The Linux Command Line by William Shotts
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
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:
| 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. I’ll discuss this in more detail in a later lesson.
less
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.
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 multimedia 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:
| 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 occurrence of the specified characters |
| n | Repeat the previous search |
| h | Display a complete list less commands and options |
| q | Quit |
file
As you wander around your Linux system, it is helpful to determine what kind of data 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:
| 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!