Shell

• Using a default value for a variable

  VAL=${<var>:-"<default>"}
  

  • The above sets default if var is null or unset. Use only : for skipping null check.

• Reuse previous shell command

  $ !!
  

  Example: check git root and cd if proper

  $ git rev-parse --show-toplevel
  /git/root
  $ cd $(!!)
  

• Reinstall xcode-select @MACOS

  $ sudo rm -rf $(xcode-select --print-path) && xcode-select --install
  

• Mount external ntfs hdd @MACOS

  1. brew cask install osxfuse
  2. Reboot
  3. brew install ntfs-3g
  4. Connect hdd
  5. mkdir ~/NTFS
  6. diskutil list (check MS drive num eg disk2s1)
  7. sudo umount /dev/disk2s1
  8. sudo /usr/local/bin/ntfs-3g /dev/disk2s1 ~/NTFS -olocal -oallow_other

• Mount an attached block storage (pre-formatted) to instance using fstab

  • Check currently attached volumes lsblk (ensure attached but not mounted)

  • Get UUID of volume blkid

  • Add entry in /etc/fstab UUID=<from above> /mount/point format options

  • Mount all fstab volumes sudo mount -a

  • Check currently attached volumes lsblk (ensure mounted)

  • IMPORTANT! If attaching new block storage, may need to format first

• Check size of a directory

  $ du -sh <dir>
  

  where,

  • s = summary
  • h = human readable format

  Example: Get size of all directories in current directory

  $ du -sh ./*/
  

• Move new files only (do not overwrite if exists)

  $ mv -vn <source> <target>
  

  where,

  • v = verbose
  • n = no overwrite

• if/else

  $ if [ <test> ]; then <something>; fi
  

  for command sucess | fail, simply do this (no sq brackets)

  $ if command; then <sucess>; else <fail>; fi
  

  • Check if an environment variable exists

    if [[ -z $ENV_VAR ]]; then echo "Not exists"; fi;
    

• for loop

  for ((expr1;expr2;expr3)); do <command>; done;
  

  Example: Loop from 1 to END

  END=5
  for ((i=1;i<=END;i++)); do echo $i; done;
  

• for in loop

  for <variable> in <list>; do <command>; done
  

  Example: Add all dot files to gitignore

  for file in .*; do echo $file >> .gitignore; done
  

• Arrays

  An array in shell is within () brackets and looped like so:

  arr=("a" "b" "c")
  
  for alph in ${arr[*]}; do echo $alph; done;
  

  Note that on cli, direct $arr works for looping but not in script!

• Script strict mode

  Ignore the \ in # below, kept for formatting reasons

  \#!/usr/bin/env bash
  set -euo pipefail
  IFS=$'\n\t'
  

  http://redsymbol.net/articles/unofficial-bash-strict-mode/

• Find command

  find <path> [-type | -size] -name "regex" [-exec <another cmd> | -delete]
  

  Example: find and delete all dot files in current directory

  find . -name ".*" -delete
  

  as an alternative, fd works as follows

  fd -t [d|f] [-HIL] "regex"
  

• Remove duplicate lines from a file

  awk '!l[$0]++' file > newfile
  

  if order is not important

  sort -u file > newfile
  

• Start a background job

  nohup <job> < /dev/null > /dev/null 2>&1 & disown
  

  where,

  • nohup = no hangup signal
  • < /dev/null = no stdin (input)
  • > /dev/null = no nohup file (output)
  • 2>&1 = redirect errors to stdout (which is null)
  • & = start in background
  • disown = detach from shell

  Example: To start mpv as music daemon

  nohup mpv --no-audio-display --shuffle ~/Music < /dev/null > /dev/null 2>&1 & disown
  

• Mount Ram as disk

  sudo mount -t tmpfs -o size=5g tmpfs /mnt/ramfs
  

  where a new temporary mount point /mnt/ramfs is created with 5 Gigabytes of capacity; to unmount, do

  umount /mnt/ramfs
  

• Watch a file for change and run command when changes

  while true; do watch -g ls -l <filename> && <cmd>; sleep 5; done
  

• Download a webpage and all it's linked pages and content

  wget -r -l 1 -p -k -H -D domain.com,relateddomain.com http://domain.com/page/in/domain
  

  where,

  • r = recurse
  • k = patch local links
  • H = traverse domains other than original
  • D = limit domains traversed
  • l = depth of recursion
  • p = download related content like images

• Generate a random base64 string (May be used as token or password)

  head -n1 /dev/urandom | base64
  

  • number after n changes length of string

• Lazy regex quatifier (works in js, not is sed, vim)

  Suppose there is a line like so:

  "this is some string" "this is another string"

  And the aims is to select everything within quotes as a string.

  A naive regex would be: /".*"/

  But it actually selects everything inside starting from first " to the end of line "

  To select only the content withing first closing quote, do : /.*?/

  That is, add a ? aka a lazy quatifier (as opposed to normal greedy approach)

• Add colour to console output (make sure to add Reset at the end after adding colour)

  \033[0m : Reset \033[1m : Bold \033[91m : Red \033[92m : Green \033[93m : Blue

  • JS Function

  const colorize = (...args) => ({
    black: `\x1b[30m${args.join(" ")}`,
    red: `\x1b[31m${args.join(" ")}`,
    green: `\x1b[32m${args.join(" ")}`,
    yellow: `\x1b[33m${args.join(" ")}`,
    blue: `\x1b[34m${args.join(" ")}`,
    magenta: `\x1b[35m${args.join(" ")}`,
    cyan: `\x1b[36m${args.join(" ")}`,
    white: `\x1b[37m${args.join(" ")}`,
    bgBlack: `\x1b[40m${args.join(" ")}\x1b[0m`,
    bgRed: `\x1b[41m${args.join(" ")}\x1b[0m`,
    bgGreen: `\x1b[42m${args.join(" ")}\x1b[0m`,
    bgYellow: `\x1b[43m${args.join(" ")}\x1b[0m`,
    bgBlue: `\x1b[44m${args.join(" ")}\x1b[0m`,
    bgMagenta: `\x1b[45m${args.join(" ")}\x1b[0m`,
    bgCyan: `\x1b[46m${args.join(" ")}\x1b[0m`,
    bgWhite: `\x1b[47m${args.join(" ")}\x1b[0m`,
  });
  

• Process SIGnals (man signal)

  • The INT (interrupt) signal is the weakest. It is the signal number 2. Its meaning is "stop what you're doing right now and wait for further user input". It's the signal generated by Ctrl+C in a terminal

  • The TERM (terminate) signal is the normal kill signal (i.e. the one sent when you just call kill). It is signal number 15. It tells the application to exit cleanly. This signal allows for a trap handler, which enables the receiving process to do some clean up in an orderly fashion. An application that doesn't want to be interrupted during a critical operation might ignore SIGTERM for a while.

  • The HUP signal (hang up) is about the same as SIGTERM in terms of harshness, but it has a specific role because it's automatically sent to applications running in a terminal when the user disconnects from that terminal. It is signal number 1.

  • The QUIT signal is the harshest of the ignorable signals. It's meant to be used when an application is misbehaving and needs to be killed now, and by default it traditionally left a core dump file. It is signal number 3.

  • The KILL signal does not have ANY possibility of the being resisted. It always works because a KILL signal cannot be handled. This is both its blessing and its curse. It is signal number 9.

curl

• Send an email using gmail smtp

  curl --ssl-reqd \
    --url smtp://smtp.gmail.com \
    --mail-from [email protected] \
    --mail-rcpt [email protected] \
    --user "[email protected]:apppassword" \
    --upload-file mail.txt
  

  where mail.txt:

  From: Sender Name <[email protected]>
  To: Recipient Name <[email protected]>
  Subject: an example.com example email
  Date: Mon, 5 Nov 1994 12:10:00
  
  Dear Recipient,
  This is an example email.
  

ffmpeg

• Convert a mov to mp4 (with compression):

  ffmpeg -i video.mov -vcodec h264 -acodec mp2 video.mp4
  ```
  
  

• Trim media from/to

  ffmpeg -ss <start_seconds> -i "input.mp3" -t <end_second> "output.mp3"
  

vlc [cli mode]

• Start vlc with ncurses interface

    vlc --intf ncurses --random --loop --audio --recursive --no-video ~/Music/
  

  • recursive is so subdirs in playlist expand instead of just sitting there; gui is preferences > all > playlist > subdir behav > expand

youtube-dl

• Download songs from youtube using youtube-dl

  Only audio:

  youtube-dl --ignore-errors --format bestaudio --extract-audio --audio-format mp3 --output '%(title)s.%(ext)s' [--yes-playlist] <"URL" | --batch-file  ./fileName>
  

  where fileName is a list of urls separated by blank lines;

  Best video+audio:

  youtube-dl --ignore-errors --format 'bestvideo[ext=mp4]+bestaudio[ext=m4a]/bestvideo+bestaudio' --merge-output-format mp4  -o '%(title)s.%(ext)s' <"video-link">
  

  Note the brackets for the above are part of the command.

gpg

• Encrypt file for self

  gpg --encrypt ["name" | --recipient <[email protected]>] <file>
  

  This will create an encryped <file>.gpg file in same directory. Opening it normally will show gibberish.

• Decrypt a file

  gpg --decrypt <file>.gpg
  

  This will ask for your gpg passphrase and output the original <file>.

• Export public key to server (so can encrypt files there)

  gpg --armor --export <[email protected]> > pub.key
  

  • Copy pub.key to server (via scp or copy-paste), then

  gpg --import pub.key
  

beets

• Seek help

  $ beet help [command]
  

• For all tracks in albumFolder, import metadata, fix filenames and copy to Dir:

  $ beet -d ~/Dir import [-S <url>] [-s <file> | -g <folder>]
  

  where
  -S - Source URL (musicbrainz) -s - Import as single track (Artist -> Track) -g - Import as album (Album -> Track)

Android emulator cli

• Source

• Emulator bin path (macos): $ANDROID_HOME/emulator/emulator

• List available devices: emulator -list-avds

• Start an emulator: emulator @<device_name>

• Some useful options: emulator @<device> -no-boot-anim -memory <size> -logcat <regex>

SSHFS (macos setup)

$ brew install --cask macfuse
$ brew tap gromgit/fuse && brew install gromgit/fuse/sshfs-mac
$ sshfs -V # test if installed
$ sshfs -o allow_other,default_permissions <server-dir> <local-dir> # follow prompts for permission

On linux, sshfs should be preinstalled, if not just use default package manager