7.5 Command History

bash lets you display or modify previous commands. Commands in the history list can be modified using:

• Line-edit mode

• The fc command

In addition, the command substitutions described in Chapter 8 also work in bash.

7.5.1 Line-Edit Mode

Line-edit mode lets you emulate many features of the vi and Emacs editors. The history list is treated like a file. When the editor is invoked, you type editing keystrokes to move to the command line you want to execute. On most terminals, arrow keys work in both Emacs mode and vi command mode. You can also change the line before executing it. See Table 7-23 for some examples of common line-edit commands. When you're ready to issue the command, press Return.

The default line-edit mode is Emacs. To enable vi mode, enter:

$ set -o vi

Note that vi starts in input mode; to type a vi command, press Esc first.

The mode you use for editing bash commands is entirely separate from the editor that is invoked for you automatically within many commands (for instance, the editor invoked by mail readers when you ask them to create a new mail message). To change the default editor, set the VISUAL or EDITOR variable to the filename or full pathname of your favorite editor:

$ export EDITOR=emacs

Table 7-23. Common editing keystrokes

vi	Emacs	Result
k	Ctrl-p	Get previous command.
j	Ctrl-n	Get next command.
/string	Ctrl-r string	Get previous command containing string.
h	Ctrl-b	Move back one character.
l	Ctrl-f	Move forward one character.
b	M-b	Move back one word.
w	M-f	Move forward one word.
X	Del	Delete previous character.
x	Ctrl-d	Delete one character.
dw	M-d	Delete word forward.
db	M-Ctrl-h	Delete word back.
xp	Ctrl-t	Transpose two characters.

7.5.2 The fc Command

Use fc -l to list history commands, and fc -e to edit them. See the fc built-in command for more information.

7.5.2.1 Examples

$ history            Display the command history list
$ fc -l 20 30        List commands 20 through 30
$ fc -l -5           List the last five commands
$ fc -l cat          List the last command beginning with cat
$ fc -ln 5 > doit    Save command 5 to file doit
$ fc -e vi 5 20      Edit commands 5 through 20 using vi
$ fc -e emacs        Edit previous command using Emacs
$ !!                 Reexecute previous command
$ !cat               Reexecute last cat command
$ !cat foo-file      Reexecute last command, adding foo-file to 
                   the end of the argument list

7.5.3 Command Substitution

Syntax	Meaning
!	Begin a history substitution.
!!	Previous command.
!N	Command number N in history list.
!-N	Nth command back from current command.
!string	Most recent command that starts with string.
!?string?	Most recent command that contains string.
!?string?%	Most recent command argument that contains string.
!$	Last argument of previous command.
!#	The current command up to this point.
!!string	Previous command, then append string.
!N string	Command N, then append string.
!{s1}s2	Most recent command starting with string s1, then append string s2.
^old^new^	Quick substitution; change string old to new in previous command, and execute modified command.

7.5.4 Variables in Prompt

Using the following variables, you can display information about the current state of the shell or the system in your bash prompt. Set the PS1 variable to a string including the desired variables. For instance, the following command sets PS1 to a string that includes the \w variable to display the current working directory, and the \! variable to display the number of the current command. The next line is the prompt displayed by the change.

$ PS1='\w: Command \!$ '
~/book/linux: Command 504$

Variable	Meaning
\a	Alarm (bell)
\d	Date in the format "Mon May 8"
\e	Escape character (terminal escape, not backslash)
\h	Hostname
\j	Number of background jobs (active or stopped)
\l	Current terminal name
\n	Newline inserted in the prompt
\r	Carriage return inserted in the prompt
\s	Current shell
\t	Time in 24-hour format, where 3:30 p.m. appears as 15:30:00
\u	User's account name
\v	Version and release of bash
\w	Current working directory
\A	Time in 24-hour format, where 3:30 p.m. appears as 15:30
\D{format}	Time in the specified format interpreted by strftime; an empty format displays the locale-specific current time
\H	Like \h
\T	Time in 12-hour format, where 3:30 p.m. appears as 03:30:00
\V	Version, release, and patch level of bash
\W	Last element (following last slash) of current working directory
\\	Single backslash inserted in the prompt
\!	Number of current command in the command history
\#	Number of current command, where numbers start at 1 when the shell starts
\@	Time in 12-hour format, where 3:30 p.m. appears as 03:30 p.m.
\$	Indicates whether you are root: displays # for root, $ for other users
\[	Starts a sequence of nonprinting characters, to be ended by \]
\]	Ends the sequence of nonprinting characters started by \[
\nnn	The character in the ASCII set corresponding to the octal number nnn inserted into the prompt