The shell records each command that you enter in a file under your home directory. This file is called the history file; its name is .sh_history.If you enter the command:

history

the shell displays the current contents of your history file. Each command is numbered.

For example, suppose that you are a programmer and you enter a complicated command to compile part of a program. The program contains a syntax error, so you call a text editor to edit the source code and correct the problem. Now you want to run the same compile command on the corrected program. You may save yourself a good deal of typing by using:

history

to find out the number of the previous compile command; you can then run the command with r.

Another time-saver is to specify your shell prompt as:

PS1='(!)$'

in your .profile. The shell prompt is then preceded by the number assigned to the command in the command history file.

This is how you use the command numbers to enter a command. To repeat command number 14, enter:

r 14

The shell displays the original command 14 in the output area of the screen and then runs it. If you get another error, you can correct it, and then compile again with another r 14. You can perform the operation many times, but you have to type the original only once.

If you type r followed by a space, followed by a string of characters (not beginning with a digit), the shell checks backward through the history file and runs the most recent command that begins with the given string. For example, let's look at the compilation example. Suppose you are using the c++ command to compile your program. Then:

r c++

looks back through the history and runs the most recent c++ command. You do not even have to check on the number of the command you want to enter. The shell displays the selected command in the output area of the screen and then runs it.