Useful Bash variables

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GNU Bash has a few two letter variables that may be useful when typing on the terminal.

!!: the text of the last command

The !! variable refers to the previous command, and I find useful when following chains for symlinks:

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$ which git
/run/current-system/sw/bin/git
$ readlink $(!!)
readlink $(which git)
/nix/store/5bgr1xpm4m0r72h9049jbbhagxdyrnyb-git-2.28.0/bin/git

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It is also useful when you forget to prefix sudo to a command that requires it:

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$ requires-sudo.sh
requires-sudo.sh: Permission denied
$ sudo !!
sudo ./requires-sudo.sh
# all good

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Bash prints the command expansion before executing it, so it is better for you to follow along what it is doing.

$_: most recent parameter

The $_ variable will give you the most recent parameter you provided to a previous argument, which can save you typing sometimes:

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# instead of...
$ mkdir -p a/b/c/d/
$ cd a/b/c/d/

# ...you can:
$ mkdir -p a/b/c/d/
$ cd $_

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Conclusion

I wouldn’t use those in a script, as it would make the script terser to read, I find those useful shortcut that are handy when writing at the interactive terminal.