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Here’s the sample code:
user$ touch file.txt
user$ mate !$
“mate” is calling textmate text editor, and that second command opens the previously created file.txt. what is this !$ syntax? what does ! do? what does $ do?
This is a shell feature called in various shells history expansion or history substitution.
Words starting with an exclamation mark are replaced with words from previously executed commands. Which words and which commands, it depends on the rest of the word.
In this particular example the dollar sign refers to the last word of a command, and since there is no other character between ‘!’ and ‘$’ the referenced command is the previous one, so !$ gets replaced with the last argument of the prevoius command.
What other command and word specifiers exist differ somewhat between shells.