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This article is excerpted from chapter 9 of the new Third Edition of A Practical Guide to Red Hat Linux: Fedora Core and Red Hat Enterprise Linux , copyright 2007 Pearson Education, Inc. ISBN 0132280272. Published June 2006 by Prentice Hall Professional. Reproduced by permission of Pearson Education, Inc. All rights reserved.

Variables that control history

The value of the HISTSIZE variable determines the number of events preserved in the history list during a session. A value in the range of 100 to 1,000 is normal.

When you exit from the shell, the most recently executed commands are saved in the file given by the HISTFILE variable (the default is ~/.bash_history). The next time you start the shell, this file initializes the history list. The value of the HISTFILESIZE variable determines the number of lines of history saved in HISTFILE (not necessarily the same as HISTSIZE). HISTSIZE holds the number of events remembered during a session, HISTFILESIZE holds the number remembered between sessions, and the file designated by HISTFILE holds the history list.

The Bourne Again Shell assigns a sequential event number to each command line. You can display this event number as part of the bash prompt by including \! in the PS1 variable. Examples in this article show numbered prompts when they help to illustrate the behavior of a command.

Give the following command manually or place it in ~/.bash_profile (to affect future sessions) to establish a history list of the 100 most recent events:

 $ HISTSIZE=100 

The following command causes bash to save the 100 most recent events across login sessions:

 $ HISTFILESIZE=100 

After you set HISTFILESIZE, you can log out and log in again, and the 100 most recent events from the previous login session will appear in your history list.

Give the command history to display the events in the history list. The list of events is ordered with oldest events at the top of the list. The following history list includes a command to modify the bash prompt so that it displays the history event number. The last event in the history list is the history command that displayed the list.

32 $ history | tail
   23  PS1="\! bash$ "
   24  ls -l
   25  cat temp
   26  rm temp
   27  vim memo
   28  lpr memo
   29  vim memo
   30  lpr memo
   31  rm memo
   32  history | tail

As you run commands and your history list becomes longer, it may run off the top of the screen when you use the history builtin. Pipe the output of history through less to browse through it, or give the command history 10 to look at the 10 most recent commands.

You can reexecute any event in the history list. Not having to reenter long command lines allows you to reexecute events more easily, quickly, and accurately than you could if you had to retype the entire command line. You can recall, modify, and reexecute previously executed events in three ways: You can use the fc builtin (covered next); the exclamation point commands; or the Readline Library, which uses a one-line vi- or emacs-like editor to edit and execute events.

If you are more familiar with vi or emacs and less familiar with the C or TC Shell, use fc or the Readline Library. If you are more familiar with the C or TC Shell and less familiar with vi and emacs, use the exclamation point commands. If it is a toss-up, try the Readline Library; it will benefit you in other areas of Linux more than learning the exclamation point commands will.

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