kv-ates: a list of handy solutions.

ansible, end playbook early:

• use meta module: - meta: end_play

lightdm, testing mode:

lightdm --test-mode

systemd, add service:

sudo vim /etc/systemd/system/<serviceName>.conf
sudo systemctl daemon-reload
sudo systemctl <serviceName> enable
sudo systemctl <serviceName> start 

general, use feh to quickly navigate svg files inside a folder:

• feh --conversion-timeout 1 -g 640x480 -d -S filename .

general, install yay:

git clone https://aur.archlinux.org/yay.git
cd yay
makepkg -si

general, configure suspend on lid close:

• edit /etc/systemd/logind.conf, reboot

general, generate random string with openssl:

• openssl rand -base64 <size>

general, list swap partitions:

• swapon -s or cat /proc/swaps

general, compact XOR (source):

• A XOR B = NOT (NOT A == NOT B)

general, xargs delimiter flag (in case there is the possibility of having whitespace in a line):

• xargs -d '\n' <input>

general, sort by a given column:

• sort -k <column number> <input>

general, manually set keyboard layout/variant:

• setxkbmap <layout> <variant>

general, timedatectl command:

• set system timezone: timedatectl set-timezone Europe/Berlin

• list timezones: timedatectl list-timezones

general, mnemonic for little and big endian (source):

• big endian: the large part of the number is first (big first)

  • similar to common usage: 123 = 1 * 10^2 + 2 * 10^1 + 3 * 10^0

• little endian: the small part of the number is first (little first)

  • exponents order follows digit iteration order, so 123 is 321 in LE: 321 = 3 * 10^0 + 2 * 10^1 + 3 * 10^2

general, watch command:

• -n flag defines time interval: watch -n 2 nmcli

autojump, script:

• autojump on arch needs a symlink from the .bash to .sh nope, solved on .bashrc

css, the three same differences:

• .class1 .class2: means inside
• .class1.class2: means needs to have both
• .class1, .class2: means has one or the other

css, icons that pull events into the void:

• <i> elements (at least, those that have FontAwesome classes) need to have pointer-events: none; in their CSS in order to allow clicks to pass through to other elements behind them

webpack (2, not sure about other versions), ignore files in watch mode:

• Use watchIgnorePlugin, NEEDS ABSOLUTE PATHS TO WORK (could not get it to work otherwise).

npm, glob package:

• needs special option to return full paths
• needs to have symlinks flagged as such in order to follow them

npm, which logger?:

• debug, winston, log4js (order of popularity 2k18)

npm, input parser:

• commander.js:

const program = require('commander');
program.option('-d, --debug', 'extra debugging'); //etc
program.parse(process.argv);

mount, check if dir is mountpoint:

• findmnt -M, -J flag outputs json for jq

shell/Bash, syntax:

• single brakets need spaces between next characters

shell/Bash, conditional flags:

• -f: exists file
• -z: string with zero length
• -n: string with non-zero length
• -d: exists dir

shell/Bash, negate condition:

• [ ! -f filename ], for instance

shell/Bash, single line while:

• while true; do foo; sleep 2; done;

JavaScript, what the...:

• in browsers, any global variable called name declared in the global scope is INVARIABLY treated as if it is a string (credit to @joaosamouco and @joaopintomfc):

  var name = () => 1;
  console.log(typeof name) // "string"
  

JavaScript, check if value is numeric (SO answer):

• const isNumeric = n => !isNaN(parseFloat(n)) && isFinite(n); (kinda oldschool, but works)

JavaScript, difference between isNaN and Number.isNaN:

• isNaN attempts to convert the value to a number and then checks whether the result equals NaN;

• Number.isNaN checks whether the input is exactly NaN (without conversion);

Number.isNaN(void 0) === false

isNaN(void 0) === true

JavaScript, difference between isFinite and Number.isFinite:

• isFinite attempts to convert the value to a number and then checks whether it is finite;

• Number.isFinite performs the check without conversion;

Jest/Enzyme, wait for async calls/promise resolution before proceeding with test:

• when using Enzyme mount, async calls in or triggered by lifecycle methods (such as componentDidMount) will not be waited for, even if remote calls are stubbed/mocked using jest.fn().mockResolvedValue or the like. (does not mean that they will not be resolved, but they may end up resolving at a later time than expected)

• to make sure that pending promises are executed after calling 'mount', use setImmediate:

test('Foo does a network request on componentDidMount', async () => {
    const component = mount(<Foo />);

    // if you do not use this line, the assertion below may (and probably will) fail
    await new Promise(res => setImmediate(res));

    expect(fetch).toHaveBeenCalled();
})

git, show diff without header and add context:

• git diff --no-prefix -U1000 HEAD^ HEAD

git, reference to nth last commit:

• git diff HEAD~n HEAD

git, pull and init submodules:

• git submodule update --init --recursive

git, log without pager:

• git --no-pager log > log.txt

git, exclude files matching a certain pattern from diff (stackoverflow):

• git diff --cached -- . ':!spec'

git, list url for origin:

• git remote get-url <origin name>

git, caveat regarding --depth

• if --depth is set, pulling from remote does nothing.

grub, recover from command line:

set pager=1
ls
set root=(hd?,?)
linux <stuff> root=/dev/sdX
initrd <stuff>
boot

grub, grub-install:

• --efi-directory option takes the folder that CONTAINS the efi directory, not the efi directory path itself

node.js, path.normalize vs path.resolve (SO answer):

"Ironically, this means that path.resolve() produces a relative path in absolute terms (you could execute it anywhere, and it would produce the same result), whereas path.normalize() produces an absolute path in relative terms (you must execute it in the path relative to which you want to calculate the absolute result)."

• Meaning, path.resolve(__dirname, <relativePath>) results in an absolute filepath, since __dirname is always absolute.

• To get an absolute filepath from resolve, we would then pass it to normalize: path.normalize(path.resolve('home/foo', 'bar')) results in ${process.cwd}/home/foo/bar

npm, express package, default error handler middleware signature:

• (err, req, res, next) => {/*something*/}

npm, express package, set response headers:

• res.set(<header name>, <header value>)

npm, express package, correct CORS headers:

• required in all methods, but when in OPTIONS middleware, return 200 and send immediately instead of calling next.

npm, express package, use middleware on all routes:

• app.use('*', <middleware>);

npm, ajv package, custom validator:

• use ajv.addKeyword(<keyword>, <descriptor>)

• if you define a type in the descriptor that is passed to addKeyword, then the validator only tries to apply that validator to a value of that type (!!!!)

markdown, syntax:

• links: []() (square then round)

• checkboxes: - [ ] Foo bar

• more help: https://www.markdownguide.org/basic-syntax

Postman, generate values:

• uuid: {{$guid()}}
• defining custom keywords:

// in the pre-request script section:
pm.globals.set('randomNumber', Math.floor(Math.random() * 5));

// in the request body:
{{randomNumber}}

http, difference between PUT and PATCH:

I know, I know, I should know this, but I always get which is which wrong...

• PUT is a full-entity replacement of an existing entity. Idempotent.

• PATCH is a partial update of an existing entity. NOT idempotent.

virtualbox, mounting physical drive:

vboxmanage internalcommands createrawvmdk -filename $HOME/win10.vmdk -rawdisk /dev/sdb

sed, delete empty lines:

• sed '/^$/d' (delete mode on sed with d)

ssh, remember ssh key auth for a terminal session:

• eval $(ssh-agent) && ssh-add

ssh, show visual keys representation when connecting with ssh:

• use VisualHostKey yes in /etc/ssh/sshd_config

ssh, local port forwarding:

• ssh <server user>@<server host> -L <address in local host>:<address in remote server> -N

• the -N flag makes it so that a shell is not opened

xrandr, set resolution and refresh rate in the same command:

• xrandr --output <output name> --mode <resolution> --rate <refresh rate>

postgres, local host access

• change postgres config to allow any host access (if accessing from docker)
• add user/db/pwd as necessary
• change pg_hba to allow access as that user to the db