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Running the following command works as expected, creating a symlink to src from include/bb:

ln -sf ../src include/bb

However, calling the same command again will cause an additional symlink to src to be created in include/bb aka src/src.

What can I do to prevent it?

Edit:

Apparently this happens when ln is called twice, possibly the semicolon has nothing to do with it after all.

Here's a sequence of commands that will cause this to happen:

mkdir test
cd test
mkdir src
touch src/main.cpp
mkdir include
ln -sf ../src include/bb
ln -sf ../src include/bb
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    I don't think that does happen; ln doesn't even get to know about the semicolon. Can you edit in a brief script reproducing the issue you're having? Commented Mar 4, 2016 at 8:55

2 Answers 2

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ln -s ../src include/bb

either creates a link named include/bb referring to ../src (relative to its location), or it creates a file include/bb/src referring to ../src (relative to its location). The latter is the case if include/bb exists and is a directory (actually or as a link to a directory).

Thus, when you've done the command once, there is now a link include/bb that points out the directory src, sibling to include. I.e., include/bb is now a directory. Therefore, when you do the command the subsequent time, a new link named src is added to that directory.

It has nothing to do with ;.

You might have intended to use

ln -sTf ../src include/bb

instead, so as to (by -T) tell ln to treat any existing include/bb as a plain file (even though it's a link that refers to a directory), and (by -f) force that file to be replaced.

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When you run

ln -s ../src include/bb
ln -s ../src include/bb

the first line makes a link include/bb to the directory ../src (which is what you expected).

The second one finds that include/bb already exists. If it were a regular file, or a symlink to a regular file, that would be an error (unless you gave -f as well). Instead, it is a symlink to the directory ../src, created in the previous step. When ln's target operand is a directory:

the ln utility shall create a new [...] symbolic link, for each file specified by a source_file operand, at a destination path in the existing directory named by target_dir.

So this time around you're asking to create a link to ../src inside include/bb/, which is (a link to) a directory. Those links are named by default according to their existing base names, which is src in this case. If you ran it a third time, you'd get an error because ../src/src exists now.

You can avoid this by deleting the include/bb link beforehand every time with rm include/bb. The semicolon isn't involved in this.

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