merge of forgotten link

advanced/learning_bash_scripting.md

@@ -668,7 +668,90 @@ fi
 
 ## How does it work behind the scenes?
 
-TODO - explain `test` and exit status
+## exit status
+
+Every command you execute on the command line has an **exit code**.
+You can [read up](https://en.wikipedia.org/wiki/Exit_status) a bit on what they are but the most important things to know are:
+
+* the code is always a **number**
+* on our systems it's a [uint8](https://en.wikipedia.org/wiki/Integer_(computer_science)#Common_integral_data_types) which means a value between **0** and **255**
+* the [**convention**](https://tldp.org/LDP/abs/html/exitcodes.html) is that `0` means **success**, everything else is an **error**
+
+In your bash shell, the variable `$?` always references the *last* exit status code.
+We can discover it's behaviour as follows.
+
+```
+waldek@debian:~$ ls does_exist 
+does_exist
+waldek@debian:~$ echo $?
+0
+waldek@debian:~$ ls does_not_exist
+ls: cannot access 'does_not_exist': No such file or directory
+waldek@debian:~$ echo $?
+2
+waldek@debian:~$ echo $?
+0
+waldek@debian:~$ 
+```
+
+The **first** `echo $?` prints the exit code of `ls does_exist`.
+The **second** prints the exit code of the *failed* command `ls does_not_exist`.
+The **third** prints the exit code of the `echo $?` that failed!
+
+## `test`
+
+The presence of exit codes means we can **evaluate** their value and make **decisions** based on the outcome.
+The main workhorse for this is a builtin called `test`.
+
+```
+waldek@debian:~$ whatis test
+test (1)             - check file types and compare values
+waldek@debian:~$
+```
+I **highly** recommend you take some time to read the `man test`.
+Because the convention of exit codes is *no news, good news* there are two tiny programs that just serve to output `true` and `false`, where true is `0` and false is `1`.
+A little demonstration.
+
+```
+waldek@debian:~$ which true
+/usr/bin/true
+waldek@debian:~$ which false
+/usr/bin/false
+waldek@debian:~$ man true
+waldek@debian:~$ whatis true
+true (1)             - do nothing, successfully
+waldek@debian:~$ whatis false
+false (1)            - do nothing, unsuccessfully
+waldek@debian:~$ true
+waldek@debian:~$ echo $?
+0
+waldek@debian:~$ false
+waldek@debian:~$ echo $?
+1
+waldek@debian:~$ test true == true
+waldek@debian:~$ echo $?
+0
+waldek@debian:~$ test true == false
+waldek@debian:~$ echo $?
+1
+waldek@debian:~$ 
+```
+
+Again, I highly advise you to read the `man test`.
+If you did this then the following will make a lot of sense.
+
+```
+waldek@debian:~$ test -a does_exist 
+waldek@debian:~$ echo $?
+0
+waldek@debian:~$ test -d does_exist 
+waldek@debian:~$ echo $?
+1
+waldek@debian:~$ test -a does_not_exist
+waldek@debian:~$ echo $?
+1
+waldek@debian:~$ 
+```
 
 The table below is taken from the bash reference manual you can find [here](https://www.gnu.org/software/bash/manual/bash.html#Bash-Conditional-Expressions).
 
@@ -709,8 +792,6 @@ The table below is taken from the bash reference manual you can find [here](http
 | string1 < string2 | True if string1 sorts before string2 lexicographically. |
 | string1 > string2 | True if string1 sorts after string2 lexicographically. |
 
-
-
 ## Nested `if` statements
 
 It's worth pointing out we can *nest* `if` statements *inside* other `if` statements.
@@ -789,8 +870,18 @@ fi
 
 ## A *modern* version of `test`
 
+I'll be the first to admit that the syntax of `bash` can be confusing and is rarely reader friendly.
+A nice, but brief, explication of the nuances of single and double brackets can be found in [this](https://unix.stackexchange.com/questions/306111/what-is-the-difference-between-the-bash-operators-vs-vs-vs) stack overflow post.
+The double bracket command are called [compound commands](https://www.gnu.org/software/bash/manual/html_node/Compound-Commands.html).
+
 ### `[[  ]]`
 
+```
+waldek@debian:~$ [[ 3 = [[:digit:]] ]] ; echo $?
+0
+waldek@debian:~$ [ 3 = [[:digit:]] ] ; echo $?
+1
+```
 TODO
 
 ### `((  ))`
@@ -799,7 +890,13 @@ TODO
 
 ### `&&` and  `||`
 
-TODO
+```
+waldek@debian:~$ test true == true && echo "yes sir!" || echo "nope..."
+yes sir!
+waldek@debian:~$ test true == false && echo "yes sir!" || echo "nope..."
+nope...
+waldek@debian:~$ 
+```
 
 [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-if-statements.php)
 
@@ -813,11 +910,21 @@ Rename all files in a folder with an prefix or postfix.
 
 # Functions - Reuse code to make life easier.
 
+## defining a function
+
+## global vs local variable
+
+## return values
+
+## the `command` builtin
+
 [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-functions.php)
 
 # User Interface - Make your scripts user friendly.
 
-[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-user-interfaces.php)
+* [dialog tutorial](https://www.linuxjournal.com/article/2807)
+* [better dialog tutorial](https://linuxcommand.org/lc3_adv_dialog.php)
+* [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-user-interfaces.php)
 
 ## Python
 

advanced/learning_shells.md

@@ -0,0 +1,44 @@
+# `bash` login
+
+TODO
+
+## which files are sources when
+
+TODO
+
+# Prompt customization
+
+TODO
+
+# Alternative shells
+
+TODO
+
+## `zsh`
+
+TODO
+
+## `fish`
+
+TODO
+
+## `xonsh`
+
+TODO
+
+# Shell completion
+
+TODO
+
+# Frameworks
+
+TODO
+
+## oh my zsh
+
+TODO
+
+## oh my bash
+
+TODO
+

links.md

@@ -1,12 +1,21 @@
 # links and ideas
 
 * [info](https://un.curl.dev/) about working in open source
+* grep and regex [exercises](https://github.com/learnbyexample/Command-line-text-processing/tree/master/exercises/GNU_grep)
 * systemd-nspawn doc and exercises
 	* [hub](https://hub.nspawn.org/images/)
+	* [tutorial](https://blog.selectel.com/systemd-containers-introduction-systemd-nspawn/)
+	* [docker nspawn](https://seanmcgary.com/posts/run-docker-containers-with-systemd-nspawn)
+	* [debian](https://wiki.debian.org/nspawn)
+	* [arch](https://wiki.archlinux.org/title/systemd-nspawn#Use_a_%22macvlan%22_or_%22ipvlan%22_interface)
 	* [systemd-sysext](https://0pointer.net/blog/testing-my-system-code-in-usr-without-modifying-usr.html)
 * [history of command line arguments](https://blog.liw.fi/posts/2022/05/07/unix-cli/)
-* do a mini class on barshrc customisation, prompts, zsh and ohmyzsh
 * class on dpkg, apt, and .deb introspection
 * class on compiling a simple program, maybe recompiling a kernel
 * exercise on how to take a physical system and convert it to a VM
 * munin plugin design exercise
+* advanced [bash](https://tldp.org/LDP/abs/html/index.html) scripting
+
+## started
+
+* do a mini class on barshrc customisation, prompts, zsh and ohmyzsh