Creating links in UNIX filesystems

This is should be pretty straightforward, yet sometimes I find myself reading for the millionth time the manpage. Perharps this is more memorable.

Symbolic links (AKA symlinks) and hard links are both created with the same command (ln) and it follows the same syntax as the cp or mv commands. In most cases that would be:

<command> <“-s”-option-if-symlink> <original-or-source> <where-to-place-the-thing-in>
e.g;

• ln -s /etc/someapp/somefile.conf /home/serveradmin/tempbackups

The source/original/pre-existing-file though is called in the manpage the “TARGET” which makes the whole thing very confusing; but if it helps, think about [web- or hyper]links how they have a “target” [webpage].

Single-argument form

If you are in the pwd is the same where you wish to create the link i.e. if you’re already on the destination, you may use drop the second argument e.g;

serveradmin@www:~/tempbackups$ ln -s /etc/someapp/somefile.conf
serveradmin@www:~/tempbackups$ ln /etc/someapp/somefile.conf

would create the file ~/tempbackups/somefile.conf. ~, as you should know, is the shorthand for the user’s home folder. Usually it’s a directory with their username located in /home, /homes, /Users, etc. depending on the system.

Send-to-a-container forms

• ln /etc/someapp/somefile.conf /etc/someapp/somefile.ini /home/serveradmin/tempbackups
• ln -t /home/serveradmin/tempbackups /etc/someapp/somefile.conf /etc/someapp/somefile.ini

The first of these is the most commonly used syntax (mentioned at the beginning). Both of there create one or more pseudo-copies on a destination directory (/home/serveradmin/tempbackups), which is the main difference from the following form,

Specifying a new name

• ln -T /etc/someapp/somefile.conf /home/serveradmin/tempbackups/app-parameters-change-here

Rules and background

• [Hard] Links are also known as inodes (index nodes,) associates a name to a file on a filesystem. For each file there must be at least one hard link.
• Symbolic or soft links, point to the path where a file resides. Unlike hard links, they do not link to the data itself.
• For hard links, the source (or target) must exist.
• For both, the destination should not exist. You should in theory be able to overcome this with the -i or -f options but it never seems to work.
• You can only hard link files, not directories. It may be doable to hard link directories but it will not be trivial.
• You can change the permissions of a symlink itself with chmod in macOS but not in every other OS.
• File management commands such as mv and rm work on the symlink itself, which if viewed in some file managers in macOS appears as a file with a little arrow that may be dragged & drop & renamed & deleted as any other file keeping its associated file or directory unaffected.
  • However, be careful as some commands, e.g, cp, rsync can recurse through symlinks if using the appropriate [and very common] options.