Symbolic links

Symbolic links are also called soft links, or just symlinks. It is a pointer to another file or directory (called the “target”).

  • It is useful both for ease
    • you avoid using a long path each time you change to a directory, like your project directory
  • As well as to avoid changing hard links within other scripts or programs.
    • This is good if you for instance install a program or use a script that assumes the library it uses is called libcoolness.a and not libcoolness.2.0.a. You can then just update the symlink instead of renaming the library or updating potentially many instances where it is mentioned in the program.

Hard links and soft links

You may have heard symbolic links referred to as “soft links”. So what is a hard link?

Hard links are also shortcuts for files (not directories), but a hard link cannot be created for a file in a different file system.

Syntax

Soft link/symbolic link:

ln -s real-file-folder-or-lib link-name

Hard link:

ln source-file linked-file

Verify

To verify a soft or hard link, run:

ls -l source link

Examples

Hint

You can type along to most of this! Use the files in “exercises” -> “redirects-env-links” for examples.

Hard links

Hint

You can code along!

Either use one of the files/directories in “exercises” -> “redirect-env-link” or create a file to play with. This should work:

echo 'This is my test file' > testfile

Create a hard link, files

ln testfile linkfile

To check what happened, do this: 

ls -li testfile linkfile

My output: 

$ ls -li
total 8
808864781 -rw-r--r-- 2 bbrydsoe folk 21 May 27 16:36 linkfile
808864781 -rw-r--r-- 2 bbrydsoe folk 21 May 27 16:36 testfile

808864781 is the inode, which is identical. An inode (index node) is a concept in Linux/Unix. Each object in a filesystem is represented by an inode. It is a data structure which stores basic info about files, directories, or other file system objects. More here if you are interested: Understanding UNIX / Linux filesystem Inodes.

Soft/symbolic links

Create a symbolic link to a directory

ln -s /path/to/home/exercises/redirects-env-link $HOME/exer

This creates a symbolic link named “exer” in the home directory, pointing to the location /path/to/home/exercises/redirects-env-link. For me, this would look like this:

bbrydsoe@enterprise:~/exercises/redirects-env-link$ pwd
/home/bbrydsoe/exercises/redirects-env-link
bbrydsoe@enterprise:~/exercises/redirects-env-link$ ln -s /home/bbrydsoe/exercises/redirects-env-link $HOME/exer
bbrydsoe@enterprise:~$ cd
bbrydsoe@enterprise:~$ ls -l
...
drwxrwxr-x  6 bbrydsoe bbrydsoe       4096 nov 13  2023  course-intro-git
drwxr-xr-x  8 bbrydsoe bbrydsoe       4096 maj 25 14:53  Desktop
drwxr-xr-x  6 bbrydsoe bbrydsoe       4096 jun 20  2024  Documents
drwxr-xr-x 22 bbrydsoe bbrydsoe      20480 maj 26 15:13  Downloads
drwxrwxr-x  2 bbrydsoe bbrydsoe       4096 maj 21 09:10  examples
lrwxrwxrwx  1 bbrydsoe bbrydsoe         43 maj 26 19:36  exer -> /home/bbrydsoe/exercises/redirects-env-link
drwxr-xr-x  8 bbrydsoe bbrydsoe       4096 maj 26 15:43  exercises
-rw-rw-r--  1 bbrydsoe bbrydsoe     296399 jan 16 23:33  exercises.tar.gz
drwxrwxr-x 31 bbrydsoe bbrydsoe       4096 maj 23 10:54  hpc2n
drwxrwxr-x  5 bbrydsoe bbrydsoe       4096 apr  5 12:56  HPC2Ndocs
-rw-rw-r--  1 bbrydsoe bbrydsoe         85 okt 23  2023  hpc2n-faktura.txt
...

Create a symbolic link to a file in the same directory as the file

ln -s /path/to/file/the-file /path/to/file/<new-name>

Create a symbolic link to a library

This is to give the library the name is is presumably assumed to have in some program that uses it. Here it is in the same directory.

This is how it looks for me:

ln -s /home/bbrydsoe/exercises/redirects-env-link/libcoolness.2.0.a /home/bbrydsoe/exercises/redirects-env-link/libcoolness.a

Create a symbolic link to a library in a different directory

Sometimes a program expects a library to be in another directory than it is. An easy way of solving this is with symlinks - then you do not have to copy the library to perhaps several directories, and redo when it is updated.

ln -s /usr/lib64/libfancy.a /usr/lib/libfancy.a

Remove a symlink

You can remove a symbolic link either with

unlink <path-to-symlink>

or

rm <path-to-symlink>

The main benefit of rm over unlink is that you can remove multiple symlinks at once, like you can with files.

For me, it looks like this if I remove the symbolic link libcoolness.a I created above:

bbrydsoe@enterprise:~/exercises/redirects-env-link$ rm libcoolness.a
bbrydsoe@enterprise:~/exercises/redirects-env-link$

Warning

When removing a symlink to a folder, do not include the / at the end since that will make bash think the symbolic link is a folder that you cannot remove directly like this.

Summary

Keypoints

  • Symbolic links are pointers to another file or directory
  • there are both soft/symbolic links and hard links
  • they are useful both for ease and to avoid renaming/copying files
  • you can remove symbolic links with rm or unlink