Links in /usr/bin or /usr/local/bin: hard or symbolic

hard linksymlink

For the links located in /usr/bin and in /usr/local/bin, are they better to be hard or symbolic? They seem to be all symbolic? Why?

Best Answer

When links are put in /usr/bin or /usr/local/bin, this is often because the actual binary is living somewhere else. Why is it living somewhere else? Usually because it is a part of a group of files, often in its own subdirectory, which it depends on to run. Why can't all those files be dumped into /usr/bin or /usr/local/bin? Because those locations are only for binaries, the binaries live at the top level of those directories, and no subdirectories are allowed.

So, why don't we use a hard link there? Because a hard link to a file is considered to be of equal status to the original file (it shares the same inode) and the file will not be deleted while there is a hard link to it. In contrast, a symbolic link is simply a pointer to "the real thing". In practice, putting a hard link to a binary file that is living somewhere else wouldn't make much sense, because if those other files in that other location/subdirectory were removed, the file could not function by itself.

An example of this are the various TeX utilities, which live in /usr/bin. If you look at them, you will see that many of them point to files that are actually in /usr/share/texlive/texmf-dist/scripts/. You can see these, for example, by doing ls -la | grep texmf.

As muru mentions, another reason is that hard links don't work across filesystems.

Another situation in which links are used for files in /usr/bin or /usr/local/bin is Debian's alternatives system. In this case, a binary may be a symbolic link pointing to a symbolic link in /etc/alternatives, which then points to the actual binary, often again itself in /usr/bin.

Example:

ls -la /usr/bin/awk 
lrwxrwxrwx 1 root root 21 Jul 31  2013 /usr/bin/awk -> /etc/alternatives/awk
ls -la /etc/alternatives/awk
lrwxrwxrwx 1 root root 13 Aug 11  2013 /etc/alternatives/awk -> /usr/bin/gawk

It is fairly obvious in this case that a symbolic link is the correct mechanism, because it is pointing to any of a range of possible binaries.

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Symbolic Links – How to Keep Track of Symbolic Links

Concering your second question, if you make the symlink using a relative path and then move the whole directory structure, it still should work. Consider the following terminal session:

~$ mkdir test
~$ cd test/
~/test$ mkdir test2
~/test$ cd test2/
~/test/test2$ touch testfile; echo "hello, world" > testfile
~/test/test2$ cat testfile 
hello, world
~/test/test2$ cd ..
~/test$ ln -s ./test2/testfile testfileln
~/test$ ls -l
total 8
drwxr-xr-x 2 xxxx xxxx 4096 2010-09-09 09:18 test2
lrwxrwxrwx 1 xxxx xxxx   16 2010-09-09 09:18 testfileln -> ./test2/testfile
~/test$ cd ..
~$ mv test/ testfoo
~$ cd testfoo/
~/testfoo$ ls -l
total 8
drwxr-xr-x 2 xxxx xxxx 4096 2010-09-09 09:18 test2
lrwxrwxrwx 1 xxxx xxxx   16 2010-09-09 09:18 testfileln -> ./test2/testfile
/testfoo$ cat testfileln 
hello, world

As for your first question, if you really want a link that will refer to the same file no matter what you do with the original location of the file, a hard link is probably what you want. A hard link is basically just another name refering to the same inode. Thus, there is no difference between the hard link and the "original file." However, if you need to link across file systems, hard links often do not work and you usually cannot make hard links to directories. Further, you will notice some differences when performing some file operations. Most notably, removing the original will not remove the file. The hard link will still point to the file and be accessible.

Executable symbolic link results in “command not found”

You forgot the initial slash before bin/python. This means /usr/bin/prj-python now points to /usr/bin/bin/python. What would you like it to point to exactly?

Best Answer

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Symbolic Links – How to Keep Track of Symbolic Links

Executable symbolic link results in “command not found”

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