, and shell functions (which are related to shell scripting) will be covered in . in your environment.

.

.

, but for now we will translate:. To demonstrate how this works, try the following:

. The emacs editor was originally written by Richard Stallman. It is a gigantic, all-purpose, does-everything programming environment. Though readily available, it is seldom installed on most Linux systems by default. lists the meanings of our additions.

.

export HISTCONTROL=ignoredups

Causes the shell’s history recording feature to ignore a command if the same command was just recorded.

export HISTSIZE=1000

Increases the size of the command history from the default of 500 lines to 1000 lines.

alias l.='ls -d .* --color=auto'

Creates a new command called l., which displays all directory entries that begin with a dot.

alias ll='ls -l –color=auto'

Creates a new command called ll, which displays a long-format directory listing.

As we can see, many of our additions are not intuitively obvious, so it would be a good idea to add some comments to our .bashrc file to help explain things to the humans. Using the editor, change our additions to look like this:

# Change umask to make directory sharing easier umask 0002  # Ignore duplicates in command history and increase # history size to 1000 lines export HISTCONTROL=ignoredups export HISTSIZE=1000  # Add some helpful aliases alias l.='ls -d .* --color=auto' alias ll='ls -l --color=auto'

Ah, much better! With our changes complete, press ctrl-O to save our modified .bashrc file and ctrl-X to exit nano.

Activating Our Changes

The changes we have made to our .bashrc will not take effect until we close our terminal session and start a new one, because the .bashrc file is only read at the beginning of a session. However, we can force bash to reread the modified .bashrc file with the following command:

[me@linuxbox ˜]$ source .bashrc

After doing this, we should be able to see the effect of our changes. Try out one of the new aliases:

[me@linuxbox ˜]$ ll

Why Comments Are Important

Whenever you modify configuration files, it’s a good idea to add some comments to document your changes. Sure, you will remember what you changed tomorrow, but what about six months from now? Do yourself a favor and add some comments. While you’re at it, it’s not a bad idea to keep a log of what changes you make.

Shell scripts and bash startup files use a # symbol to begin a comment. Other configuration files may use other symbols. Most configuration files will have comments. Use them as a guide.

You will often see lines in configuration files that are commented out to prevent them from being used by the affected program. This is done to give the reader suggestions for possible configuration choices or examples of correct configuration syntax. For example, the .bashrc file of Ubuntu 8.04 contains these lines:

# some more ls aliases #alias ll='ls -l' #alias la='ls -A' #alias l='ls -CF'

The last three lines are valid alias definitions that have been commented out. If you remove the leading # symbols from these three lines, a technique called uncommenting, you will activate the aliases. Conversely, if you add a # symbol to the beginning of a line, you can deactivate a configuration line while preserving the information it contains.

Final Note

In this chapter we learned an essential skill—editing configuration files with a text editor. Moving forward, as we read man pages for commands, take note of the environment variables that commands support. There may be a gem or two. In later chapters we will learn about shell functions, a powerful feature that you can also include in the bash startup files to add to your arsenal of custom commands.