Bash Basics

bashisms.md

Manipulating Strings

# safety
set -euo pipefail

# root directory
ROOT_DIR="$(cd $(dirname $0)/.. ; pwd)"

# directory of this script
CURRENT_DIR="$(cd $(dirname $0) ; pwd)"

# null checks

[ -z "$FOO" ] # null
[ -n "$FOO" ] # not null

# Use variable expansion only for side-effects
: "${var:=$1}"

# Literal special characters
$'\t' 
$'\n'
# etc.

# get options http://wiki.bash-hackers.org/howto/getopts_tutorial
# When you want getopts to expect an argument for an option, just 
# place a : (colon) after the proper option flag. If you want -A 
# to expect an argument (i.e. to become -A SOMETHING) just do:
# getopts fA:x VARNAME
while getopts ":e:" opt; do
  case "${opt}" in
    e)
      ENVIRONMENT=${OPTARG}
      [ $ENVIRONMENT == "sandbox" -o $ENVIRONMENT == "production" ] || usage
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2 ; usage
      ;;
  esac
done
shift $((OPTIND-1))

# usage
usage () {
  echo "Usage: $(basename $0) [-e <sandbox|production>] [<script>]" 1>&2
  exit 1
}

#fail
fail () {
  echo "$@"
  exit 1
}

# prompt for y/n
echo
echo -n "Add user to \"admins\" group (y/n)? "
old_stty_cfg=$(stty -g)
stty raw -echo
ANSWER=$( while ! head -c 1 | grep -i '[ny]' ;do true ;done )
stty $old_stty_cfg
echo $ANSWER

# prompt for other input
while [ "$USER_PASSWORD" = "" ]; do
  # -s is "secure", because this is a password
  read -s -p "Enter a password for the user: " USER_PASSWORD
done

# increment a variable
var=$((var+1))
((var=var+1))
((var+=1))
((var++))

# bind keybindings to history-search
# when they're not in your .inputrc
bind "\e[A": history-search-backward            # arrow up
bind "\e[B": history-search-forward             # arrow down

See also Commands For History (Bash Reference Manual)

# display keybindings
bind -p
# They can be queried and removed! [8 Linux Bash Shell Readline Bind Command Examples](https://linux.101hacks.com/unix/bind/)

# Previous commands
# !^      first argument
# !$      last argument
# !*      all arguments
# !:2     second argument

# !:2-3   second to third arguments
# !:2-$   second to last arguments
# !:2*    second to last arguments
# !:2-    second to next to last arguments

# !:0     the command
# !!      repeat the previous line