Event History

By Danny Kalev, ITworld |  How-to

This week, we use the BASH history utility to retrieve previously
executed shell commands.

BASH automatically stores a limited number of previously typed
commands, the default limit being 500 commands. Assigning a different
value to the variable HISTSIZE changes the default limit. Each command,
technically referred to as an "event", associates with a number
according to their sequence (i.e., the most recent event has the
highest number). The "history" command lists events and their
associated numbers:

$ history
1 ls
2 mv payroll febpayroll
3 vi myprog.c

In this example, the user executed three commands before
typing "history". BASH enables you to traverse previously typed
commands by pressing the UP-ARROW key (to move up to the previous event
in the history list) and DOWN-ARROW (to move down to the next event in
the history list). In most cases, sequential access is sufficient;
however, repeatedly pressing UP-ARROW can be tedious when scrolling
through a lengthy list of commands. Instead, you can use the ! operator
followed by the command's associated number to retrieve a specific
command.

$ history
1 ls
2 mv payroll febpayroll
3 vi myprog.c

$ !1
ls

In the above example, typing "!1" retrieves event number 1. You can
also use an initial substring that appears in the command you wish to
retrieve. For example:

$ !mv
mv payroll febpayroll

In this case, following ! with the substring "mv" retrieves the
command "mv payroll febpayroll", which begins with "mv".

You may also use a negative number after the ! operator to reference an
event using an offset from the list's end. In other words, you can
instruct the history utility to access n commands from the most recent
one. For example:

$ !-2
mv payroll febpayroll

Let's examine how the "!-2" is processed. The following table contains
the list of events and their negative offsets:

event # offset
1 typing !-3 puts us here
2 typing !-2 puts us here
3 typing !-1 puts us here

As you can see, typing "!-2" retrieves event number 2, which is "mv
payroll febpayroll".

If you type only ! without a number or an initial substring, the last
command is assumed:

$!
vi myprog.c

Join us:
Facebook

Twitter

Pinterest

Tumblr

LinkedIn

Google+

Answers - Powered by ITworld

ITworld Answers helps you solve problems and share expertise. Ask a question or take a crack at answering the new questions below.

Join us:
Facebook

Twitter

Pinterest

Tumblr

LinkedIn

Google+

Ask a Question