merge of forgotten link
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waldek
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@ -668,7 +668,90 @@ fi
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## How does it work behind the scenes?
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TODO - explain `test` and exit status
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## exit status
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Every command you execute on the command line has an **exit code**.
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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:
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* the code is always a **number**
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* 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**
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* the [**convention**](https://tldp.org/LDP/abs/html/exitcodes.html) is that `0` means **success**, everything else is an **error**
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In your bash shell, the variable `$?` always references the *last* exit status code.
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We can discover it's behaviour as follows.
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```
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waldek@debian:~$ ls does_exist
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does_exist
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waldek@debian:~$ echo $?
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0
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waldek@debian:~$ ls does_not_exist
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ls: cannot access 'does_not_exist': No such file or directory
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waldek@debian:~$ echo $?
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2
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waldek@debian:~$ echo $?
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0
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waldek@debian:~$
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```
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The **first** `echo $?` prints the exit code of `ls does_exist`.
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The **second** prints the exit code of the *failed* command `ls does_not_exist`.
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The **third** prints the exit code of the `echo $?` that failed!
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## `test`
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The presence of exit codes means we can **evaluate** their value and make **decisions** based on the outcome.
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The main workhorse for this is a builtin called `test`.
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```
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waldek@debian:~$ whatis test
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test (1) - check file types and compare values
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waldek@debian:~$
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```
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I **highly** recommend you take some time to read the `man test`.
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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`.
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A little demonstration.
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```
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waldek@debian:~$ which true
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/usr/bin/true
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waldek@debian:~$ which false
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/usr/bin/false
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waldek@debian:~$ man true
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waldek@debian:~$ whatis true
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true (1) - do nothing, successfully
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waldek@debian:~$ whatis false
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false (1) - do nothing, unsuccessfully
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waldek@debian:~$ true
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waldek@debian:~$ echo $?
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0
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waldek@debian:~$ false
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waldek@debian:~$ echo $?
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1
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waldek@debian:~$ test true == true
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waldek@debian:~$ echo $?
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0
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waldek@debian:~$ test true == false
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waldek@debian:~$ echo $?
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1
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waldek@debian:~$
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```
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Again, I highly advise you to read the `man test`.
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If you did this then the following will make a lot of sense.
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```
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waldek@debian:~$ test -a does_exist
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waldek@debian:~$ echo $?
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0
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waldek@debian:~$ test -d does_exist
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waldek@debian:~$ echo $?
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1
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waldek@debian:~$ test -a does_not_exist
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waldek@debian:~$ echo $?
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1
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waldek@debian:~$
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```
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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).
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@ -709,8 +792,6 @@ The table below is taken from the bash reference manual you can find [here](http
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| string1 < string2 | True if string1 sorts before string2 lexicographically. |
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| string1 > string2 | True if string1 sorts after string2 lexicographically. |
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## Nested `if` statements
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It's worth pointing out we can *nest* `if` statements *inside* other `if` statements.
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@ -789,8 +870,18 @@ fi
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## A *modern* version of `test`
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I'll be the first to admit that the syntax of `bash` can be confusing and is rarely reader friendly.
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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.
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The double bracket command are called [compound commands](https://www.gnu.org/software/bash/manual/html_node/Compound-Commands.html).
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### `[[ ]]`
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```
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waldek@debian:~$ [[ 3 = [[:digit:]] ]] ; echo $?
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0
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waldek@debian:~$ [ 3 = [[:digit:]] ] ; echo $?
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1
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```
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TODO
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### `(( ))`
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@ -799,7 +890,13 @@ TODO
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### `&&` and `||`
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TODO
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```
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waldek@debian:~$ test true == true && echo "yes sir!" || echo "nope..."
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yes sir!
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waldek@debian:~$ test true == false && echo "yes sir!" || echo "nope..."
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nope...
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waldek@debian:~$
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```
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[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-if-statements.php)
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@ -813,11 +910,21 @@ Rename all files in a folder with an prefix or postfix.
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# Functions - Reuse code to make life easier.
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## defining a function
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## global vs local variable
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## return values
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## the `command` builtin
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[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-functions.php)
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# User Interface - Make your scripts user friendly.
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[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-user-interfaces.php)
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* [dialog tutorial](https://www.linuxjournal.com/article/2807)
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* [better dialog tutorial](https://linuxcommand.org/lc3_adv_dialog.php)
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* [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-user-interfaces.php)
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## Python
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@ -0,0 +1,44 @@
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# `bash` login
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TODO
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## which files are sources when
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TODO
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# Prompt customization
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TODO
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# Alternative shells
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TODO
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## `zsh`
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TODO
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## `fish`
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TODO
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## `xonsh`
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TODO
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# Shell completion
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TODO
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# Frameworks
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TODO
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## oh my zsh
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TODO
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## oh my bash
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TODO
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11
links.md
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links.md
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@ -1,12 +1,21 @@
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# links and ideas
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* [info](https://un.curl.dev/) about working in open source
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* grep and regex [exercises](https://github.com/learnbyexample/Command-line-text-processing/tree/master/exercises/GNU_grep)
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* systemd-nspawn doc and exercises
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* [hub](https://hub.nspawn.org/images/)
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* [tutorial](https://blog.selectel.com/systemd-containers-introduction-systemd-nspawn/)
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* [docker nspawn](https://seanmcgary.com/posts/run-docker-containers-with-systemd-nspawn)
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* [debian](https://wiki.debian.org/nspawn)
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* [arch](https://wiki.archlinux.org/title/systemd-nspawn#Use_a_%22macvlan%22_or_%22ipvlan%22_interface)
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* [systemd-sysext](https://0pointer.net/blog/testing-my-system-code-in-usr-without-modifying-usr.html)
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* [history of command line arguments](https://blog.liw.fi/posts/2022/05/07/unix-cli/)
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* do a mini class on barshrc customisation, prompts, zsh and ohmyzsh
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* class on dpkg, apt, and .deb introspection
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* class on compiling a simple program, maybe recompiling a kernel
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* exercise on how to take a physical system and convert it to a VM
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* munin plugin design exercise
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* advanced [bash](https://tldp.org/LDP/abs/html/index.html) scripting
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## started
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* do a mini class on barshrc customisation, prompts, zsh and ohmyzsh
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