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117 lines
3.8 KiB
Markdown
117 lines
3.8 KiB
Markdown
# Shell
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A shell is a command-line interpreter that provides a command-line interface to
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interact with the many unix-like system.
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## Unix Shells
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The following is a list of Unix shells that are POSIX compliant.
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- [Bash](https://www.gnu.org/software/bash/) is the GNU Projects shell.
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It is probably the most used and well known shell.
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- [DASH](http://gondor.apana.org.au/~herbert/dash/) is a small implementation of
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`/bin/sh`. It is significantly faster than Bash and it is recommende to set
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DASH to be [the system shell](#changing-the-system-shell-binsh).
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- [Zsh](https://www.zsh.org/) is a shell designed to be interactive but also a
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powerful scripting language. It is often set to be the
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[default user shell](#changing-the-default-user-shell) because of its user
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friendliness and a more features like the possibility for
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[syntax highlighting](https://github.com/zsh-users/zsh-syntax-highlighting)
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and [auto suggestions](https://github.com/zsh-users/zsh-autosuggestions).
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## Usage
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This section addresses various different functions by and actions that can be
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taken with shell commands.
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### For Loops
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For loops can be especially useful when scripting.
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A function could be called on all the files in a directory.
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The following command shows the basic functionality and syntax of a for loop in
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a POSIX compliant shell.
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The command appends `-1` to all files in the current working directory.
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It consists of the loop structure that cycles through all files `*`.
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and appends `-1` using `mv "$f" "${f}-1"`.
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The command aswell as the scope could be change to only cycle through specific
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filetypes or use another command.
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```sh
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for f in *
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do mv "$f" "${f}-1"
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done
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```
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This command can also be shortened to a single line by using `;` like the
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following shows.
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```sh
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for f in *; do mv "$f" "${f}-1"; done
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```
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### Changing the Default User Shell
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The default user shell can be changed with the following command.
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Change `<path to shell>` according to the shell to set.
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For [ZSH](#unix-shells) this would be `/bin/zsh`.
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```sh
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chsh -s <path to shell>
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```
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### Changing the System Shell /bin/sh
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The system shell in some distributions is made up by a POSIX compliant mode of
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Bash.
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This can be slow at times and a shell like
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A shell like [DASH](#unix-shells) would be much faster.
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This is a guide on how to persistently change the symlink /bin/sh from Bash to
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DASH.
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Run the following commands to install and link `/bin/sh` to the DASH shell.
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```sh
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yay -S dash
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sudo ln -sfT /bin/dash /bin/sh
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```
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Afterwards create the file `/usr/share/libalpm/hooks/bash-update.hook` and fill
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it with the following text.
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```txt
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[Trigger]
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Type = Package
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Operation = Install
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Operation = Upgrade
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Target = bash
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[Action]
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Description = Re-pointing /bin/sh symlink to dash...
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When = PostTransaction
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Exec = /usr/bin/ln -sfT dash /usr/bin/sh
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Depends = dash
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```
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### Disown a Command
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This section is based on a guide by
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[baeldung](https://www.baeldung.com/linux/detach-process-from-terminal).
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Disowning a command removes it from the current shell.
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Commands that are detached from a shell will not terminate upon its closure.
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A command can be disowned simply by appending `& disown` at the end of it.
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It will then provide the PID of the command that got disowned.
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### Silencing a Command
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This section is based on a guide by
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[baeldung](https://www.baeldung.com/linux/silencing-bash-output).
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If the output of a command is unwanted it can be silenced.
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There are two parts that can be silenced independently.
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Non-error outputs (stdout) can be silenced by appending `1> /dev/null` to the
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command.
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Due to the `1` being ommitable `> /dev/null` has the same effect.
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Error outputs (stderr) can be silenced by appending `2> /dev/null`.
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The complete output of both stderr and stdout can be silenced by appending
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`> /dev/null 2>&1` to the command.
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A shortened version of it not possible on all shells is `&> /dev/null`.
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