In UNIX, filenames are case-sensitive. All that means is that UNIX treats upper and lowercase letters as distinct in a filename.
For example, the filename Aunt_Betty is a different filename than aunt_betty.
File Extensions
Often, you'll see filenames that include something called an extension. For example, if I had a set of documents that I created with my favorite word processor I might give them filenames like:
paper1.doc
paper2.doc
short_essay.doc
resume.doc
These files have names just like the files mentioned before, the only difference is that these filenames all end in .doc. The .doc part of the filename is what we call the extension and the part before .doc is called the base: . Note that paper1.doc is just a filename just as paper1 is a filename and, in that sense, the extension is nothing special.
We use extensions in order to give information about the format in which information is stored in a file as part of its filename itself. For example, I used .doc above to indicate files that were prepared by my word processor.
Extensions, by convention, begin with a period and are usually followed by a short abbreviation. For example, if I prepared some text with an editor (like Emacs) and then wanted to store that in a file, I might name that file something.txt.
Historically, it has become common to use extensions in this way. For certain file formats, common extensions are often used. For example,
.doc
a file prepared by a word processor.
.txt
a plain text file.
.c
source code for a program written in the C language.
.bak
a backup copy of another file.
It should be noted that you can often choose whatever extensions you want; however, there are some common ones that most people use for certain file formats and you should use them. Furthermore, some programs will require you to put certain extensions on files in order to deal with them properly. After time, you will become familiar with what extensions are commonly used, and when certain extensions are required.
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