This is what Jayesh Bhoot looks like, with a lopsided smile on its right, which almost closes his right eye while smiling, which strongly implies that he needs to work on his lazy left-side cheek muscles.

cp - copying all files of a directory, including hidden ones

Posted on in ,

I outline my exploration of why cp -R src/. dest copies contents of src – that too all its files, including hidden ones – and not src itself, as src/. had initially led me to believe. This is because of how the algorithm of cp is defined.

Table of contents

  1. Table of contents
  2. Solution - cp -R src/. dest
  3. Why does it work?
  4. Why not just use cp -R src/* dest/?
  5. ChatGPT took me for a ride

There are two directories - a non-empty src and an empty dest. My aim is to copy all the files in src inside dest, including hidden files. 

$ ls -a src
.  ..  .hidden  unhidden

$ ls -a dest
. ..
src dir contains a normal file named unhidden and a hidden file called .hidden

Solution - cp -R src/. dest

I want to cut to the chase, so here is the command: cp -R src/. dest. 

$ cp -R src/. dest

$ ls -a dest
. .. .hidden unhidden
The src/. construct performs the magic.

I was more interested in figuring out why src/. makes this happen. It felt counter-intuitive to me – src/. simply refers to the . entry in the src directory, i.e., it refers to src itself. A command like cp -R src dest should have copied the entire src directory inside dest, ending up with a structure like dest/src/{.hidden,unhidden} instead of dest/{.hidden,unhidden}.

Why does it work?

After some faffing about, I found the algorithm used by cp as documented in POSIX. Below is the relevant portion of the algorithm.

cp -R source_file target

If target exists and names an existing directory, the name of the corresponding destination path for each file in the file hierarchy rooted in each source_file shall be the concatenation of target, a single >slash< character if target did not end in a >slash<, and the pathname of the file relative to the directory containing source_file. [Emphasis added]

I tried to understand the algorithm by building path for src/.hidden file. According to the algorithm, the path of a target file is a concatenation of:

  1. target (dest in our case) +
  2. / if needed (which we do) +
  3. relative pathname of the target file, starting from the directory that contains the source_file. I am interpreting heavily now – in our case of src/., the directory containing . is src. So the pathname for the file .hidden , relative from src, would become: ./.hidden.

Thus, the built path for the hidden file .hidden becomes: dest/./.hidden, which is equal to dest/.hidden.

Similarly, for the normal file unhidden, the built path becomes dest/./unhidden, which is equal to dest/unhidden.