Controlling Shell Operations

The BASH shell has several features that enable you to control the way different shell operations work. For example, setting the noclobber feature prevents redirection from overwriting files. You can turn these features on and off like a toggle, using the set command. The set command takes two arguments: an option specifying on or off and the name of the feature. To set a feature on, you use the -o option, and to set it off, you use the +o option. Here is the basic form:

$ set -o feature        turn the feature on
$ set +o feature        turn the feature off

Three of the most common features are ignoreeof, noclobber, and noglob. Table 9-1 lists these different features, as well as the set command. Setting ignoreeof enables a feature that prevents you from logging out of the user shell with CTRL-D. CTRL-D is not only used to log out of the user shell, but also to end user input entered directly into the standard input. CTRL-D is used often for the Mail program or for utilities such as cat. You could easily enter an extra CTRL-D in such circumstances and accidentally log yourself out. The ignoreeof feature prevents such accidental logouts. In the next example, the ignoreeof feature is turned on using the set command with the -o option. The user can now log out only by entering the logout command.

$ set -o ignoreeof
$ ctrl-d
Use exit to logout
$

Table 9-1: BASH Shell Special Variables and Features

BASH Shell Special Variables	Description
HOME	Pathname for user's home directory
LOGNAME	Login name
USER	User name
SHELL	Pathname of program for type of shell you are using
BASH_ENV	Holds name of BASH initialization script executed whenever a BASH shell script is run or BASH shell entered; usually $HOME/.bashrc
PATH	List of pathnames for directories searched for executable commands
PS1	Primary shell prompt
PS2	Secondary shell prompt
IFS	Interfield delimiter symbol
MAIL	Name of mail file checked by Mail utility for received messages
MAILCHECK	Interval for checking for received mail
MAILPATH	List of mail files to be checked by Mail for received messages
TERM	Terminal name
CDPATH	Pathnames for directories searched by cd command for subdirectories
EXINIT	Initialization commands for Ex/Vi editor
BASH Shell Features
$ set -+o feature	Bash shell features are turned on and off with the set command; -o sets a feature on and +o turns it off: $ set -o noclobber set noclobber on $ set +o noclobber set noclobber off
ignoreeof	Disabled CTRL-D logout
noclobber	Does not overwrite files through redirection
noglob	Disables special characters used for filename expansion: *, ?, ~, and []