How to regularly update symbolic link (ln -sf) via crontab

Question

Background

The PNG image files I want to use is stored in directories according to date, for example:
/NAS-mein/data/201812/
PNG stored within it like /NAS-mein/data/201812/foo/bar/20181231_1500.png

So I created a symbolic link PNG_path in my home directory
ln -s /NAS-mein/data/201812/ PNG_path

and I'm able to update it manually through:
ln -sf /NAS-mein/data/201812/ PNG_path
which works fine and returns `PNG_path' -> `/NAS-mein/data/201812'

I'm in a CentOS 6.7 environment and I don't have superuser privilege.
The destination directory is created by the others but granted 777 permission, i.e.:
drwxrwxrwx /NAS-mein/
drwxrwxrwx /NAS-mein/data/
drwxrwxrwx /NAS-mein/data/201812/

With Crontab

Then I tried to automatically update this symbolic link on the first day of month, so it will always redirect me to the directory of current date.
I tried start a job in crontab like:
0 0 1 * * ln -sf /NAS-mein/data/$(date "+%Y%m") /home/me/PNG_path >>/home/me/.pngln.log 2>>&1
but this does not work, even without giving any information to the log.
So I tried:
0 0 1 * * cd /home/me/ && ln -sf /NAS-mein/data/$(date "+%Y%m") PNG_path >>.pngln.log 2>>&1
and wrap it into a Bash script like:

#!/bin/bash  
/bin/unlink  "/home/me/PNG_path"  
/bin/ln -sf /NAS-mein/data/$(date "+%Y%m")  PNG_path >>/home/me/.pngln.log  2>>&1  

but all of above seem not working as the symbolic link does not change, and no any information was logged (i.e. .pngln.log is not created anyway.)

I'm not sure where I did it wrong, or using ln in crontab is just not a legit use?

Edit: I notice that I didn't write the most suspicious part: using date function in ln expression.

Answer 1

The percent sign is special in crontab and needs to be escaped if you put your date command there (see man 5 crontab).

Your symbolic link points to a directory. When you run ln again, it will put the link inside that directory.

Example:

$ mkdir real
$ ln -sf real link
$ tree
.
|-- link -> real
`-- real

1 directory, 1 file

$ ln -sf real link
$ tree
.
|-- link -> real
`-- real
    `-- real -> real

1 directory, 2 files

The solution is to use ln with -n (or --no-dereference) on Linux or on any system with GNU coreutils' ln, and with -h on BSD. This would cause ln to not descend into the directory that the link points to before creating the new link.

A portable solution would be to first explicitly remove the link using rm:

ln -s some_directory link

Later:

rm link && ln -s some_directory link

Answer 2

I want to sum up the mistakes that cause of the failure of my crontab ln schedule in this answer.
This is much thanks to Kusalananda's answer, I just organize them for record.

First, as mentioned by Kusalananda, if one is trying to update the symlink of a directory, the symlink will be created inside the directory, so one will need to set -n flag for ln, so the algorithm will not follow into the reference.

Good:

ln -nsf  /NAS-mein/data/$(date "+%Y%m")  PNG_path

Not working:

ln -nsf  /NAS-mein/data/$(date "+%Y%m")  PNG_path

Second, as mentioned by Kusalananda, the % has special meaning in cron, so it needs to be escaped.

Good:

0 0 1 * *  /bin/ln -nsf  /NAS-mein/data/$(date "+\%Y\%m")  PNG_path

Not working:

0 0 1 * *  /bin/ln -nsf  /NAS-mein/data/$(date "+%Y%m")  PNG_path

Third and the last, though this is actually not really related to cron or ln, that for stderr to append to the same destination of stdout, it cannot be written as 2>>&1. Instead, 2>&1 is correct. (Actually without -v flag for ln, it will output nothing, but one will at least see an empty log is created.)

Good:

0 0 1 * *  /bin/ln -nsf  /NAS-mein/data/$(date "+\%Y\%m")  PNG_path  >>.log  2>&1

Not working:

0 0 1 * *  /bin/ln -nsf  /NAS-mein/data/$(date "+\%Y\%m")  PNG_path  >>.log  2>>&1