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+# ZSH
+
+## TL;DR
+
+Startup file sequence:
+
+1. `/etc/zshenv` and `${ZDOTDIR}/.zshenv`
+1. **login** shells only: `/etc/zprofile` and `${ZDOTDIR}/.zprofile`
+1. **interactive** shell only: `/etc/zshrc` and `${ZDOTDIR}/.zshrc`
+1. **login** shells only: `/etc/zlogin` and `${ZDOTDIR}/.zlogin`
+1. **upon exit**: `/etc/zlogout` and `${ZDOTDIR}/.zlogout`
+
+Aliases are expanded when the function definition is parsed, not when the function is executed. Define aliases **before** functions to avoid problems.
+
+```shell
+# create a function
+function_name () { … }
+function function_name { … }
+function function_name () { … }
+
+# regex match
+[[ $OSTYPE =~ "darwin" ]]
+[[ $OSTYPE -regex-match "darwin" ]]
+
+# find broken symlinks in the current directory
+ls **/*(-@)
+
+# print all shell and environment variables
+( setopt posixbuiltin; set; )
+```
+
+## Alias expansion
+
+When one writes an alias, one can also press `ctrl-x` followed by `a` to see the expansion of that alias.
+
+## Arrays
+
+```shell
+# get a slice (negative are backwards)
+echo ${ARRAY[2,-1]}
+
+# get all folders up to a non folder, backwards
+local COMMAND
+local FOLDERS=()
+for (( I = $# ; I >= 0 ; I-- )); do
+  if [[ -d "${@[$I]}" ]]; then
+    FOLDERS+="${@[$I]}"
+  else
+    COMMAND="${@[1,-$((${#FOLDERS}+1))]}"
+    break
+  fi
+done
+```
+
+```shell
+# make entries in PATH unique
+# see https://til.hashrocket.com/posts/7evpdebn7g-remove-duplicates-in-zsh-path
+typeset -aU path
+```
+
+## Tests
+
+```shell
+# regex match
+[[ $OSTYPE =~ "darwin" ]]
+[[ $OSTYPE -regex-match "darwin" ]]
+```
+
+## Find broken symlinks in the current directory
+
+```shell
+ls **/*(-@)
+```
+
+## Configuration
+
+### Config files read order
+
+1. `/etc/zshenv`; this cannot be overridden  
+   subsequent behaviour is modified by the `RCS` and `GLOBAL_RCS` options:
+
+   - `RCS` affects all startup files
+   - `GLOBAL_RCS` only affects global startup files (those shown here with an path starting with a /)
+
+   If one of the options is unset at any point, any subsequent startup file(s) of the corresponding type will not be read.  
+   It is also possible for a file in `$ZDOTDIR` to re-enable `GLOBAL_RCS`.  
+   Both `RCS` and `GLOBAL_RCS` are set by default
+
+1. `$ZDOTDIR/.zshenv`
+1. if the shell is a login shell:
+
+   1. `/etc/zprofile`
+   1. `$ZDOTDIR/.zprofile`
+
+1. if the shell is interactive:
+
+   1. `/etc/zshrc`
+   1. `$ZDOTDIR/.zshrc`
+
+1. if the shell is a login shell:
+
+   1. `/etc/zlogin`
+   1. `$ZDOTDIR/.zlogin`
+
+1. when a login shell exits:
+
+   1. `$ZDOTDIR/.zlogout`
+   1. `/etc/zlogout`
+
+   This happens with either an explicit exit via the `exit` or `logout` commands, or an implicit exit by reading `end-of-file` from the terminal.  
+   However, if the shell terminates due to exec’ing another process, the files are not read. These are also affected by the `RCS` and `GLOBAL_RCS` options.  
+   The `RCS` option affects the saving of history files, i.e. if `RCS` is unset when the shell exits, no history file will be saved.
+
+If `ZDOTDIR` is unset, `HOME` is used instead. Files listed above as being in `/etc` may be in another directory, depending on the installation.
+
+`/etc/zshenv` is run for **all** instances of zsh.  
+it is a good idea to put code that does not need to be run for every single shell behind a test of the form `if [[ -o rcs ]]; then ...` so that it will not be executed when zsh is invoked with the `-f` option.
+
+When `/etc/zprofile` is installed it will override `PATH` and possibly other variables that a user may set in `~/.zshenv`. Custom `PATH` settings and similar overridden variables can be moved to `~/.zprofile` or other user startup files that are sourced after the `/etc/zprofile`.  
+If `PATH` must be set in `~/.zshenv` to affect things like non-login ssh shells, one method is to use a separate path-setting file that is conditionally sourced in `~/.zshenv` and also sourced from `~/.zprofile`.
+
+### History
+
+```shell
+# The maximum number of events stored in the internal history list.
+# If you use the HIST_EXPIRE_DUPS_FIRST option, setting this value larger than
+# the SAVEHIST size will give you the difference as a cushion for saving
+# duplicated history events.
+HISTSIZE=1000
+
+# The file to save the history in when an interactive shell exits.
+# If unset, the history is not saved.
+HISTFILE=~/.histfile
+
+# The maximum number of history events to save in the history file.
+SAVEHIST=1000
+```
+
+### Completion
+
+```shell
+# Enable completion
+autoload -U compinit
+compinit
+
+# Enable cache for the completions
+zstyle ':completion::complete:*' use-cache true
+```
+
+### Prompt management
+
+```shell
+# Enable prompt management
+autoload -U promptinit
+promptinit; prompt gentoo
+```
+
+### Automatic source of files in a folder
+
+```shell
+# Configuration modules.
+# All files in the configuration folder will be automatically loaded in
+# numeric order. The last file setting a value overrides the previous ones.
+# Links are only sourced if their reference exists.
+ZSH_MODULES_DIR="${ZSH_MODULES_DIR:-$HOME/.zshrc.d}"
+if [[ -d "${ZSH_MODULES_DIR}" ]]
+then
+	for ZSH_MODULE in ${ZSH_MODULES_DIR}/*
+	do
+		[[ -r ${ZSH_MODULE} ]] && source "${ZSH_MODULE}"
+	done
+	unset ZSH_MODULE
+fi
+```
+
+## Frameworks
+
+- [antibody]
+- [antigen]
+- [ohmyzsh]
+- [zplug]
+
+## Plugins
+
+- [fzf]
+- [zsh-autosuggestions]
+- [zsh-history-substring-search]
+- [zsh-syntax-highlighting]
+
+## Troubleshooting
+
+> zsh compinit: insecure directories and files, run compaudit for list
+
+```shell
+compaudit | xargs chmod g-w
+```
+
+## Further readings
+
+- [Substitutions]
+- [ZSH compinit: insecure directories and files, run compaudit for list]
+- [Pattern matching in a conditional expression]
+- [Eemove duplicates in ZSH path]
+- [Completion config example]
+- [What should/shouldn't go in .zshenv, .zshrc, .zlogin, .zprofile, .zlogout?]
+- [The Z Shell Manual]
+- [Gentoo Wiki]
+- [How can I convert an array into a comma separated string?]
+- [How to list all variables names and their current values?]
+
+[completion config example]: https://github.com/ThiefMaster/zsh-config/blob/master/zshrc.d/completion.zsh
+[gentoo wiki]: https://wiki.gentoo.org/wiki/Zsh
+[How can I convert an array into a comma separated string?]: https://stackoverflow.com/questions/53839253/how-can-i-convert-an-array-into-a-comma-separated-string
+[pattern matching in a conditional expression]: https://unix.stackexchange.com/questions/553607/pattern-matching-in-a-zsh-conditional-expression
+[remove duplicates in zsh path]: https://til.hashrocket.com/posts/7evpdebn7g-remove-duplicates-in-zsh-path
+[substitutions]: http://zsh.sourceforge.net/Guide/zshguide05.html
+[the z shell manual]: http://zsh.sourceforge.net/Doc/Release/
+[what should/shouldn't go in .zshenv, .zshrc, .zlogin, .zprofile, .zlogout?]: https://unix.stackexchange.com/questions/71253/what-should-shouldnt-go-in-zshenv-zshrc-zlogin-zprofile-zlogout#487889
+[zsh compinit: insecure directories and files, run compaudit for list]: https://github.com/zsh-users/zsh-completions/issues/433#issuecomment-619321054
+[how to list all variables names and their current values?]: https://askubuntu.com/questions/275965/how-to-list-all-variables-names-and-their-current-values#275972
+
+[antigen]: https://github.com/zsh-users/antigen
+[fzf]: https://github.com/junegunn/fzf
+[ohmyzsh]: https://github.com/ohmyzsh/ohmyzsh
+[zplug]: https://github.com/zplug/zplug
+[zsh-autosuggestions]: https://github.com/zsh-users/zsh-autosuggestions
+[zsh-history-substring-search]: https://github.com/zsh-users/zsh-history-substring-search
+[zsh-syntax-highlighting]: https://github.com/zsh-users/zsh-syntax-highlighting