Some bash examples, including how to start a script

bash.sh

#!/usr/bin/env bash
set -o errexit -o nounset -o pipefail
IFS=$'\n\t\v'
cd `dirname "${BASH_SOURCE[0]:-$0}"`

echo test >/dev/null 2>&1 # these
echo test &>/dev/null     # all
&>/dev/null echo test     # do the same

# echo to stderr
>&2 echo test

#functions
function cleanup() {
  rm -rf junk
}
function fail() {
  >&2 echo $1  # echo the first arg to stderr
  exit ${2:-1} # exit $2 or exit 1
}
cleanup

# variables & default values
X=${1:-"default"}   # X=$1, or "default" if $1 not set
Y=${2:-}            # Y=$2, or nothing (use to bypass nounset)
: ${Z:="default"}   # Z="default" if it was unset before

# https://stackoverflow.com/a/16753536/4415398
# +--------------------+----------------------+-----------------+-----------------+
# |   Expression       |       parameter      |     parameter   |    parameter    |
# |   in script:       |   Set and Not Null   |   Set But Null  |      Unset      |
# +--------------------+----------------------+-----------------+-----------------+
# | ${parameter:-word} | substitute parameter | substitute word | substitute word |
# | ${parameter-word}  | substitute parameter | substitute null | substitute word |
# | ${parameter:=word} | substitute parameter | assign word     | assign word     |
# | ${parameter=word}  | substitute parameter | substitute null | assign word     |
# | ${parameter:?word} | substitute parameter | error, exit     | error, exit     |
# | ${parameter?word}  | substitute parameter | substitute null | error, exit     |
# | ${parameter:+word} | substitute word      | substitute null | substitute null |
# | ${parameter+word}  | substitute word      | substitute word | substitute null |
# +--------------------+----------------------+-----------------+-----------------+

# indirection
A=one
B=A
C=${!B} # C=one

# test for empty variable
# this is equivalent to `test -z "$Y"`
# so you can get help with `man test`
if [[ -z "${Y:-}" ]]; then
  echo "Y not set"
fi

# does a command exist
if ! CMD_PATH=$(command -v commandname); then
  fail "commandname is not installed"
fi

# retrieve exit code from individual pipe sections
# '!' bypasses pipefail
! echo "my command" | grep "whatever"
if [[ ${PIPESTATUS[1]} -ne 0 ]]; then
  echo "grep failed" >&2 # echo to stderr
  >&2 echo "grep failed" # works too
fi

# start & stop a background process
sleep 10s &
PID=$! # PID of last started background process
! ps -p $PID &> /dev/null # this will fail if background process failed to start
if [[ ${PIPESTATUS[0]} -ne 0 ]]; then
  fail "no background process to kill" 1
else
  kill $PID
fi

# ask for input
DEFAULT_Z="Z"
read -p "Type a Z (default ${DEFAULT_Z}): " MY_Z
MY_Z=${MY_Z:-${DEFAULT_Z}}

# math
NUM=$((NUM+1))
NUM=$((${NUM}**2))

# search and replace in one or more files
FILELIST=$'a.sh\tb.sh'
! sed -i -- "s|my search|${MY_Z}|g" ${FILELIST} &>/dev/null # ignore both stdout and stderr

# search for a regex in a file, only print the match, trim newline at the end
cat myfile | grep -Eo 'http://[a-zA-Z0-9-]*.domain.com' | tr -d '\012\015'