linux system calls
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Julia's Reruns Bot
@b0rk-reruns.jvns.ca
261 followers 1 following 323 posts
This is a bot that I (@b0rk.jvns.ca) made to post reruns of my comics. Posts ~3x/week. Almost all of these are part of zines that you can buy at wizardzines.com, and you can find all of them at wizardzines.com/comics.
![Let's talk about an advanced SQL feature: window functions!
Normally SQL expressions only (et you refer to information in a single row.
`SELECT CONCAT(firstname, ' ', lastname) as full_name`
(`firstname` and `lastname` are 2 columns from the same row)
person 1, bald and with a worried expression: can I refer to other rows though? Like subtract the value in the previous row?
person 2, happy with curly hair: yes, with `*` window functions `*`
Window functions are SQL expressions that let you reference values in other rows. The syntax (explained on the next page!) is:
`[expression] OVER ([window definition])`
Example: use `LAG()` to find how long since the last sale
```
SELECT item,
day - LAG(day) OVER (ORDER BY day)
FROM sales
```
sales:
| item | day |
|](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:ygxqgtpypjuaf2b3oyrdn44x/bafkreifkm3b4kx4anwb5jjd66cfeeeyhvjf2jpdxaydt7yidtk3rizqifm)
![
### every process has command line arguments
`$ ls 1 /usr/bin`
(`ls`, `-l`, and `usr` are arguments!)
### they're passed to the program as an array
example from Python:
```
import sys
print(sys.argv)
```
`['test.py', 'file.txt' ]`
### arguments can be any sequence of bytes
`$ python program.py ♥`
(emoji are totally allowed!)
### the first argument is the executable's name
```
[
'ls'
'-1',
'/usr/bin/'
]
```
(`ls` is the executable name)
### the total length of the arguments is limited
you can find the limits on your system with `xargs -show-limits`
It's usually ~2MB
### you can decide how you parse arguments
- `-flag`: single dash!
- `--flag`: 2 dashes!
- `♥♥flag`: weird emoji scheme that will be very annoying to use!](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:ygxqgtpypjuaf2b3oyrdn44x/bafkreifixmtykscfmoug57qywymbrapunhwt53zp26d6kexxrqaizcaj3i)
![### You might have heard that DNS updates need time to "propagate".
What's actually happening is that there are old cached records which need to expire.
### DNS records are cached in many places
- browser caches
- DNS resolver caches
- operating system caches
google.com, represented by a box with a smiley face: my DNS records are cached on billions of devices!
### let's see what happens when you update an IP
bananas.com A▾
300 [changed to] 60
1.2.3.4 [changed to] 5.6.7.8
beware: even if you change the TTL to 60s, you still have to wait 300 seconds for the old record to expire
### 30 seconds later...
(you go to bananas.com in your browser)
Illustration of a resolver, represented by a box with a smiley face holding a magnifying glass, and a browser, represented by the Firefox logo of a fox wrapped around a globe
browser: hey what's the IP for bananas.com?
resolver, thinking: let's check my cache for bananas.com... found it!!
resolver: it's 1.2.3.4!
### 400 second](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:ygxqgtpypjuaf2b3oyrdn44x/bafkreidj6tnwrxt2wqkkazselj4psk244joa2kfoj37tjh2srhows4u3la)
![### HTTP responses have:
- a status code (200 OK! 404 not found!)
- headers
- a body (HTML, an image, JSON, etc)
### Here's the HTTP response from `examplecat.com/cat.txt`:
```
HTTP/1.1 200 OK
status
Accept-Ranges: bytes
Cache-Control: public, max-age=0
Content-Length: 33
Content-Type: text/plain; charset=UTF-8 Date: Mon, 09 Sep 2019 01:57:35 GMT
Etag: "ac5affa59f554a1440043537ae973790-ssl"
Strict-Transport-Security: max-age=31536000
Age: 0
Server: Netlify
[ASCII image of a cat, labelled "cat!" with a smiley face]
```
The first line, `HTTP/1.1 200 OK` is the status code. "200" is the status.
The lines from `Accept-Ranges` to `Server` are the headers.
The cat picture is the body.
### There are a few kinds of response headers:
- when the resource was sent/modified:
```
Date: Mon, 09 Sep 2019 01:57:35 GMT
Last-Modified: 3 Feb 2017 13:00:00 GMT
```
- about the response body:
```
Content-Language: en-US
Content-Length: 33
Content-Type: text/plain; charset=UTF-8
Content-Encoding: g](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:ygxqgtpypjuaf2b3oyrdn44x/bafkreiddcdprzhtnwxvofkmdifyesdu2jfvdweyyosp6heqe6wnnx3ylx4)
![### shellcheck finds problems with your shell scripts
`$ shellcheck my-script.sh`
shellcheck: oops, you can't use in an `if [ ... ]`!
### it checks for hundreds of common shell scripting errors
shellcheck: hey, that's a bash- only feature but your script starts with `#!/bin/sh`
### every shellcheck error has a number (like "SC2013")
and the shellcheck wiki has a page for every error with examples! I've learned a lot from the wiki.
### it even tells you about misused commands
shellcheck: hey, it looks like you're not using `grep` correctly here
person: wow I'm not! thanks!
### your text editor probably has a shellcheck plugin
shellcheck: I can check your shell scripts every time you save!
### basically, you should probably use it
bash has too many weird edge cases for me to remember, I love that shellcheck can help me out!](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:ygxqgtpypjuaf2b3oyrdn44x/bafkreidpubhfrjtxplyzpfm3up22hkz42sgbhtzkfrvghznwtwxmkvwfgy)
![### some bash features aren't in the POSIX spec
Illustration of a smiling stick figure with curly hair.
Person: here are some examples! These won't work in POSIX shells like `dash` and `sh`.
### arrays
POSIX shells only have one array: `$@` for arguments
### [[ $DIR=/home/*]]
POSIX alternative: match strings with `grep`
### [[ ... ]]
POSIX alternative:
`[ ... ]`
### diff <(./cmd1) <./cmd2)
this is called "process substitution", you can use named pipes instead
### the local Keyword
in POSIX shells, all variables are global
### for ((i=0; i <3; i++))
`sh` only has for `x` in ... loops, not C-style loops
### a. {png, svg}
you'll have to type
`a.png a.svg`
### {1..5}
POSIX alternative:
`$(seq 1 5)`
### $'\n'
POSIX alternative:
`$(printf "\n")`
### ${var//search/replace}
POSIX alternative:
pipe to `sed`](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:ygxqgtpypjuaf2b3oyrdn44x/bafkreiaozbeoudiceuacg5gfk4bzjzxovwg7zsjo5dk4l3kooqfkfgqdqe)
