This week I faced a very odd bug at work.

A build process failed - on one machine, but not on the other.

Identical setup - of course :-/

The build script consists of a bash script, which wraps a call to the C binary.

The bash script was innocent.

When I had a closer look at the C source code, it turned out there were about 30 lines of code, just dealing with traversing directories and copying files and folders.

As I usually don’t do a lot of C, I asked a colleague to have a look and finally we figured out what is going wrong:

The C code was reading the directory layout at a very low level, and the files/subdirectories were actually not sorted alphabetically, but in the order they appeared in the directory file.

I even did not know this existed!

Notes

The index of a directory is written in a directory file for e.g. ext file systems. As another example, ReiserFS uses a B+ Tree.

Having a closer look at the 30 lines of C code, we noticed that the recursion had a bug, as it relied on the order of the files and sub-folders in the directory.

Ok, but how can you find out the “real” order of files in a directory?

Turns out there is an option for ls - from its man page:

-U     do not sort; list entries in directory order

Have a look at these two (unrelated) file listings:

❯ ls -la
total 28
drwxr-xr-x  2 jugmac00 jugmac00 4096 Jun  8 09:52 .
drwxr-xr-x 20 jugmac00 jugmac00 4096 Jun  8 09:53 ..
-rw-rw-r--  1 jugmac00 jugmac00 1949 Mai  9 15:49 conf.py
-rw-rw-r--  1 jugmac00 jugmac00 7526 Mai  9 10:58 index.rst
-rw-rw-r--  1 jugmac00 jugmac00   29 Mai 21 18:28 requirements.in
-rw-rw-r--  1 jugmac00 jugmac00 1247 Jun  8 09:52 requirements.txt

❯ ls -laU
total 28
-rw-rw-r--  1 jugmac00 jugmac00   29 Mai 21 18:28 requirements.in
drwxr-xr-x  2 jugmac00 jugmac00 4096 Jun  8 09:52 .
drwxr-xr-x 20 jugmac00 jugmac00 4096 Jun  8 09:53 ..
-rw-rw-r--  1 jugmac00 jugmac00 1949 Mai  9 15:49 conf.py
-rw-rw-r--  1 jugmac00 jugmac00 1247 Jun  8 09:52 requirements.txt
-rw-rw-r--  1 jugmac00 jugmac00 7526 Mai  9 10:58 index.rst

By the way - I never had to do copy a directory tree in Python, but I’d probably use shutil.copytree - what about you?