The Linux Command Line (2012)

Part I. Learning the Shell

Chapter 3. Exploring the System

Now that we know how to move around the filesystem, it’s time for a guided tour of our Linux system. Before we start, however, we’re going to learn some more commands that will be useful along the way:

· ls—List directory contents.

· file—Determine file type.

· less—View file contents.

More Fun with ls

ls is probably the most used command and for good reason. With it, we can see directory contents and determine a variety of important file and directory attributes. As we have seen, we can simply enter ls to see a list of files and subdirectories contained in the current working directory:

[me@linuxbox ˜]$ ls
Desktop Documents Music Pictures Public Templates Videos

Besides the current working directory, we can specify the directory to list, like so:

me@linuxbox ˜]$ ls /usr
bin games kerberos libexec sbin src
etc include lib local share tmp

or even specify multiple directories. In this example we will list both the user’s home directory (symbolized by the ˜ character) and the /usr directory:

[me@linuxbox ˜]$ ls ˜ /usr
/home/me:
Desktop Documents Music Pictures Public Templates Videos
/usr:
bin games kerberos libexec sbin src
etc include lib local share tmp

We can also change the format of the output to reveal more detail:

[me@linuxbox ˜]$ ls -l
total 56
drwxrwxr-x 2 me me 4096 2012-10-26 17:20 Desktop
drwxrwxr-x 2 me me 4096 2012-10-26 17:20 Documents
drwxrwxr-x 2 me me 4096 2012-10-26 17:20 Music
drwxrwxr-x 2 me me 4096 2012-10-26 17:20 Pictures
drwxrwxr-x 2 me me 4096 2012-10-26 17:20 Public
drwxrwxr-x 2 me me 4096 2012-10-26 17:20 Templates
drwxrwxr-x 2 me me 4096 2012-10-26 17:20 Videos

By adding -l to the command, we changed the output to the long format.

Options and Arguments

This brings us to a very important point about how most commands work. Commands are often followed by one or more options that modify their behavior and, further, by one or more arguments, the items upon which the command acts. So most commands look something like this:

command -options arguments

Most commands use options consisting of a single character preceded by a dash, such as -l. But many commands, including those from the GNU Project, also support long options, consisting of a word preceded by two dashes. Also, many commands allow multiple short options to be strung together. In this example, the ls command is given two options, the l option to produce long format output, and the t option to sort the result by the file’s modification time:

[me@linuxbox ˜]$ ls -lt

We’ll add the long option --reverse to reverse the order of the sort:

[me@linuxbox ˜]$ ls -lt --reverse

The ls command has a large number of possible options. The most common are listed in Table 3-1.

Table 3-1. Common ls Options

Option	Long Option	Description
-a	--all	List all files, even those with names that begin with a period, which are normally not listed (i.e., hidden).
-d	--directory	Ordinarily, if a directory is specified, ls will list the contents of the directory, not the directory itself. Use this option in conjunction with the -l option to see details about the directory rather than its contents.
-F	--classify	This option will append an indicator character to the end of each listed name (for example, a forward slash if the name is a directory).
-h	--human-readable	In long format listings, display file sizes in human-readable format rather than in bytes.
-l		Display results in long format.
-r	--reverse	Display the results in reverse order. Normally, ls displays its results in ascending alphabetical order.
-S		Sort results by file size.
-t		Sort by modification time.

A Longer Look at Long Format

As we saw before, the -l option causes ls to display its results in long format. This format contains a great deal of useful information. Here is the Examples directory from an Ubuntu system:

-rw-r--r-- 1 root root 3576296 2012-04-03 11:05 Experience ubuntu.ogg
-rw-r--r-- 1 root root 1186219 2012-04-03 11:05 kubuntu-leaflet.png
-rw-r--r-- 1 root root 47584 2012-04-03 11:05 logo-Edubuntu.png
-rw-r--r-- 1 root root 44355 2012-04-03 11:05 logo-Kubuntu.png
-rw-r--r-- 1 root root 34391 2012-04-03 11:05 logo-Ubuntu.png
-rw-r--r-- 1 root root 32059 2012-04-03 11:05 oo-cd-cover.odf
-rw-r--r-- 1 root root 159744 2012-04-03 11:05 oo-derivatives.doc
-rw-r--r-- 1 root root 27837 2012-04-03 11:05 oo-maxwell.odt
-rw-r--r-- 1 root root 98816 2012-04-03 11:05 oo-trig.xls
-rw-r--r-- 1 root root 453764 2012-04-03 11:05 oo-welcome.odt
-rw-r--r-- 1 root root 358374 2012-04-03 11:05 ubuntu Sax.ogg

Let’s look at the different fields from one of the files and examine their meanings in Table 3-2.

Table 3-2. ls Long Listing Fields

Field	Meaning
-rw-r–r--	Access rights to the file. The first character indicates the type of file. Among the different types, a leading dash means a regular file, while a d indicates a directory. The next three characters are the access rights for the file’s owner, the next three are for members of the file’s group, and the final three are for everyone else. The full meaning of this is discussed in Chapter 9.
1	File’s number of hard links. See the discussion of links at the end of this chapter.
root	The user name of the file’s owner.
root	The name of the group that owns the file.
32059	Size of the file in bytes.
2012-04-03 11:05	Date and time of the file’s last modification.
oo-cd-cover.odf	Name of the file.

Determining a File’s Type with file

As we explore the system, it will be useful to know what files contain. To do this, we will use the file command to determine a file’s type. As we discussed earlier, filenames in Linux are not required to reflect a file’s contents. For example, while a filename like picture.jpg would normally be expected to contain a JPEG compressed image, it is not required to in Linux. We can invoke the file command this way:

file filename

When invoked, the file command will print a brief description of the file’s contents. For example:

[me@linuxbox ˜]$ file picture.jpg
picture.jpg: JPEG image data, JFIF standard 1.01

There are many kinds of files. In fact, one of the common ideas in Unix-like operating systems such as Linux is that “everything is a file.” As we proceed with our lessons, we will see just how true that statement is.

While many of the files on your system are familiar, for example MP3 and JPEG files, many kinds are a little less obvious, and a few are quite strange.

Viewing File Contents with less

The less command is a program to view text files. Throughout our Linux system, there are many files that contain human-readable text. The less program provides a convenient way to examine them.

Why would we want to examine text files? Because many of the files that contain system settings (called configuration files) are stored in this format, being able to read them gives us insight about how the system works. In addition, many of the actual programs that the system uses (calledscripts) are stored in this format. In later chapters, we will learn how to edit text files in order to modify system settings and write our own scripts, but for now we will just look at their contents.

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, understand only numbers, and all data is converted to numeric representation.

Some of these representation systems are very complex (such as compressed video 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 simple encoding scheme was first used on Teletype machines.

Text is a simple one-to-one mapping of characters to numbers. It is very compact. Fifty characters of text translate to fifty bytes of data. It is not the same as text in a word processor document such as one created by Microsoft Word or OpenOffice.org Writer. Those files, in contrast to simple ASCII text, contain many non-text elements that are used to describe their structure and formatting. Plain ASCII text files contain only the characters themselves and a few rudimentary control codes like tabs, carriage returns, and linefeeds.

Throughout a Linux system, many files are stored in text format, and many Linux tools work with text files. Even Windows recognizes the importance of this format. The well-known Notepad program is an editor for plain ASCII text files.

The less command is used like this:

less filename

Once started, the less program allows you to scroll forward and backward through a text file. For example, to examine the file that defines all the system’s user accounts, enter the following command:

[me@linuxbox ˜]$ less /etc/passwd

Once the less program starts, we can view the contents of the file. If the file is longer than one page, we can scroll up and down. To exit less, press the Q key.

Table 3-3 lists the most common keyboard commands used by less.

Table 3-3. less Commands

Command	Action
page up or b	Scroll back one page.
page down or Spacebar	Scroll forward one page.
Up Arrow	Scroll up one line.
Down Arrow	Scroll down one line.
G	Move to the end of the text file.
1G or g	Move to the beginning of the text file.
/characters	Search forward to the next occurrence of characters.
n	Search for the next occurrence of the previous search.
h	Display help screen.
q	Quit less.

Less Is More

The less program was designed as an improved replacement of an earlier Unix program called more. Its name is a play on the phrase “less is more”—a motto of modernist architects and designers.

less falls into the class of programs called pagers, programs that allow the easy viewing of long text documents in a page-by-page manner. Whereas the more program could only page forward, the less program allows paging both forward and backward and has many other features as well.

A Guided Tour

The filesystem layout on your Linux system is much like that found on other Unix-like systems. The design is actually specified in a published standard called the Linux Filesystem Hierarchy Standard. Not all Linux distributions conform to the standard exactly, but most come pretty close.

Next, we are going to wander around the filesystem ourselves to see what makes our Linux system tick. This will give you a chance to practice your navigation skills. One of the things we will discover is that many of the interesting files are in plain, human-readable text. As we go about our tour, try the following:

1. cd into a given directory.

2. List the directory contents with ls -l.

3. If you see an interesting file, determine its contents with file.

4. If it looks as if it might be text, try viewing it with less.

Note

Remember the copy-and-paste trick! If you are using a mouse, you can double-click a filename to copy it and middle-click to paste it into commands.

As we wander around, don’t be afraid to look at stuff. Regular users are largely prohibited from messing things up. That’s the system administrator’s job! If a command complains about something, just move on to something else. Spend some time looking around. The system is ours to explore. Remember, in Linux, there are no secrets!

Table 3-4 lists just a few of the directories we can explore. Feel free to try more!

Table 3-4. Directories Found on Linux Systems

Directory	Comments
/	The root directory, where everything begins.
/bin	Contains binaries (programs) that must be present for the system to boot and run.
/boot	Contains the Linux kernel, initial RAM disk image (for drivers needed at boot time), and the boot loader. Interesting files: · /boot/grub/grub.conf or menu.lst, which are used to configure the boot loader · /boot/vmlinuz, the Linux kernel
/dev	This is a special directory that contains device nodes. “Everything is a file” also applies to devices. Here is where the kernel maintains a list of all the devices it understands.
/etc	The /etc directory contains all of the system-wide configuration files. It also contains a collection of shell scripts that start each of the system services at boot time. Everything in this directory should be readable text. Interesting files: While everything in /etc is interesting, here are some of my all-time favorites: · /etc/crontab, a file that defines when automated jobs will run · /etc/fstab, a table of storage devices and their associated mount points · /etc/passwd, a list of the user accounts
/home	In normal configurations, each user is given a directory in /home. Ordinary users can write files only in their home directories. This limitation protects the system from errant user activity.
/lib	Contains shared library files used by the core system programs. These are similar to DLLs in Windows.
/lost+found	Each formatted partition or device using a Linux filesystem, such as ext3, will have this directory. It is used in the case of a partial recovery from a filesystem corruption event. Unless something really bad has happened to your system, this directory will remain empty.
/media	On modern Linux systems the /media directory will contain the mount points for removable media such as USB drives, CD-ROMs, etc. that are mounted automatically at insertion.
/mnt	On older Linux systems, the /mnt directory contains mount points for removable devices that have been mounted manually.
/opt	The /opt directory is used to install “optional” software. This is mainly used to hold commercial software products that may be installed on your system.
/proc	The /proc directory is special. It’s not a real filesystem in the sense of files stored on your hard drive. Rather, it is a virtual filesystem maintained by the Linux kernel. The “files” it contains are peepholes into the kernel itself. The files are readable and will give you a picture of how the kernel sees your computer.
/root	This is the home directory for the root account.
/sbin	This directory contains “system” binaries. These are programs that perform vital system tasks that are generally reserved for the superuser.
/tmp	The /tmp directory is intended for storage of temporary, transient files created by various programs. Some configurations cause this directory to be emptied each time the system is rebooted.
/usr	The /usr directory tree is likely the largest one on a Linux system. It contains all the programs and support files used by regular users.
/usr/bin	/usr/bin contains the executable programs installed by your Linux distribution. It is not uncommon for this directory to hold thousands of programs.
/usr/lib	The shared libraries for the programs in /usr/bin.
/usr/local	The /usr/local tree is where programs that are not included with your distribution but are intended for system-wide use are installed. Programs compiled from source code are normally installed in /usr/local/bin. On a newly installed Linux system, this tree exists, but it will be empty until the system administrator puts something in it.
/usr/sbin	Contains more system administration programs.
/usr/share	/usr/share contains all the shared data used by programs in /usr/bin. This includes things like default configuration files, icons, screen backgrounds, sound files, etc.
/usr/share/doc	Most packages installed on the system will include some kind of documentation. In /usr/share/doc, we will find documentation files organized by package.
/var	With the exception of /tmp and /home, the directories we have looked at so far remain relatively static; that is, their contents don’t change. The /var directory tree is where data that is likely to change is stored. Various databases, spool files, user mail, etc. are located here.
/var/log	/var/log contains log files, records of various system activity. These are very important and should be monitored from time to time. The most useful one is /var/ log/messages. Note that for security reasons on some systems, you must be the superuser to view log files.

Symbolic Links

As we look around, we are likely to see a directory listing with an entry like this:

lrwxrwxrwx 1 root root 11 2012-08-11 07:34 libc.so.6 -> libc-2.6.so

Notice how the first letter of the listing is l and the entry seems to have two filenames? This is a special kind of a file called a symbolic link (also known as a soft link or symlink). In most Unix-like systems it is possible to have a file referenced by multiple names. While the value of this may not be obvious now, it is really a useful feature.

Picture this scenario: A program requires the use of a shared resource of some kind contained in a file named foo, but foo has frequent version changes. It would be good to include the version number in the filename so the administrator or other interested party could see what version of foo is installed. This presents a problem. If we change the name of the shared resource, we have to track down every program that might use it and change it to look for a new resource name every time a new version of the resource is installed. That doesn’t sound like fun at all.

Here is where symbolic links save the day. Let’s say we install version 2.6 of foo, which has the filename foo-2.6, and then create a symbolic link simply called foo that points to foo-2.6. This means that when a program opens the file foo, it is actually opening the file foo-2.6. Now everybody is happy. The programs that rely on foo can find it, and we can still see what actual version is installed. When it is time to upgrade to foo-2.7, we just add the file to our system, delete the symbolic link foo, and create a new one that points to the new version. Not only does this solve the problem of the version upgrade, but it also allows us to keep both versions on our machine. Imagine that foo-2.7 has a bug (damn those developers!) and we need to revert to the old version. Again, we just delete the symbolic link pointing to the new version and create a new symbolic link pointing to the old version.

The directory listing above (from the /lib directory of a Fedora system) shows a symbolic link called libc.so.6 that points to a shared library file called libc-2.6.so. This means that programs looking for libc.so.6 will actually get the file libc-2.6.so. We will learn how to create symbolic links in the next chapter.

Hard Links

While we are on the subject of links, we need to mention that there is a second type of link called a hard link. Hard links also allow files to have multiple names, but they do it in a different way. We’ll talk more about the differences between symbolic and hard links in the next chapter.