The History Mechanism

History

The history mechanism lets you recall and rerun earlier commands. To see what it’s remembered, use the history command or its usual alias h, which might show you something like the following:

127 D% history 12
   116  (this is the inline data)
   117  eof
   118  end
   119  cat << ***
   120  The ^$home directory is $home.
   121  Today's date is `date`.
   122  ***
   123  cat << ^***
   124  The ^$home directory is $home.
   125  Today's date is `date`.
   126  ***
   127  history 12
128 D% _

The history list won’t be quite what you typed: it’ll be broken into separate words wherever one of the special tokens, &, |, ;, >, <, (, ), &&, ||, >> or <<, occurs. Only inline data escapes being broken up into words this way. Each command may be thought of as an array of words, indexed from 0.

To reuse the text or maybe just a few words from a previous command, you type an exclamation point, “!”, followed by a few characters to identify what you want to reuse. You can do this anywhere and whatever you select is just stuffed back on the command line to be interpreted as whatever the context suggests. For convenience, the exclamation point is not treated as a history reference if it’s followed by white space (a space, tab or newline) or by “=“, “~” or “(“.

Retrieving a Whole Command Line

There several ways of picking up a whole command line. You already know how to do it interactively with arrow keys and command completion. You can also use a shorthand notation that can be more convenient if you want to do something a bit more complex. The simplest shorthand is “!!”, which picks up the text of the immediately preceding command:

128 D% echo !!
echo history 12
history 12
129 D% !!
echo history 12
history 12
130 D% _

The shell first echoes your command showing the effects of the substitutions and then runs it. The other quick ways of referring to a whole command line from history are by the command number,

130 D% !104
ls -l `whereis ls`
---A-- Jun 24  8:28      86016  d:\Program Files\Hamilton C shell 2003\Bin\ls.exe
relative to the immediately preceding command ,
131 D% echo one
one
132 D% echo two
two
133 D% echo three
three
134 D% !-1
echo two
two
135 D% _

or by mentioning some of the text to look for. A question mark after the exclamation point means you’ll accept the match anywhere on the line; otherwise it has to be at the start.

135 D% !h
history 12
   124  The ^$home directory is $home.
   125  Today's date is `date`.
   126  ***
   127  history 12
   128  echo history 12
   129  echo history 12
   130  ls -l  `whereis ls`
   131  echo one
   132  echo two
   133  echo three
   134  echo two
   135  history 12

A search string ends at the first word boundary. This is so it’s convenient to type additional text following without having it be confused as part of the search string. For example:

136 D% !?one;!?two;!?thr
echo one ; echo two ; echo three
one
two
three
137 D% _

Retrieving Individual Words

To pick off individual words of the immediately preceding command, there’s some convenient shorthand. “!*” gets all the argument words:

137 D% echo now is the time
now is the time
138 D% echo Finally, !* to begin
echo Finally, now is the time to begin
Finally, now is the time to begin
139 D% _
“!$” gets just the last word:
139 D% echo the last word was !$.
echo the last word was begin.
the last word was begin.
140 D% _
and “!^” gets just the first argument word:
140 D% echo ===!^=== time is here
echo ===the=== time is here
===the=== time is here
141 D% _

Notice that a history substitution can be smashed right up against other literal text.

In the chapter on editing, additional facilities for selecting individual words or doing a search/replace will be introduced.

History Short-Form

Recognizing how frequently one would like to make a simple change to the immediately preceding command to correct a typo, the history mechanism provides a short form for just that purpose. “%” typed as the first character on the command line indicates that a search/replace pair follows:

141 D% echo hello world
hello world
142 D% %world%friends%
echo hello friends
hello friends

Typing “%%” matches the beginning of the line:

143 D% %%echo %
echo echo hello friends
echo hello friends

It’s also possible to refer to the search string in the replacement string by using an ampersand. (This example also illustrates that the trailing “%” isn’t required unless you want to explicitly mark the end of the replacement.)

144 D% %friends%family, & and neighbors
echo echo hello family, friends and neighbors
echo hello family, friends and neighbors
145 D% _

Obviously, that raises the question: how do you put a literal ampersand in the replacement? Simple. Just quote it with “^”, the shell escape character.

145 D% %and%^&
echo hello family, friends & neighbors
hello family, friends & neighbors
146 D% _