2009.10_Bash Bootcamp-Grasping the Basics of Bash.pdf

Grasping the basics of Bash

BASH Bootcamp

Even experts forget the fundamentals. Learn more about - or get a refresher on - the basics of Bash.

By Bruce Byfield

Andrey Tsidvintsev, 123RF

One disadvantage of the modern emphasis on the desktop is that people learn about the command line only

when they need it. As a result, their knowledge of is often haphazard and full of gaps. For example, for years

I've been using the su - command several times a day to log in as root from a virtual terminal on the desktop

of my everyday account. I always thought that when you were finished as root, you couldn't return to the

everyday account in the same terminal; instead, you had to close the window and open another one. Then I

learned, purely by accident, that all I really needed to do was type exit .

Since then, I've discovered that, if pressed, even experts would confess to a similar gap in their knowledge at

one time or the other.

Back to the Basics

In this article, I'll discuss a topic I should have covered months ago: the basics of Bash, the command line

used by most GNU/Linux distributions. I'll discuss the basic tools and sources for information, as well as the

basics of navigation. Even if you know some of this information, a systematic discussion might fill in gaps in

your knowledge. If not, you can always pass on the information to a desktop user to further their education.

The command line is one of the most basic interfaces ever invented. All you have to do is enter the correct

string of characters to get the results you need. Right?

Well, not entirely.

This approach works if you approach the command line like a series of magical spells, each of which you

must carefully copy before pressing the Enter key; however, if you add any of your own input, even the name

to call a saved file, you frequently need to know more.

Case-Sensitive

For one thing, you need to know that, unlike the Windows command line, Bash is case sensitive, treating

uppercase letters differently from lowercase letters. For this reason, experienced users tend to avoid

capitalizing anything; they don't want to bother dealing with the extra keystrokes.

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More importantly, this also means that a file called taxsummary is not the same as one called Taxsummary ; if

you forget this basic fact, you can waste time trying to locate a file.

Character Choice

Additionally, certain characters are not allowed in the command line in order to avoid confusion. Prohibited

characters include a space, which separates out parts of a command; the forward slash, which separates out

directory names in a file path (not a backward slash, as in Windows); and characters such as a question mark

or asterisk, which are used in regular expressions or wild cards.

These restrictions can cause some conflicts with the desktop, which has more relaxed rules, especially about

the use of spaces in file names. However, you can get around these restrictions in several ways. The easiest

way is simply to avoid using anything more exotic than a period in the file name and to use an underscore or

hyphen instead of a space, the way that old Unix hands do. Almost as easily, you can use single or double

quotation marks around file names with spaces or restricted characters in them as a signal that the names

should not be read as they normally would.

Or, if you prefer, you can do what is called escaping and place a forward slash in front of a character. The

forward slash is a signal that the next character - and only the next character - is not to be read normally. For

instance, if you entered Grocery\ list , you would be telling Bash to read the space between the two words as

part of a continuous file name. Without the forward slash, Bash would read your entry as two files, Grocery

and list .

Command Structure

A typical command has three parts that appear in a set order:

The command - The basic action that you are performing. Sometimes, entering just a command will

do something basic. For instance, if you type just cal , then the output is a calendar for the current

month. More often, however, the command by itself does nothing.

Options, arguments, variables or parameters - Variations on what the command does. Options that

were developed for Unix or by unaffiliated developers are a single letter with a hyphen in front them.

Options developed by the GNU project are whole words with two hyphens in front of them. Often, a

command will have one of each that does the same thing, and you can choose whether you want the

speed of a single-hyphen option or the clarity of a two-hyphen option. Some options also end in a

specification; for example, if you use -tvfat with the mount command, you are indicating that you

want to add a device to your system that is formatted for VFAT or FAT32, the old format once used

by Windows. Options vary between commands, but you will find some frequently used conventions,

such as -r or R , which mean that the command affects subdirectories and their contents, and -v , which

tells the command to display an explanation of what it is doing.

Input - The working material for a command. This may be a single file, as when you are using the rm

command to delete a file. At other times, as with the mv command, it may be an original file name

followed by a space and the new path for the file. It may also be a string of characters for which you

are searching or a specification, such as the format used by the date command.

A few commands also have sub-commands that come directly after the command, such as the install or

remove sub-commands used by apt-get, the tool used for installing packages in Debian and Ubuntu.

You'll probably notice, too, that many commands are abbreviations that describe their function. For instance,

cp copies a file, and ls lists a file. This information comes in handy when you have a purpose in mind, but are

unsure what command you need.

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Figure 1: Tab completion can help you find the command you want.

Command-Line Shortcuts

One reason that Bash is easier to use than the Windows command line is that it has several built-in tools for

making life easier. These tools can be tremendous time-savers - to say nothing of memory aids.

The first of these tools is a command history, which stores the commands that you enter. You can use the Up

and Down arrow keys to move through the history and save yourself having to retype long or

hard-to-remember commands.

The command history includes all sorts of interesting tricks, most of which are topics for another day.

However, one basic tip is to refer back to a previous entry by entering !-[number] and pressing Ctrl+j. For

example, if you ran the ls command three entries ago, typing !-3 would automatically display the ls command,

but not run it. Alternatively, you could use the structure ![string] to find the last entry in the history that

matched the string.

Another time-saver is tab completion. When you have entered part of a path or a command, you can press the

Tab key to complete it. If more than one possibility exists, Bash will display them in multi-column form, and

you can continue entering letters until only one possibility remains.

For example, if you enter ch , then press the Tab key, you see a list of 22 possible completions. Continue by

typing an a , and press Tab again, and the possibilities are reduced to five. Add a t and press Tab, and, this

time, the only choice is chattr , whose final letters are added after your typing.

If, like many people, you are using a virtual terminal, you also have basic copy and paste functionality

available. You can use the basic commands listed in the Edit menu of your terminal, but, if you prefer, you

can highlight some text, then move the cursor to the position where you want to repeat the text and press the

left mouse button.

Basic Navigation

When you are new to GNU/Linux, you soon realize that it has a directory structure completely different from

Windows. You will probably quickly learn your way around your home directory, because you can set it up as

you please, but, beyond the fact that it is a subdirectory of /home, you may have little idea of the rest of the

directory tree. Fortunately, you can use the command man here to read a summary of the main directories.

From that, you can learn, for example, whether a command is more likely to be in /bin or in /sbin, where only

the root user can run it.

Another point that you soon learn is that your home directory is full of configuration files that start with a

period and are not ordinarily listed when you run the ls command. These are not listed because most of the

time you do not want them - and, perhaps, because if they are out of sight, you are less likely to tamper with

them. The fact they are not listed does not mean that you cannot use the cd command to move into them, but,

if you need to see their names, then ls -A will reveal them.

Where and Who?

If you get lost in the directory structure while in the command line, you can use pwd , which is short for

BASH Bootcamp

"present working directory" to get your bearings. If you have been switching accounts and want to be sure, for

example, that you are not running a dangerous command as root, then the whoami command will tell you the

name of the account that you are currently using. If, however, you find yourself continually forgetting the

current account name, you might want to research how to add it to your command prompt (Figure 2).

Figure 2: The pwd and whoami commands can tell you where you are in the directory.

Save Time

As you enter commands, Bash also has several ways to save time and reduce typing. Unless a full path is

given, any files or sub-directories that you type are assumed to be in the present directory. However, if you

want to be absolutely sure (which is never a bad idea), you can use ./ to indicate the present directory. For

instance, if you are in a user account named bruce , then typing /home/bruce/download is the same as typing

./download . If your sub-directories are several levels deep, then this piece of shorthand can significantly

reduce your typing.

There are also several different shortcuts that you can use with the cd (change directory) command:

cd .. - Moves you to the directory directly above the current one.

cd - - Returns you to the previous directory.

cd ~ - Returns you to the home directory for the current account.

cd ~[directory name] - Changes to the directory specified within the home directory for the current

account.

These last two shortcuts explain why you often see references to the home directory reduced to a tilde. For

example, if you see a reference to ~/.bashrc , it is indicating the .bashrc configuration file in your home

directory.

Finding Command Help

As you work at the command line, you will undoubtedly come across unfamiliar commands, many with more

options than you can easily remember. If all you want is a quick explanation of a command, you can use

apropos or what is . The only trouble is that, while these commands can serve as reminders for veteran users,

their output is often cryptic to the less experienced. For example, if you enter apropos chmod , the definition

you receive is "change file mode bits," which is geeky enough to scare off most beginners (Figure 3).

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Figure 3: Man pages can be intimidating in their thoroughness, as the start of the page for apt-get shows.

You can get more detailed information by typing man followed by the name of the command. However, the

quality of so-called man pages varies wildly. Although some pages, especially more recent ones, read easily

enough, others are written for an expert audience. In fact, some may have remained almost unchanged for a

decade or more.

In theory, info pages - the explanatory notes you receive when you enter info followed by a command name -

are more user-friendly than man pages. And, in practice, they often are. However, they are sometimes little

better than the worst man pages and can vary almost as much. Moreover, the original impetus to help info

pages evolve into a replacement for man pages seems to have weakened in recent years, and the quality varies.

Often, the most useful help you can find is to use the -h or --help option for a command, which produces a

short summary of options. What is missing, however, is any sort of built-in help that allows you to enter a

description of the action you would like to perform and receive a list of possible commands. Unfortunately,

the options here are the only ones available to you without searching on the web.

Just the Beginning

Naturally, there is more to Bash than detailed here. Bash is such a large subject that it takes years to learn its

intricacies. In fact, it is so large that few people can claim to know it completely.

However, the points I mention here should serve as a general orientation. If nothing else, they should be

enough to show that Bash is a flexible program with countless ways to save you effort - if only you take the

time to learn. If you familiarize yourself with these basics, you should know enough to really take advantage

of the usefulness of Bash.

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