shell: different shells and for loops

wiki/linux/shell.md

@@ -3,20 +3,95 @@
 A shell is a command-line interpreter that provides a command-line interface to
 interact with the many unix-like system.
 
-## Change the standard shell
+## Unix Shells
 
-The Shell in some distributions is made up by a POSIX compliant mode of bash.
-Problem with that is the speed.
-A shell like dash would be much faster.
-This is a guide on how to persistently change the symlink /bin/sh from bash to
-dash.
+The following is a list of Unix shells that are POSIX compliant.
 
-- `yay -S dash` - Install dash
-- `sudo ln -sfT /bin/dash /bin/sh` - link /bin/sh to dash
-- `sudo vim /usr/share/libalpm/hooks/bash-update.hook` - setup a hook to avoid
-bash resetting the symlink
+- [Bash](https://www.gnu.org/software/bash/) is the GNU Projects shell.
+  It is probably the most used and well known shell.
+- [DASH](http://gondor.apana.org.au/~herbert/dash/) is a small implementation of
+  `/bin/sh`. It is significantly faster than Bash and it is recommende to set
+  DASH to be [the system shell](#changing-the-system-shell-binsh).
+- [Zsh](https://www.zsh.org/) is a shell designed to be interactive but also a
+  powerful scripting language. It is often set to be the
+  [default user shell](#changing-the-default-user-shell) because of its user
+  friendliness and a more features like the possibility for
+  [syntax highlighting](https://github.com/zsh-users/zsh-syntax-highlighting)
+  and [auto suggestions](https://github.com/zsh-users/zsh-autosuggestions).
 
-## Disown a Command
+## Usage
+
+This section addresses various different functions by and actions that can be
+taken with shell commands.
+
+### For Loops
+
+For loops can be especially useful when scripting.
+A function could be called on all the files in a directory.
+The following command shows the basic functionality and syntax of a for loop in
+a POSIX compliant shell.
+The command appends `-1` to all files in the current working directory.
+It consists of the loop structure that cycles through all files `*`.
+and appends `-1` using `mv "$f" "${f}-1"`.
+The command aswell as the scope could be change to only cycle through specific
+filetypes or use another command.
+
+```sh
+for f in *
+  do mv "$f" "${f}-1"
+done
+```
+
+This command can also be shortened to a single line by using `;` like the
+following shows.
+
+```sh
+for f in *; do mv "$f" "${f}-1"; done
+```
+
+### Changing the Default User Shell
+
+The default user shell can be changed with the following command.
+Change `<path to shell>` according to the shell to set.
+For [ZSH](#unix-shells) this would be `/bin/zsh`.
+
+```sh
+chsh -s <path to shell>
+```
+
+### Changing the System Shell /bin/sh
+
+The system shell in some distributions is made up by a POSIX compliant mode of
+Bash.
+This can be slow at times and a shell like
+A shell like [DASH](#unix-shells) would be much faster.
+This is a guide on how to persistently change the symlink /bin/sh from Bash to
+DASH.
+Run the following commands to install and link `/bin/sh` to the DASH shell.
+
+```sh
+yay -S dash
+sudo ln -sfT /bin/dash /bin/sh
+```
+
+Afterwards create the file `/usr/share/libalpm/hooks/bash-update.hook` and fill
+it with the following text.
+
+```txt
+[Trigger]
+Type = Package
+Operation = Install
+Operation = Upgrade
+Target = bash
+
+[Action]
+Description = Re-pointing /bin/sh symlink to dash...
+When = PostTransaction
+Exec = /usr/bin/ln -sfT dash /usr/bin/sh
+Depends = dash
+```
+
+### Disown a Command
 
 This section is based on a guide by
 [baeldung](https://www.baeldung.com/linux/detach-process-from-terminal).
@@ -25,7 +100,7 @@ Commands that are detached from a shell will not terminate upon its closure.
 A command can be disowned simply by appending `& disown` at the end of it.
 It will then provide the PID of the command that got disowned.
 
-## Silencing a Command
+### Silencing a Command
 
 This section is based on a guide by
 [baeldung](https://www.baeldung.com/linux/silencing-bash-output).