Home > Articles > Apple > Operating Systems

The book is fully up-to-date with the 10.4 (Tiger) release of OS X which is built on top of a UNIX system. Building upon Mark Sobell's A Practical Guide to the UNIX System, this book provides step-by-step tutorials for OS X users eager to get the most from the underlying UNIX system. Shell Programming in Unix, Linux and OS X is a thoroughly updated revision of Kochan and Wood’s classic Unix Shell Programming tutorial.Following the methodology of the original text, the book focuses on the POSIX standard shell, and teaches you how to develop programs in this useful programming environment, taking full advantage of the underlying power of Unix and Unix-like operating systems.

Learning Unix for OS X: Going Deep With the Terminal and Shell Dave Taylor. 4.1 out of 5 stars 51.

1. Shell Settings

Shell Settings

Your shell uses a variety of settings that you will want to change from time to time. You should read the man page for your shell to learn about dozens of possible settings.

You may want your shell prompt to be shorter, or to display the date, time, or some other specific piece of information. This is easy to do by adding a line to your shell con-figuration file.

Another setting you may want to change is umask, which controls the permissions given to any new file you create. (Chapter 8, 'Working with Permissions and Ownership,' goes into detail on permissions.)

Customizing your shell prompt

Both tcsh and bash provide ways to customize your shell prompt. The simplest customization would be to simply have your prompt be a word or phrase, such as 'Type something:,' but far more interesting is the ability to have the prompt include information about what is going on in your shell. For example, the default prompt for tcsh uses the formatting pattern

Table 7.4 lists the common formatting patterns available for the tcsh shell prompt.

If your computer's name is violet, and your user name is vanilla, and your current directory is /usr/share.man/man1, then the prompt would be [violet:share/man/man1] vanilla%

See how the %m gets replaced with violet, the %c3 gets replaced with share/man/man1, the %n gets replaces with vanilla, and the %# gets replaced with %.

Table 7.4 Some tcsh Prompt Macros

See man tcsh for the complete list.

The following tasks show you how to customize your shell prompt so that it shows the current time (in 12-hour a.m./p.m. format), followed by your current directory, followed by your user name.

Don't worry about making a mistake—the changes you make will only be in effect for the single Terminal window in which you perform this task. Once you are satisfied that you have it right, you can add the setting to your ~/.tcshrc file to have it take effect for all future shells you start up (that is, all future Terminal windows).

To temporarily customize your prompt in tcsh:

• set prompt='%t:%c3 %# '

  Your prompt will immediately change, as shown in Figure 7.12. Note the space at the end of the prompt—right after the # and before the last '. That space is actually part of the prompt, so when you type a command, it is visually separated from the prompt.

  Figure 7.12 Changing your tcsh shell prompt on the command line.

To make a durable customization of your tcsh shell prompt:

1. Open your ~/.tcshrc file.

2. Add a line with the new setting.

3. Using the example from the previous task, add set prompt='%t:%c3 %# '

4. Save the file.

5. Quit the editor.

   The change will take effect with the next Terminal window you open.

6. Open a new Terminal window to see your new prompt.

The procedure for customizing your bash shell prompt is similar to that for the tcsh shell. The bash shell also uses a set of special formatting codes to allow the inclusion of things like the current directory and date in the prompt.

The bash shell calls these escape sequences because they all start with the backslash character, which is frequently used in Unix to alter the meaning of the following character, usually by removing some special meaning ('escaping' the character). But in this case, the use of the backslash creates a special meaning for the following character, so d becomes the date, and u becomes your user name.

Table 7.5 shows the common escape sequences for the bash shell prompt.

The default bash shell prompt uses the format

and produces a prompt that looks like

The s becomes bash; the v becomes 2.05.

In the tasks below you will set your prompt to show the current time in 12-hour a.m./p.m. format, a colon, and the current directory, followed by the > character and a space.

Table 7.5 Some bash Prompt Escape Sequences

See man bash for the complete list.

To temporarily customize your bash shell prompt:

• PS1='t:w > '

  Make sure there are no spaces on either side of the equal sign. Your shell prompt will immediately change, as in Figure 7.13. Note that even though this looks as if you are setting an environment variable (because PS1 is capitalized), you aren't. You are setting a variable (the PS1 variable), but you are setting it for this shell only. Unlike environment variables, this variable will not be passed on to child processes of this shell.

  Figure 7.13 Changing your bash shell prompt on the command line.

To make a durable customization of your bash shell prompt:

1. Open your ~/.bash_profile file.

2. Add a line with the new setting. Using the example from the previous task, you would add PS1='t:w > '

3. Save the file.

4. Quit the editor.

   The change will take effect with the next Terminal window you open.

5. Open a new Terminal window. You will see your new prompt.

Changing your umask

When you create a new file or directory, the initial permissions are determined by the umask (user mask) setting of the shell that created the file. So the umask is a shell configuration that affects the permissions of any file you create. (See Chapter 8, 'Working with Permissions and Ownership,' for more on permissions.)

Your umask setting can be either temporary or durable, like your shell prompt, which you customized in the tasks above.

You use the umask command to set (and view) your umask. Used with no arguments, umask simply displays your current umask setting. To set your umask, you supply one argument, which is a three-digit octal (base eight) number. If you use fewer than three digits, zeros are added to the left to bring the total up to three digits—so 2 becomes 002, and 22 becomes 022, for example. See Chapter 8 for an explanation of how the umask is actually applied to determine file permissions, and of what those permissions mean.

Your default umask is 022, which means that any new files you create will be readable by every user of your Mac, but only writable by you. A common change is to set one's umask so that newly created files are writable not only by oneself but also by other users in the same 'group' (see Chapter 8 for more on groups). The umask setting for this is 002 (which can be abbreviated as simply 2).

To temporarily change your umask:

1. umask 002

   The change takes effect at once and will last until you log out of the shell or change it again.

2. Verify that the change took effect:

   Figure 7.14shows an example.

   Figure 7.14 Changing your umask on the command line.

   Changing your umask for all future shells is simply a matter of putting the same command in your shell configuration file.

To make a durable change to your umask:

1. Open the startup file for your shell. For tcsh, edit ~/.tcshrc. For bash, edit ~/.bash_profile.

2. Add a line with the umask command and the new umask.

   For example, to have your umask set to 002, add a line that says umask 002

3. Save the file.

4. Quit the editor.

   The change will take effect on the next shell you start.

5. Open a new Terminal window.

6. umask

   to confirm it's correct.

Related Resources

• Book $31.99

• eBook (Watermarked) $38.39

• eBook (Watermarked) $41.59

Home > Articles > Apple > Operating Systems

1. Finding Configuration Files

Many user-configurable Unix programs (such as your shell) read configuration files when they start up. These configuration files contain settings and commands that determine how the programs will behave—for instance, the files can modify the list of places your shell looks for the commands you enter (that list is called your PATH).

You change your Unix environment to have it more closely match your personal preferences and to shape it more closely to the way you work.

Examples of configuring your Unix environment include:

• Customizing your shell prompt so that it displays information you want to see.

• Creating shortcuts for commonly used command/option combinations (these can be 'aliases,' but they are distinct from the aliases you create in the Finder, and from the shell functions, which are short scripts that are part of your private configuration).

• Making it easier to use additional software you install; for example, if you add /Developer/Tools to your PATH, then you can use the commands in the /Developer/Tools directory without typing their full pathnames.

• Configuring specific programs such as vi to turn on various options whenever you use them, much the same way that traditional Mac programs often have a preferences dialogue box.

The first program to configure is your shell, since your shell is the primary program you use to interact with Unix. We will also show you how to configure the vi editor by editing a configuration file it uses (see Chapter 6, 'Editing and Printing Files,' to learn how to use vi).

It should come as no surprise by now that you configure your shell by editing text files.

Finding Configuration Files

User-configurable Unix programs (including your shell, the vi editor, and others) look for configuration files in your home directory when they start up. Most of the commands you have learned so far are not user-config-urable; neither the ls nor the cd command uses configuration files, for example. They do accept options on the command line but do not read any configuration files when you run them.

Many configuration filenames begin with a dot (.), so they are called dot files (use ls -a to see them). Often the filenames end in 'rc' (for resource). For example, the main config-uration file for the tcsh shell is ~/.tcshrc, a configuration file for the bash shell is ~/.bash_profile, and the configuration file for the vi editor is called ~/.exrc (ex is an older editor, and vi provides a 'visual interface' for it). There are actually several config-uration files available for each shell, and Table 7.1 lists the more common ones. In this chapter, we will concentrate on the ones you would change in the course of normal use. These files each have settings and commands for the particular program being configured. (Remember that ~ [the tilde] is a synonym for your home directory.)

Configuration files for shells are actually scripts. This means they are a series of commands written in the scripting language for the corresponding shell. They make use of variables, if-then conditions, and other scripting elements such as loops. (See Chapter 9, 'Creating and Using Scripts,' for more on scripts.)

Table 7.1 A Summary of Common Shells

Am I Configuring the Terminal Application or My Shell?

There's an important distinction to understand here.

The Terminal application you are using to access the command line in Mac OS X is not the same as your shell.

Terminal is a regular Mac OS X graphical application, like your Web browser or word processor. When you open a new window in Terminal, the application runs the appropriate Unix shell (determined by the Terminal application's preferences). Terminal is the program that is handling the screen display and keyboard input for the shell. When you type something in Terminal, the Terminal application passes that to the shell, and when the shell produces output, Terminal draws it on your screen.

The subtle point here is that there are actually other ways besides Terminal in which you can use your shell. One example: You can connect to your Mac using the command line over a network from another machine, which we'll cover in Chapter 10, 'Interacting with Other Unix Machines.' So when we tell you in this chapter that a change you make will take effect 'in the next Terminal window you open,' that is really a shorthand way of saying that the change will take effect in the next instance of your shell that you run, and that the easiest way to see it is to open a new Terminal window.

Related Resources

Book Unix For Os X Pdf

• Book $31.99

• eBook (Watermarked) $38.39

Book Unix For Os X Mac

• eBook (Watermarked) $41.59