Looking Around
Now that you know how to move from working directory to
working directory, we're going to take a tour of your Linux system and,
along the way, learn some things about what makes it tick. But before
we begin, I have to teach you some tools that will come in handy during
our adventure. These are:
- ls (list
files and directories)
- less (view
text files)
- file
(classify a file's contents)
The ls command is used to list the contents of a
directory. It is probably the most commonly used Linux command. It can
be used in a number of different ways. Here are some examples:
Examples of the ls command
| Command |
Result |
| ls |
List the files in the working directory
|
| ls /bin |
List the files in the /bin directory (or any other
directory you care to specify)
|
| ls -l |
List the files in the working directory in long
format
|
| ls -l /etc /bin |
List the files in the /bin directory and the /etc
directory in long format
|
| ls -la .. |
List all files (even ones with names beginning
with a period character, which are normally hidden) in the parent of
the working directory in long format
|
These examples also point out an important concept about
commands. Most commands operate like this:
command -options arguments
where command is the name of the command, -options
is one or more adjustments to the command's behavior, and arguments
is one or more "things" upon which the command operates.
In the case of ls, we see that ls is the name of the
command, and that it can have one or more options, such as -a and -l,
and it can operate on one or more files or directories.
A Closer Look At Long Format
If you use the -l option with ls, you will get a file
listing that contains a wealth of information about the files being
listed. Here's an example:
-rw------- 1 bshotts bshotts 576 Apr 17 1998 weather.txt drwxr-xr-x 6 bshotts bshotts 1024 Oct 9 1999 web_page -rw-rw-r-- 1 bshotts bshotts 276480 Feb 11 20:41 web_site.tar -rw------- 1 bshotts bshotts 5743 Dec 16 1998 xmas_file.txt ---------- ------- ------- -------- ------------ ------------- | | | | | | | | | | | File Name | | | | | | | | | +--- Modification Time | | | | | | | +------------- Size (in bytes) | | | | | +----------------------- Group | | | +-------------------------------- Owner | +---------------------------------------------- File Permissions
- File Name
- The name of the file or directory.
- Modification Time
- The last time the file was modified. If the last
modification occurred more than six months in the past, the date and
year are displayed. Otherwise, the time of day is shown.
- Size
- The size of the file in bytes.
- Group
- The name of the group that has file permissions in
addition to the file's owner.
- Owner
- The name of the user who owns the file.
- File Permissions
- A representation of the file's access permissions.
The first character is the type of file. A "-" indicates a regular
(ordinary) file. A "d" indicates a directory. The second set of three
characters represent the read, write, and execution rights of the
file's owner. The next three represent the rights of the file's group,
and the final three represent the rights granted to everybody else.
less is a program that lets you view text files. This is
very handy since many of the files used to control and configure Linux
are human readable (as opposed to the legacy operating systems).
What is "text"?
There are many ways to represent information on a
computer. All methods involve defining a relationship between the
information and some numbers that will be used to represent it.
Computers, after all, only understand numbers and all data is converted
to numeric representation.
Some of these representation systems are very complex
(such as compressed image files), while others are rather simple. One
of the earliest and simplest is called ASCII text. ASCII
(pronounced "As-Key") is short for American Standard Code for
Information Interchange. This is a simple encoding scheme that was
first used on Teletype machines to map keyboard characters to numbers.
Text is a simple one-to-one mapping of characters to
numbers. It is very compact. Fifty characters of text translates to
fifty bytes of data. Throughout a Linux system, many files are stored
in text format and there are many Linux tools that work with text
files. Even the legacy operating systems recognize the importance of
this format. The well-known NOTEPAD.EXE program is an editor for plain
ASCII text files.
The less program is invoked by simply typing:
less text_file
This will display the file.
Controlling less
Once started, less will display the text file one page
at a time. You may use the Page Up and Page Down keys to move through
the text file. To exit less, type "q". Here are some commands that less
will accept:
Keyboard commands for the less program
| Command |
Action |
|
Page Up or b
|
Scroll back one page
|
|
Page Down or space
|
Scroll forward one page
|
|
G
|
Go to the end of the text file
|
|
1G
|
Go to the beginning of the text file
|
|
/characters
|
Search forward in the text file for an occurence
of the specified characters
|
|
n
|
Repeat the previous search
|
|
q
|
Quit
|
As you wander around your Linux system, it is helpful to
determine what a file contains before you try to view it. This is where
the file command comes in. file will examine a file and tell you what
kind of file it is.
To use the file program, just type:
file name_of_file
The file program can recognize most types of files, such
as:
Various kinds of files
| File Type |
Description |
Viewable as text? |
|
ASCII text
|
The name says it all
|
yes
|
|
Bourne-Again shell script text
|
A bash script
|
yes
|
|
ELF 32-bit LSB core file
|
A core dump file (a program will create this when
it crashes)
|
no
|
|
ELF 32-bit LSB executable
|
An executable binary program
|
no
|
|
ELF 32-bit LSB shared object
|
A shared library
|
no
|
|
GNU tar archive
|
A tape archive file. A common way of storing
groups of files.
|
no, use tar tvf to view listing.
|
|
gzip compressed data
|
An archive compressed with gzip
|
no
|
|
HTML document text
|
A web page
|
yes
|
|
JPEG image data
|
A compressed JPEG image
|
no
|
|
PostScript document text
|
A PostScript file
|
yes
|
|
RPM
|
A Red Hat Package Manager archive
|
no, use rpm -q to examine contents.
|
|
Zip archive data
|
An archive compressed with zip
|
no
|
While it may seem that most files cannot be viewed as
text, you will be surprised how many can. This is especially true of
the important configuration files. You will also notice during our
adventure that many features of the operating system are controlled by
shell scripts. In Linux, there are no secrets!
Linux® is a registered trademark of Linus Torvalds.
|