wiki/wiki/linux/shell.md

Shell

A shell is a command-line interpreter that provides a command-line interface to interact with the many unix-like system.

Unix Shells

The following is a list of Unix shells that are POSIX compliant.

• Bash is the GNU Projects shell. It is probably the most used and well known shell.
• 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.
• Zsh is a shell designed to be interactive but also a powerful scripting language. It is often set to be the default user shell because of its user friendliness and a more features like the possibility for syntax highlighting and auto suggestions and even an improved version of auto completion.

Usage

This section addresses various different functions by and actions that can be taken with shell commands. The focus is mostly on POSIX-compliant shells and scripts that are compatible with POSIX.

A helpful resource for shell scripts is the pure sh bible. There are similar repositories and resources for functions that are outside the scope of POSIX shells. Because of that and their issue regarding compatibility they will not be mentioned here.

Symbolic Links using ln

ln is a command to create links between files and folders. The most simple way to create a link between two folders or files is running the following command. Replace <source folder/file> and <target folder/file> as needed.

ln -s <source folder/file> <target folder/file>

ls and its Alternatives

ls is a command to list files and directories. Its most important flags are the -l option to create a list view and -a to show files that are normally hidden.

A popular alternative until its discontinuation was exa. A fork of it called eza is still actively maintained.

Renaming Files

Files can be renamed by using the mv command like in the following example.

mv old_name.ext new_name.ext

Another way to rename files that is especially useful if renaming a lot of files is needed is done by using the command-line utility rename. In the following example the string string1 will be substituted by string2 for a given file file.ext.

rename 's/string1/string2/g' file.ext

Additionally find can be used to recursively substitute substrings in all files inside a folder folder (including subfolders).

find folder -type f -exec rename 's/string1/string2/g' {} +

For renaming many files in bulk the qmv to move or qcp or copy can be used. Usually both of these commands are packaged withing the renameutils package. For more information visit the renameutils website.

Another option for bulk renaming is vidir which is part of the moreutils package for most distributions. vidir is able to create folders which qmv is not. More information can be found on the moreutils website.

Expansion

This section is based on the TLDP Bash guide for beginners. Although it is centered on Bash it can also be used in other shells (Zsh) while being unsupported by others (DASH).

In most shells curly braces can be used to expand a term. Commands can be expanded like the following example.

echo sp{el,il,al}l

The previous command will expand to the following command.

echo spell spill spall

The example shows a brace expansions of depth 1. Using multiple curly brace pairs the depth can be increased as needed.

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.

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.

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 this would be /bin/zsh.

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 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.

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.

[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. Disowning a command removes it from the current shell. 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

This section is based on a guide by baeldung. If the output of a command is unwanted it can be silenced. There are two parts that can be silenced independently. Non-error outputs (stdout) can be silenced by appending 1> /dev/null to the command. Due to the 1 being ommitable > /dev/null has the same effect. Error outputs (stderr) can be silenced by appending 2> /dev/null. The complete output of both stderr and stdout can be silenced by appending > /dev/null 2>&1 to the command. A shortened version of it not possible on all shells is &> /dev/null.

cd Into Next Or Previous Sibling Directory

Quickly switching to the alphabetically next or previous directory can in some cases be useful. For this the following commands can be used. The first one navigates to the alphabetically previous sibling directory. The second one to the next.

cd ../"$(ls -F .. | grep '/' | grep -B1 -xF "${PWD##*/}/" | head -n 1)"
cd ../"$(ls -F .. | grep '/' | grep -A1 -xF "${PWD##*/}/" | tail -n 1)"

Error solving

This section addresses various problems that can occur and are related to the shell.

Pressing Enter Produces ^M

This error can easily be fixed by running the following command. It is related to a terminal line setting problem.

stty sane