Wednesday, December 26, 2012

Linux tip: Using an exclamation point (!) to reference events

The C Shell history mechanism uses an exclamation point to reference events. This technique, which is available under bash and tcsh, is frequently more cumbersome to use than fc but nevertheless has some useful features. For example, the !! command reexecutes the previous event, and the shell replaces the !$ token with the last word from the previous command line.
You can reference an event by using its absolute event number, its relative event number, or the text it contains. All references to events, called event designators, begin with an exclamation point (!). One or more characters follow the exclamation point to specify an event.
You can put history events anywhere on a command line. To escape an exclamation point so the shell interprets it literally instead of as the start of a history event, precede it with a backslash (\) or enclose it within single quotation marks.

Event Designators

An event designator specifies a command in the history list. Table 8-8 lists event designators.

Table 8-8 Event designators

Designator Meaning
! Starts a history event unless followed immediately by SPACE, NEWLINE, =, or (.
!! The previous command.
!n Command number n in the history list.
!–n The nth preceding command.
!string The most recent command line that started with string.
!?string[?] The most recent command that contained string. The last ? is optional.
!# The current command (as you have it typed so far).
!{event} The event is an event designator. The braces isolate event from the surrounding text. For example, !{–3}3 is the third most recently executed command followed by a 3.

!! reexecutes the previous event

You can reexecute the previous event by giving a !! command. In the following example, event 45 reexecutes event 44:

44 $ ls -l text
-rw-rw-r--. 1 max pubs 45 04-30 14:53 text
45 $ !!
ls -l text
-rw-rw-r--. 1 max pubs 45 04-30 14:53 text

The !! command works whether or not your prompt displays an event number. As this example shows, when you use the history mechanism to reexecute an event, the shell displays the command it is reexecuting.

!n event number

A number following an exclamation point refers to an event. If that event is in the history list, the shell executes it. Otherwise, the shell displays an error message. A negative number following an exclamation point references an event relative to the current event. For example, the command !–3 refers to the third preceding event. After you issue a command, the relative event number of a given event changes (event –3 becomes event –4). Both of the following commands reexecute event 44:

51 $ !44
ls -l text
-rw-rw-r--. 1 max pubs 45 04-30 14:53 text
52 $ !-8
ls -l text
-rw-rw-r--. 1 max pubs 45 04-30 14:53 text

!string event text

When a string of text follows an exclamation point, the shell searches for and executes the most recent event that began with that string. If you enclose the string within question marks, the shell executes the most recent event that contained that string. The final question mark is optional if a RETURN would immediately follow it.

68 $ history 10
    59 ls -l text*
    60 tail text5
    61 cat text1 text5 > letter
    62 vim letter
    63 cat letter
    64 cat memo
    65 lpr memo
    66 pine zach
    67 ls -l
    68 history
69 $ !l
ls -l
70 $ !lpr
lpr memo
71 $ !?letter?
cat letter

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