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Basic Linux Operations FAQ

Part 3 of the Linux Newbie Administrator Guide

3.1.1 Filenames
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Related Resources
Linux Newbie Administrator Guide
0. Linux Benefit
1. Before Installation
2. Linux Resources/Help
3. Basic Operations FAQ
4. Newbie Admin FAQ
~ 4.1 Lilo
~ 4.2 Drives
~ 4.3 X-Windows
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~ 4.5 Networking
5. Shortcuts / Commands
6. Linux Applications
7. Learn Linux Commands
A. How to Upgrade Kernel?

Linux is case-sensitive. For example: myfile, Myfile, and myFILE are three different files. Your password and login name are also case-sensitive. (This follows tradition since both UNIX and the "c" programming language are case-sensitive.) Naming conventions for files and directories are identical. All the files and directories which I create (for myself, as a user) are lower-case, unless there is a very special reason to make it different. Most of Linux commands are also all lower case.
Filenames under Linux can be up to 256 characters long and they normally contain letters, numbers, "." (dots), "_" (underscores) and "-" (dashes). Other characters are possible but not recommended. In particular, it is not recommended to use special metacharacters: "*" (asterisk), "?" (question mark), " " (space), "$" (dollar sign), "&" (ampersand), any brackets, etc. This is because metacharacters have special meaning to the Linux shell (shell is something like COMMAND.COM, the command processor under DOS). It is possible to have a space in the filename, but we don't recommend it either--we use underscore "_" instead.
It is not possible at all to have '/' (slash) as a part of the filename because '/' is used to represent the top of the directory tree, and as a separator in the pathnames (the same as '\' is in DOS).

Like in DOS, I cannot have a file called . or a file called.. (dot or two dots)--they mean "current" and "parent to the current" directory respectively, exactly like in DOS.

Here is the meaning of some metacharacters:
* = Matches any sequence of zero or more characters, except for "." (a dot) at the beginning of a filename.
? = Matches any single character.
[abC1] = Matches a single character in the enumerated set. In this example the set contains: 'a', 'b', 'C', and '1'.
[a-z] = Matches any lower-case letter.
[A-F] = Matches any upper-case letter from A to F.
[0-9] = Matches any single digit.
[a-zA-Z0-9] = Matches any letter (lower or upper case) or any digit.

The character \ (backslash) is also special. It makes the subsequent special character aquire literal meaning (read on).

Examples. This command will list any filename in the current directory, with the exception of filenames starting with "." (dot):

ls *

An equivalent to this command is to type just ls or dir (without the "*"). Files with names starting with "." are not shown because "." as the first character of a filename is not matched by "*". Think of files with names starting with "." as an equivalent of DOS hidden files. Use ls -a (list with the option "all") or ls .* to see these "dot" files. The "dot-files" are common in the user home directories and they typically contain user-level configurations.
This command will list any file (in the current directory) that contains a dot (except files starting with a dot):

ls *.*

This command will list any filename that contains two dots (except those starting with a dot):

ls *.*.*

Please note that Linux does not have "filename extensions" the way DOS does, but you can still use them. For example, I can have a file my_text.txt.zip. Some other DOS-kind file-naming features are completely absent ("Micros~1.doc" comes to mind).
This command will find (on the whole filesystem) any file with the extension "htm" optionally followed by any one more character:

locate *.htm?

This command will show all filenames in the current directory that start with "a" or "b", or any capital letter:

ls [abA-Z]*

This command will list any file starting with "a" and ending with "n"

ls a*n

Command line autocompletion. This is a great command line feature--I use the [Tab] key a lot to save on typing. It makes it brisk to deal with long and complicated filenames. For example using such a filename on the command line is really not a problems, if I use autocompletion:

dir Eurosong\ 2000\ Olson\ Brothers\ -\ Fly\ on\ the\ wings\ of\ love\ \(denmark\).mp3

I just type

dir Eu

and if there are no other files starting with "Eu", the rest of the filename is automatically typed for me. Otherwise, I would have to look at my choices (which are printed for me) and type one or two more characters to make the filename unambiguous. The backslashes in the name of the example song above show that the spaces are "literal", i.e., they spaces are part of the filename.

Problems with weird filenames. Most of these problems can be solved using autocompletion. Additionally, to manipulate files with names that do contain metacharacters, I may use a pair of ' ' (two apostrophes), so that the metacharacters are quoted and therefore the shell does not interpret their meaning. For example, to rename a file my file* (contains space and asterisk), I would issue:

mv 'my file*' filename_without_weird_characters.txt

Please note that I use a pair of ' (apostrophes) for quoting. Quoting with a pair of " " (quotation marks) is generally weaker than quoting with ' ' . If you use " (quotation marks) some metacharacters may get interpreted by the shell (altering their meaning).

Following UNIX tradition, on Linux, one may create files with names contaning almost any character, including non-printable (control) characters. Those are very infrequent, but if you encounter such a file, it can make you feel really weird. I would rename such a file using a carefully positioned metacharacter. I would use ls first to try if my action indeed targets the desired file, and then rename the file (using the move "mv" command):

ls -l myfile*y.html
mv myfile*y.html myfile.html

(I assume that the non-standard character(s) are between the letters e and y.)

As an example of the perhaps weirdest problems that you might face when using non-recommended characters in a filename, try creating a file with a name starting with a dash and then remove it--there seems to be no way to do it (because a dash normally introduces command options). E.g., the command

dir > -junk

will create such a funny file (like in DOS, the symbol ">" redirects the output from the dir command to a file named "-junk"). Since the regular way of removing the file -junk does not work, I use:

rm ./-junk

The "dot slash" at the beginning means "the current directory" and here serves just the purpose of hiding the leading dash so it is not interpreted as introducing an option to the rm command. The point here is that I would rather stick to traditional naming conventions than face the occasional complications.

Besides using autocompletion, apostrophes and quotes, I can manipulate files with weird names using \ (backslash). Backslash hides the special meaning of the subsequent character. For example, i can create a weird file with the name *?[ using the following command:

touch \*\?\[

(The touch command creates an empty file or, if the file exists, updates its date/time of last modification.)

Next > 3.1.2 What are the different directories for?

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