jiahaog / notes

Cheatsheet for all the things

Notes

Shell

Creating RSA Key Pair

ssh-keygen -t rsa

Git

Git undo last commit and keep working changes

git reset --soft HEAD~

Git squash last 2 commits Will undo the last 2 commits and keep the changes staged and ready for another commit

git reset --soft HEAD~2
git commit -m "Some feature"

Rebase local branch with remote master

# while on the local branch

git fetch origin
git rebase origin/master

Replace messed up local branch with remote master

git reset --hard origin/master

Delete branches

# delete local branch
git branch -d <branchName>

# delete remote branch
git push origin --delete <branchName>

Submodules

# git clone with submodules
git clone --recursive <submodule repository>

# update submodule from upstream
git submodule update --remote <submodule>

# To pull in changes from submodules after the root repository remote has been updated
git pull
git submodule update

Sync Files

Use rSync to sync files over ssh

# note the trailing slash at the end of `~/Desktop/` to indicate all contents
rsync -avzP ~/Desktop/ awesome@192.168.1.1:/home/awesome/files

Also can use scp

scp -r user@192.168.1.1:/home/data/  ~/Desktop/

Run a shell command forever

while true; do npm start; done

Bash

Quotations

SOME_VAR="Hello"

echo "${SOME_VAR}" # 'Hello'
echo '${SOME_VAR}' # '${SOME_VAR}'
echo `${SOME_VAR}` # '-bash: Hello: command not found'

Best Practice

• Immediately exits script on error
• Performs clean up on error

#!/bin/sh

# exit the shell script on error immediately
set -e

TEMP_DIR="SOME_TEMP_DIR"

function cleanUp() {
  rm -rf "${TEMP_DIR}"
}

# Set up cleanUp to trigger on EXIT
trap cleanUp EXIT

mkdir -p "${TEMP_DIR}"

# Do things
# Error here will trigger `cleanUp()` and exit the script immediately
echo 'I am awesome'

# Done, remove the cleanup trap
trap - EXIT

# Manual Cleanup
cleanUp

Executing Scripts

source runs the script in the current process, does not require the script to be an executable

$ source some_script
$ . some_script # same as the above

./some_script spawns a new shell and executes the script, according to the shebang line

bash some_script also spawns a new shell and executes the file with bash

Multiple Commands in One Line

A; B    # Run A and then B, regardless of success of A
A && B  # Run B if A succeeded
A || B  # Run B if A failed
A &     # Run A in background.

Source

Redirecting Output

do_stuff > /dev/null 2>&1

Redirects stdout to /dev/null, redirects stderr to stdout

Source

Test TCP Connection

Helpful snippet to test network connections that do not respond to cUrl

(echo > /dev/tcp/$HOST/$PORT) # returns a status code of whether the connection is successful
echo $? # prints the status code of the last command

Find Process Running on Port

lsof -i tcp:8002

OSX Clipboard

Copy stdout to clipboard

$ echo 'Hello!' | pbcopy

Caffeinate

Prevent OSX from sleeping

$ caffeinate

# Or prevent sleep until script has completed
$ caffeinate long_running_script.sh

Simple HTTP Server

Creates a HTTP server for files in the local directory. Either an index.html file (if present) or the other files in the directory wil be served.

$ python -m SimpleHTTPServer 8080

Find Files

Find files by filename or wildcard

$ find ./test -name "*.php"

Download a Static Website

$ wget -r -N http://example.com

Keyboard Shortcuts

Shortcut	Purpose
ctrl + a	Go to the beginning of the line
ctrl + e	Go to the end of the line
ctrl + u	Clear everything before the cursor
ctrl + w	Delete the word before the cursor
ctrl + l	Clear the screen

Miscellaneous Commands

# show hex and ascii values
$ xxd some_file.mp3

# DNS address lookup
$ nslookup google.com

# show local cached IP-MAC address table
$ arp -a

# copy contents of one directory to another
$ cp directory_with_desired_contents/* ~/destination_directory

OSX

Taking screenshots

Shortcut	Purpose
⌘ + ⌃ + ⇧ + 4	Select area and save to clipboard
⌘ + ⇧ + 4	Select area and save to desktop
⌘ + ⌃ + ⇧ + 3	Save screen to clipboard
⌘ + ⇧ + 3	Save screen to desktop

While selecting area, press space to select a window.

Javascript

Factory Pattern for Objects

function createObject(objectParams) {
  const prototype = {
    instanceMethod(params) {
      return params;      
    }
  };

  const instance = Object.create(prototype);

  const instanceProps = {
    data: objectParams.data
  };
  return Object.assign(instance, instanceProps);
}

See also:

• Different ways to create objects

Python

Run a prompt after running a python file

python -i hello.py

Function Arbitrary arguments *args

def foo(*args):

    def bar(*args):
        return args
        
    return bar(*args)

>>> foo(1,2,3) # (1, 2, 3)

MongoDB

MongoDB Authentication

# first login with admin credentials
use admin
db.auth('admin', 'password')

use myDatabase

# create a user local to `myDatabase`
db.createUser({ user: "myuser",
  pwd: "password",
  roles: [
    "readWrite"
  ]
})

Connect to the database using the following:

MONGO_URL=mongodb://${DATABASE_USER}:${DATABASE_PASSWORD}@${MONGO_IP}/${DATABASE_NAME}

Docker

Docker Compose

Rebuild and recreate all containers

docker-compose up -d --force-recreate --build

Connecting to host (not VM) inside the docker container

On host (Mac):

ipconfig getifaddr en0

Performance

Benchmark a command

perf stat -r 10 $command