Adding or Changing Variables

After you’ve poked around in your environment, you might determine that you want to set a variable that’s currently not available or change one to make it better meet your needs. In general, you won’t randomly specify variables; you’ll do it because a certain program requires a specific variable in order to run.

Variables You Can Mess With

The following table includes some of the variables you can safely change. Keep in mind that the shell itself might not use a specific variable, like NNTPSERVER, while programs running under the shell might. Sometimes shells assign default variables, while in other cases you’ll have to manually set the value.

ZSH, BASH, AND KSHCSHDESCRIPTION
CDPATHcdpathSpecifies the search path for directories specified by cd. This is similar to PATH.
COLUMNS Specifies width of the edit window in characters.
EDITOR Specifies the default editor.
ENV Specifies where to look for configuration files.
HISTFILE Specifies the name of the file containing the command history.
HISTFILESIZEsavehistSpecifies the maximum number of lines to keep in the history file.
HISTSIZEhistorySpecifies the number of commands to keep in the command history.
HOSTFILE Specifies the name of the file containing host name aliases for expansion.
IGNOREEOFignoreeofSpecifies that should not log out of the shell. Use IGNOREEOF=.
LINES Specifies the number of lines on the screen.
MAILmailSpecifies the location of incoming mail so bash can notify you of mail arrival.
MAILCHECKmailSpecifies how often (in seconds) bash checks for mail.
MAIL_WARNING Specifies the message to be displayed if you have read mail but not unread mail.
noclobbernoclobberSpecifies that the shell should not overwrite an existing file when redirecting output.
PATHpathSpecifies the search path for commands, including multiple paths separated by colons.
PROMPT_COMMAND Specifies the command to be run before displaying each primary prompt (does not apply to ksh).
PS1promptSpecifies the primary prompt.
PS2 Specifies the default second-level prompt.
PS3 Specifies the prompt for the select command in scripts.
PS4 Specifies the prompt used when tracing execution of a script.
TMOUT Specifies time in seconds to wait for input before closing the shell.
VISUAL Specifies the default visual editor—usually the same as EDITOR, but referenced by different programs.


Code Listing 8.3. In the zsh, bash, and ksh shells, you can add a new environment variable by specifying the variable and its value, then exporting the variable to the system.
  [ejr@hobbes ejr]$
  → NNTPSERVER=news.xmission.com
  [ejr@hobbes ejr]$ export NNTPSERVER
  [ejr@hobbes ejr]$ echo $NNTPSERVER
  news.xmission.com
  [ejr@hobbes ejr]$

Fill in Your bash System Configuration Files

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Fill in Your zsh System Configuration Files

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By following the steps in this section, you can add or change environment variables for the current session. As Code Listing 8.3 shows, for example, you can specify a news server environment variable (called NNTPSERVER)that some Usenet news readers require to access the news (nntp) server.

To add or change a variable in zsh, bash, or ksh:

1.
NNTPSERVER=news.xmission.com

At the shell prompt, type the name of the variable (in this case, NNTPSERVER), followed by = and the value you want for the variable (here, news.xmission.com), as shown in Code Listing 8.3. In this step, you’re setting up the variable and its value and making it available to all programs and scripts that run in the current shell session. If the value contains spaces or special characters, put the value in quotes.

2.
export NNTPSERVER

Type export followed by the name of the variable. By exporting the variable, you make it available to all programs and scripts that run in the current shell session (again, Code Listing 8.3).

Until it is exported, it is a shell variable, which will not be available to other processes that this shell starts.

3.
echo $NNTPSERVER

Optionally, type echo followed by a $ and the name of the variable to have the shell tell you what the variable is set to.

✓ Tip

  • In bash, ksh, or zsh, save a step by typing export NNTPSERVER=news.xmission.com.


To Add or Change a Variable in csh:

1.
setenv NNTPSERVER news.xmission.com

Type setenv followed by the name of the variable, a space, and the value of the variable. In this step, you’re setting up the variable and its value and making the environment variable available to all programs and scripts that run in the current shell session (Code Listing 8.4).

Code Listing 8.4. The process for the csh shell is similar to the process for the bash and ksh shells.
  xmission> setenv NNTPSERVER
  → news.xmission.com
  xmission> echo $NNTPSERVER
  news.xmission.com

If the value contains spaces or special characters, put the value in quotes.

2.
echo $NNTPSERVERx

Optionally, type echo followed by a $ and the name of the variable to have the shell tell you what the variable is set to.

✓ Tips

  • If you want to change or add to your variables so that the new settings exist from session to session, use the instructions for changing the environment variables in your configuration files, as described throughout the rest of this chapter.

  • Find out more about news readers in Chapter 12.


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