punctuation

#! rule

At its lowest level, the Unix kernel executes programs with the exec(2) system call. Typically the file to execute is one of two kinds:
- a binary executable file
- a text script file

In the case of a textual script file, if the first two characters of the file are the characters # and then !, the shell program that's used to interpret the file is taken from the rest of that line. (If not, the file is thrown at the default shell for that user. Since shell languages differ syntactically, this is to be avoided.)

Interpreters that are not considered shells can also be used if that script is written in the corresponding language.

Here are some typical examples you might find at the top of a script:
#!/bin/sh       # run with borne shell
#!/bin/csh       # run with C shell
#!/usr/bin/perl       # run a perl program
#!/usr/bin/perl -w       # run a perl program, with warnings turned on
#!/bin/bash       # run specifically with Bash shell

Pronounciation: this rule is usually pronounced The she-bang rule.

. (the dot directory)

In the Unix filesystem, every directory is supposed to have a subdirectory named with a single period. This is a sortof alternate name for the directory itself. It is actually a special kind of Hard Link to the directory.

In unusual circumstances, you might encounter a directory that has no . entry. This might happen in special virtual directories created on the fly, such as entries in the /proc directory. Usually this is not a cause for concern unless you need to reference them.

You might also see directories with two or more . entries. Some software adds in the dot and/or the dotdot entries, artificially, because it anticipates that they don't already exist naturally. When they do, you see two of each, or more. This is harmless and you can, for instance, click on either to get your work done. Applications such as Tint cannot distinguish between them anyway. Because all of the system calls take a character string name, if you somehow end up with duplicate filenames in a directory (a highly irregular situation), all operations on that name will always work on the same instance.

.. (the dot dot directory)

In the Unix filesystem, every directory is supposed to have a subdirectory named with two periods. This is a sortof alternate name for the parent directory itself. (The exception is that the root of the filesystem has a .. file that points to itself.) It is actually a special kind of Hard Link to the directory.

See also . (dot directory)

.DS_Store

The .DS_Store file that is found in many directories is installed by the Mac OS Finder. It stores per-directory settings, mostly affecting the way the directory appears in the Finder, as in the View Options box.

Outside of Mac OS, the file is ignored.

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