Pass `.` the current directory symbol as an argument

Question

I read Windows - What are "." and ".." in a directory? - Super User

It explains:

The single dot . is also used if you want to pass the current directory as an argument to a command.

I am very confused. Can somebody give an example?

Answer 1

The question is about Windows but you tagged linux so my answer applies to Linux.

Every time you invoke a command, there's some working directory your're in at the moment; . denotes this directory. pwd is a general command to know what this directory is. While working in a shell like bash you can "go" to another directory with cd /another/directory/path.

Some commands (like cd above) accept directories (i.e. their paths) as argument(s). Now I'm feeling like talking about obvious basics; but your question seems to be basic, so here we go. The general syntax to pass an argument to some_command is like:

some_command argument

or even arbitrary number of arguments (four in this example):

some_command argument1 argument2 "argument3 with spaces" argument4

You can use . as an argument instead of the path returned by pwd. If the command interprets it as a path to directory then it should be understood as the current working directory. Although cd . has little sense because it makes your shell "go" to the same directory it's already in, the syntax is perfectly valid. Useful example is

df .

It reports disk space usage of the filesystem where your current working directory resides.

There are commands that operate on current working directory by default: ls is equivalent to ls ., du is equivalent to du .. Note this is not always the case: df prints all available filesystems while df . prints just one of them.

---

I wrote "if the command interprets it as a path to directory" because some commands don't. E.g. in bash the command

tr . , <<< foo.bar

will print foo,bar (tr replaces . with , in the given foo.bar string). This is because tr interprets . as a single-character string.

This shows . is not translated to current directory path by the shell. Compare this to ~ which is translated to the value of $HOME (try echo ~ in bash). The interpretation of . as a certain path lays way deep inside the design of directory tree in Linux (broader: in UNIX).

So every command can interpret . on its own. But when it's meant to be a directory path, the convention is it should be the current working directory. Linux understands this and provides tools to support this "conversion".

---

Also note ls -a almost always prints . among the "real" directory content. It gets interesting when your current working directory gets deleted. In this case ls -a prints nothing, but ls . or df . still work, as if . was there.

Answer 2

A slightly contrived Windows example: you can use . to set the attributes of the current directory:

C:\Path\to\Some\Directory> md sub1 sub2

C:\Path\to\Some\Directory> attrib * /s /d
             C:\Path\to\Some\Directory\sub1
             C:\Path\to\Some\Directory\sub2

C:\Path\to\Some\Directory> cd sub2

C:\Path\to\Some\Directory\sub2> attrib +r .

C:\Path\to\Some\Directory\sub2> cd ..

C:\Path\to\Some\Directory> attrib * /s /d
             C:\Path\to\Some\Directory\sub1
     R       C:\Path\to\Some\Directory\sub2

Here we set the read-only attribute on the sub2 directory while we’re in it.  attrib +r . is shorter and easier to type than attrib +r ..\sub2 or attrib +r C:\Path\to\Some\Directory \sub2.