/iptables-1.4.7 => iptables-multi

syg00 · 10-12-2014, 08:45 PM

realpath ?.

jpollard · 10-12-2014, 09:16 PM

Quote:

Originally Posted by rknichols

I vaguely recall seeing something like that before, but it's pretty easy to write a script:

Code:

#!/bin/sh
# Show the chain of symlinks to a target
X="$1"
while [ -L "$X" ]; do
    echo -n "$X => "
    X="$(readlink "$X")"
done
echo "$X"

Sample output:

Code:

$ showlink /sbin/iptables
/sbin/iptables => /etc/alternatives/iptables.x86_64 => /sbin/iptables-1.4.7 => iptables-multi

Only problem is when there is a symbolic link loop... But for most things that won't happen

rknichols · 10-12-2014, 10:57 PM

Quote:

Originally Posted by jpollard

Only problem is when there is a symbolic link loop... But for most things that won't happen

True. It's also misleading if $1 doesn't exist (no link, no file, ... nothing). It just echoes back $1 as though it found an ordinary file there. That's an easy fix. Detecting the loop would be somewhat more involved.

Hmmm, it also mishandles relative links -- tries to evaluate them relative to the current directory instead of the directory where the link resides. A bit more complicated now:

Code:

#!/bin/sh
# Show the chain of symlinks to a target
stat "$1" >/dev/null || exit
Y="$1"
X="${1##*/}"
Dir="${1%$X}"
while [ -L "$Dir$X" ]; do
    echo -n "$Y => "
    Y="$(readlink "$Dir$X")"
    X="${Y##*/}"
    case "$Y" in
    /*)
	Dir="${Y%$X}";;
    *)
	Dir="$Dir${Y%$X}";;
    esac
done
echo "$Y"

jlinkels · 10-13-2014, 08:08 AM

Thanks all. I assumed it could be scripted somehow, but knowing the command is easier so it is always avaialable even on machines not connected to my network. I keep hoping.

jlinkels

rknichols · 10-13-2014, 09:25 AM

I thought you wanted to see all the intermediate steps. AFAIK, readlink doesn't do that.

jlinkels · 10-13-2014, 12:49 PM

Well, what I'd like to see is the output you produced with your script. Which is very nice btw. However, if you regularly work on many different machine you know what the problem is managing you collection of useful scripts. They reside at your own network, and not at the host you are currently working at.

I am frustrated because a few weeks ago I saw this command in one of the threads here and I forgot the name.

Unfortunately readlink doesn't do what I want.
readlink /sbin/iptables produces iptables which is helpful.
readlink vi doesn't produce any output while it would be interesting to see where vi points to.

jlinkels

rknichols · 10-13-2014, 03:15 PM

Code:

readlink `which vi`

jlinkels · 10-13-2014, 04:29 PM

Code:

readlink -f `which vi`

gives the correct output.

Still I like the output of your script better.

But thanks for the pointer anyway.

jlinkels