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 settingsfor example, man bash .

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 configuration 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.

The bash shell uses a set of special formatting codes to allow the inclusion of things like the current directory and date in the prompt. You put these codes into a shell variable (called PS1 ) that bash reads to determine your prompt.

In Unix we call 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 7.4 shows some common escape sequences for the bash shell prompt.

Table 7.4. Some bash Prompt Escape Sequences

E SCAPE S EQUENCE	M EANING
\d	The date in "weekday month date" format (for example, "Tue May 26")
\h	Your computer's Internet hostname up to the first " . "
\H	Your computer's Internet hostname
\j	The number of jobs currently managed by the shell
\s	The name of the shell program (for example, bash )
\t	Current time in 24- hour HH:MM:SS format
\T	Current time in 12-hour HH:MM:SS format
\@	Current time in 12-hour a.m./p.m. format
\u	Your user name.
\v	The version of bash you are usingfor example, 2.05
\w	Your current directory
\W	Base name (last part) of your current directory
\\	A backslash
See man bash for the complete list.

In Mac OS X 10.4 the default bash shell prompt is set in /etc/bashrc and uses the format

'\h:\w \u\$ '

It produces a prompt that looks like

hostname:~ vanilla $

The \h becomes whatever your machine's name (called the hostname ) is; the \w becomes your current working directory; and the \u becomes your user name. The $ is reproduced literally. Notice the space included in the format, after the $ . The space ensures that whatever you type next to the prompt isn't smooshed up against the $ , which would be hard to read.

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

Don't worry about making a mistakethe changes you make will be in effect only 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 ~/.bash_profile file to have it take effect for all future shells you start up (that is, all future Terminal windows ).

To temporarily customize your bash shell prompt:

• PS1='\t:\w > '
  
  Make sure there are no spaces on either side of the equals sign. Your shell prompt will immediately change, as in Figure 7.11 .
  
  

  Figure 7.11. Changing your bash shell prompt on the command line.

   bash-2.05$ PS1='\t:\w > ' 10:17:03:~ > 

  
  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.
  
  

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.

The procedure for customizing a tcsh shell prompt is similar to that for the bash shell. The tcsh 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 tcsh shell calls these formatting codes prompt macros .

Table 7.5 lists the common prompt macros available for the tcsh shell prompt.

Table 7.5. Some tcsh Prompt Macros

F ORMATTING S EQUENCE	T URNS INTO T HIS
%/	Your current directory.
%~	Your current directory, but with your home directory shown as ~ and other home directories shown as ~user .
%c	The last part of the current directory. If followed by a number n (for example, %c3 ), then only the last n components (directories) are shown. Your home directory and other users' home directories are shown, as with %~ above.
%C	Same as %c but does not show home directories with a ~ .
%M	Your computer's Internet hostname.
%m	The hostname up to the first " . ".
%B (%b)	Start (stop) bold mode.
%U (%u)	Start (stop) underline mode.
%t	The time of day in 12-hour a.m./p.m. format.
%T	Like %t but in 24-hour format.
%%	A single % .
%n	Your user name.
%d	The weekday in day format.
%D	The day in dd format.
%w	The month in mon format.
%W	The month in mm format.
%y	The year in yy format.
%Y	The year in yyyy format.
%#	A % for normal users and a # for the root user (helps you know if you are logged in as root).
See man tcsh for the complete list.

For example, the default prompt for tcsh uses the formatting pattern

[%m:%c3] %n%#

If your computer's hostname is violet , 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 replaced with vanilla , and the %# gets replaced with % .

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.

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 immediately changes, as shown in Figure 7.12 . Note the space at the end of the promptright 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.

   localhost:~ vanilla$  set prompt='%t:%c3 %# '  11:32am:~ % 

  
  

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.

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 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 an octal (base eight) number of one to four digits. If you use fewer than four digits, zeros are added to the left to bring up the total to four digitsso 2 becomes 0002, and 22 becomes 0022, for example. See Chapter 8 for an explanation of how the umask is applied to determine file permissions, and of what those permissions mean. In most Unix documentation, umask s are shown as three-digit numbers , because the four-digit form is rarely needed. So in most cases, we will show umask s as three-digit values.

In Mac OS X your default umask is 0022 , 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 simply as 2 ).

To temporarily change your umask:

1.	umask 002 The change takes effect at once and lasts until you log out of the shell or change it again.
2.	Verify that the change took effect: umask Figure 7.13 shows an example. Figure 7.13. Changing your umask on the command line. localhost:~ vanilla$ umask 0022 localhost:~ vanilla$ umask 002 localhost:~ vanilla$ umask 0002 localhost:~ vanilla$

Tip

• In some shellsfor example, the tcsh shellthe leading zeros in your umask are not displayed, so a umask of 0022 appears as 22 .

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 bash , edit ~/.bash_profile . For tcsh , edit ~/.tcshrc .
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.