What does !$ mean?

Question

I'm following through a tutorial and it mentions to run this command:

sudo chmod 700 !$

I'm not familiar with !$. What does it mean?

Answer 1

Basically, it's the last argument to the previous command.

!$ is the "end" of the previous command. Consider the following example: We start by looking for a word in a file:

grep -i joe /some/long/directory/structure/user-lists/list-15

if joe is in that userlist, we want to remove him from it. We can either fire up vi with that long directory tree as the argument, or as simply as vi !$ Which bash expands to:

vi /some/long/directory/structure/user-lists/list-15

^{(source; handy guide, by the way)}

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It's worth nothing the distinction between this !$ token and the special shell variable $_. Indeed, both expand to the last argument of the previous command. However, !$ is expanded during history expansion, while $_ is expanded during parameter expansion. One important consequence of this is that, when you use !$, the expanded command is saved in your history.

For example, consider the keystrokes

• echo Foo Enter echo !$ Jar Enter Up Enter; and

• echo Foo Enter echo $_ Jar Enter Up Enter.

(The only characters changed are the $! and $_ in the middle.)

In the former, when you press Up, the command line reads echo Foo Jar, so the last line written to stdout is Foo Jar.

In the latter, when you press Up, the command line reads echo $_ bar, but now $_ has a different value than it did previously—indeed, $_ is now Jar, so the last line written to stdout is Jar Jar.

Another consequence is that _ can be used in other parameter expansions, for example, the sequence of commands

printf '%s '    isomorphism
printf '%s\n'   ${_%morphism}sceles

prints isomorphism isosceles. But there's no analogous "${!$%morphism}" expansion.

For more information about the phases of expansion in Bash, see the EXPANSION section of man 1 bash (this is called Shell Expansions in the online edition). The HISTORY EXPANSION section is separate.

Answer 2

Strictly speaking !$ is the last word of the last command from the history list.

word - A sequence of characters treated as a unit by the shell. Words may not include unquoted metacharacters.

metacharacter - A character that, when unquoted, separates words. A metacharacter is a blank or one of the following characters: ‘|’, ‘&’, ‘;’, ‘(’, ‘)’, ‘<’, or ‘>’

blank - A space or tab character.

Examples:

set -o history # enable command history
set -o histexpand # enable ! style history substitution
HISTSIZE=10
save all lines on the history list
HISTCONTROL=
HISTIGNORE=
date
echo !$ # prints date
date>/dev/null
echo !$ # prints /dev/null
echo a b c>/dev/null
echo !$ # prints /dev/null
HISTCONTROL=ignorespace # lines which begin with a space character are not saved in the history list
echo a b c
 echo d e f # space at the beginning
echo !$ # prints c

Answer 3

!$ will give you last command used for that particular user....

you can also find the history of commands used earlier by using history command... try it out....

NOTE: For a particular user, all the commands used earlier will be stored under bash history file.