Matt Terwilliger

The Many Meanings of the Dot

That little . character sure gets a lot of use on our keyboard. Naturally, in prose as a full stop, but I’m talking about in software. www.google.com. cd ... The list goes on. Behind each seemingly innocuous dot, hides some pretty fun abstractions. Let’s dive into a few:

In file paths

If you’ve spent any time in a shell, you’ve probably used . or .. to reference the current and parent directory, respectively. It’s a handy shortcut! How and why it works under the hood is more subtle.

To begin understanding the dot in relation to file paths we need to introduce the two different types of paths: absolute and relative. An absolute path begins with a /, the root directory. A relative path is any path that does not begin with /. It is relative to your current directory — more on that later. All relative paths can be ‘resolved’ to an absolute path.

For example, /tmp is an absolute path. If your current directory was /, you could also reference the same directory as tmp, ./tmp, or even ./././././tmp, as . in this context will always refer to the current directory.

But what provides this concept of a ‘current directory’? The shell? The file system? Turns out it’s the kernel. Each process has associated with it a working directory that can change throughout the life of the process (see man 2 getcwd / man 2 chdir). Since the kernel tracks a processes’ working directory, we hypothesize it also provides the . and .. abstractions for navigating file hierarchies. A “quick” read through the POSIX standard confirms this:

The special filename dot shall refer to the directory specified by its predecessor. The special filename dot-dot shall refer to the parent directory of its predecessor directory. As a special case, in the root directory, dot-dot may refer to the root directory itself.

An interesting property of this functionality being built in at such a low level is how it simplifies programs and scripts. When writing code, you don’t necessarily need to know (though you can always find out) where in the filesystem you are — a relative path is often just as good as an absolute path.

Aside: another common (but distinct) usage of the dot in file paths is to hide files whose name begins with a .. I’ll leave this one for you. Is there anything special about so-called dotfiles?

Shells

In POSIX-compliant shells (think bash, zsh, etc.), the command . followed by a filename executes commands from a file in the current shell.

Don’t take my word for it:

$ help .
.: . filename [arguments]
Execute commands from a file in the current shell.

Read and execute commands from FILENAME in the current shell.  The
entries in $PATH are used to find the directory containing FILENAME.
If any ARGUMENTS are supplied, they become the positional parameters
when FILENAME is executed.

Exit Status:
Returns the status of the last command executed in FILENAME; fails if
FILENAME cannot be read.

Note that we have to use help here, not man because the . is a bash builtin, meaning it’s literally built into bash, which itself is just a C program.

Normally, when a shell executes an external program it will do so in a subprocess. In order to execute a new command, the shell first asks the kernel to create a copy of itself using something like the fork system call (man 2 fork). This copy is identical, except for its process ID (PID). The new subprocess then replaces itself with the program you supplied (man 2 execve). This “program” is just a file (in *nix systems, everything is just a file) that has its executable permission bit set (if you’ve ever done a chmod +x).

Using the . (or, in many shells, its equivalent command source) skips fork-exec and instead reads commands from a file directly into your shell as if you were typing them at an interactive prompt. Somewhat counterintuitively, however, any programs themselves will still be executed in a subprocess; if you want an external program to replace the current shell there’s a builtin for that, too (see help exec).

Let’s break this down with an example. There’s one more thing to understand, first: also builtin to bash are a few special variables that we can reference. One of which, $$, will tell us our current PID.

Assume I’m in a bash shell with PID 76735:

$ echo $$
76735

Take this simple script:

$ cat /tmp/getmypid
#!/bin/bash

echo "My PID is $$"
echo "My parent's PID is $PPID"

If we run it normally as an external program you’ll notice that the PID is new (a subprocess) but the parent’s PID is in fact our shell:

$ chmod +x /tmp/getmypid
$ /tmp/getmypid
My PID is 77128
My parent's PID is 76735

Now notice the distinction when prefaced with a .:

$ . /tmp/getmypid
My PID is 76735
My parent's PID is 75379

The script is being executed in the shell and the parent PID is that of whatever process spawned the shell.

A few asides:

Networks

DNS is the protocol responsible for translating the human-readable www.google.com into an IP address. IP addresses (like 74.125.141.147) are essential to routing requests across the Internet. Both DNS and IPv4 use the dot as a separator, but each in a unique way.

DNS

In DNS, even though you don’t see it, fully qualified domain names actually end with a dot. www.google.com is actually www.google.com.. The reason for this is that DNS resolution works recursively. The authoritative answer for

What is www.google.com’s IP address?

actually comes from answering a series of questions generated by traversing the domain name from right to left.

The rightmost . represents the DNS root zone — yep, that’s a thing! The root zone knows the nameservers for all top level domains (TLDs) like com, net, org, etc. Each TLD’s nameservers in turn resolve another level down, so on and so forth.

This is a bit of an oversimplification, but this article is on the dot, not DNS. Let’s skip ahead to the action:

$ dig +trace www.google.com
; <<>> DiG 9.8.3-P1 <<>> +trace www.google.com
;; global options: +cmd
.                       29875   IN      NS      d.root-servers.net.
.                       29875   IN      NS      f.root-servers.net.
.                       29875   IN      NS      g.root-servers.net.
.                       29875   IN      NS      l.root-servers.net.
.                       29875   IN      NS      j.root-servers.net.
.                       29875   IN      NS      a.root-servers.net.
.                       29875   IN      NS      h.root-servers.net.
.                       29875   IN      NS      m.root-servers.net.
.                       29875   IN      NS      c.root-servers.net.
.                       29875   IN      NS      e.root-servers.net.
.                       29875   IN      NS      i.root-servers.net.
.                       29875   IN      NS      b.root-servers.net.
.                       29875   IN      NS      k.root-servers.net.
;; Received 228 bytes from 192.168.1.1#53(192.168.1.1) in 26 ms

com.                    172800  IN      NS      a.gtld-servers.net.
com.                    172800  IN      NS      b.gtld-servers.net.
com.                    172800  IN      NS      c.gtld-servers.net.
com.                    172800  IN      NS      d.gtld-servers.net.
com.                    172800  IN      NS      e.gtld-servers.net.
com.                    172800  IN      NS      f.gtld-servers.net.
com.                    172800  IN      NS      g.gtld-servers.net.
com.                    172800  IN      NS      h.gtld-servers.net.
com.                    172800  IN      NS      i.gtld-servers.net.
com.                    172800  IN      NS      j.gtld-servers.net.
com.                    172800  IN      NS      k.gtld-servers.net.
com.                    172800  IN      NS      l.gtld-servers.net.
com.                    172800  IN      NS      m.gtld-servers.net.
;; Received 492 bytes from 199.7.83.42#53(199.7.83.42) in 49 ms

google.com.             172800  IN      NS      ns2.google.com.
google.com.             172800  IN      NS      ns1.google.com.
google.com.             172800  IN      NS      ns3.google.com.
google.com.             172800  IN      NS      ns4.google.com.
;; Received 280 bytes from 192.41.162.30#53(192.41.162.30) in 188 ms

www.google.com.         300     IN      A       74.125.141.104
www.google.com.         300     IN      A       74.125.141.106
www.google.com.         300     IN      A       74.125.141.99
www.google.com.         300     IN      A       74.125.141.103
www.google.com.         300     IN      A       74.125.141.105
www.google.com.         300     IN      A       74.125.141.147
;; Received 128 bytes from 216.239.36.10#53(216.239.36.10) in 52 ms

Notice how the path gradually builds up from . to com. to google.com. and finally to www.google.com.. We now have an IP address (actually, 6 IP addresses) for www.google.com, which contains more dots.

These dots are, you guessed it, completely different from any we’ve seen thus far.

IPv4

Unlike DNS, the Internet Protocol (IP) doesn’t work recursively. Each IPv4 address is just a 32 bit number composed of four 8-bit octets. However, you usually see an IP represented as a set of four base-10 numbers ranging from 0 to 255, separated by dots. We’ll be using 74.125.141.147, one of Google’s IPs from above as our example.

The actual IP address that gets sent over the wire is the binary representation of these octets. In fact, we can represent an IP address any number of ways. Let’s hop into a Python interpreter to do some quick conversions:

$ python3
>>> ip_as_octets = '74.125.141.147'.split('.')
['74', '125', '141', '147']
>>> as_binary_list = ['{:08b}'.format(int(octet)) for octet in ip_as_octets]
['01001010', '01111101', '10001101', '10010011']
>>> as_binary = ''.join(as_binary_list)
'01001010011111011000110110010011'
>>> len(as_binary)
32
>>> as_decimal = int(as_binary, 2)
1249742227
>>> as_hex = hex(as_decimal)
'0x4a7d8d93'

So, 74.125.141.147 is the same as 0b01001010011111011000110110010011 in binary, 1249742227 in decimal, and 0x4a7d8d93 in hexadecimal.

Aside: I noticed when writing this post that decimal and hexadecimal IPs are actually interpreted correctly by something (we’ve gone down enough rabbit holes for today) — try passing one into ping, curl, or even your web browser. Nifty!

Recap

While we’ve covered a lot of ground, we also definitely have not hit everything; take regular expressions as another example.

Why the dot has been adopted and used in so many different ways? Maybe because it doesn’t draw attention to itself? Because of its use in language? Its position on the keyboard? I have no idea.