path: root/_tils/2020-11-12-useful-bash-variables.md

---

title: Useful Bash variables

date: 2020-11-12 1

layout: post

lang: en

ref: useful-bash-variables

---

[GNU Bash][gnu-bash] has a few two letter variables that may be useful when
typing on the terminal.

[gnu-bash]: https://www.gnu.org/software/bash/

## `!!`: the text of the last command

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

[previous-command]: https://www.gnu.org/software/bash/manual/bash.html#Event-Designators

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

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

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

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][recent-parameter] will give you the most recent parameter
you provided to a previous argument, which can save you typing sometimes:

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

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

[recent-parameter]: https://www.gnu.org/software/bash/manual/bash.html#Special-Parameters

## 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.