src/content/en/: Unpluralize collection names

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+= Useful Bash variables
+:categories: shell
+
+:bash:                   https://www.gnu.org/software/bash/
+:bash-bang-bang:         https://www.gnu.org/software/bash/manual/bash.html#Event-Designators
+:bash-dollar-underscore: https://www.gnu.org/software/bash/manual/bash.html#Special-Parameters
+
+{bash}[GNU Bash] has a few two letter variables that may be useful when typing
+on the terminal.
+
+== `!!`: the text of the last command
+
+The {bash-bang-bang}[`!!` variable] refers to the previous command, and I find
+useful when following chains for symlinks:
+
+[source,sh]
+----
+$ which git
+/run/current-system/sw/bin/git
+$ readlink $(!!)
+readlink $(which git)
+/nix/store/5bgr1xpm4m0r72h9049jbbhagxdyrnyb-git-2.28.0/bin/git
+----
+
+It is also useful when you forget to prefix `sudo` to a command that requires
+it:
+
+[source,sh]
+----
+$ 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.
+
+== `$_`: most recent parameter
+
+The {bash-dollar-underscore}[`$_` variable] will give you the most recent
+parameter you provided to a previous argument, which can save you typing
+sometimes:
+
+[source,sh]
+----
+# instead of...
+$ mkdir -p a/b/c/d/
+$ cd a/b/c/d/
+
+# ...you can:
+$ mkdir -p a/b/c/d/
+$ cd $_
+----
+
+== 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.