Daemons in Bash

preso.sh

#!/bin/bash

SLEEP=0.005
COLS=$(tput cols)
ROWS=$(tput lines)

        RED="\033[0;31m"
     YELLOW="\033[0;33m"
      GREEN="\033[0;32m"
       BLUE="\033[0;34m"
       CYAN="\033[1;36m"
    MY_BLUE="\033[1;34m"
  LIGHT_RED="\033[1;31m"
LIGHT_GREEN="\033[1;32m"
      WHITE="\033[1;37m"
 LIGHT_GRAY="\033[0;36m"
     MY_RED="\033[0;31m"
 COLOR_NONE="\e[0m"


function center_one() {
  local foo="$1"
  for (( i=0; i<$((($COLS - ${#foo}) / 2)); i++)); do
    printf " "
  done
  write_one "$foo"
}

function write_one() {
  local foo="$1"
  for (( i=0; i<${#foo}; i++ )); do

    if [ "${foo:$i:1}" == "%" ]; then
      printf "%%"
    else
      printf "${foo:$i:1}"
    fi
    if [ "${foo:$i:1}" != " " ]; then
      sleep $SLEEP
    fi
  done
  echo
}

function center() {
  echo "$1" |while read l; do
    center_one "$l"
  done
}

function write() {
  echo "$1" |while read l; do
    write_one "$l"
  done
}

function wait_key_press() {
  read -n1 -s x
  if [ "$x" == "p" -o "$x" == "P" -o "$x" == "[" -o "$x" == "{" -o "$x" == "," -o "$x" == "<" ]; then
    echo "P"
  else
    echo "N"
  fi
}

function finish_page() {
  local cur_row=$1
  while [ "$cur_row" -lt "$ROWS" ]; do
     echo
     sleep $SLEEP
     cur_row=$(($cur_row + 1))
  done
}

function breaks() {
  local cur_row=0
  while [ "$cur_row" -lt "$1" ]; do
     echo
     sleep $SLEEP
     cur_row=$(($cur_row + 1))
  done
}

function page_1() {
  write_one
  center "D A E M O N S  I N  B A S H"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 6
  center "Tomasz Janowski"
  finish_page 11
  n=$(wait_key_press)
}

function page_1a {
  write_one
  center "BASH"
  center "‾‾‾‾"
  breaks 5
  write_one "    Bash is a command processor that typically runs in a text"
  write_one "    window, where the user types commands that cause actions."
  write_one
  write_one "    Bash can also read commands from a file, called a script."
  write_one "    Like all Unix shells, it supports filename wildcarding,"
  write_one "    piping, here documents, command substitution, variables"
  write_one "    and control structures for condition-testing and iteration."
  breaks 3
  printf "${BLUE}"
  center_one "http://en.wikipedia.org/wiki/Bash_(Unix_shell)"
  printf "$COLOR_NONE"
  finish_page 20
  n=$(wait_key_press)

}

function page_2() {
  write_one
  center "PRESENTATIONS IN BASH"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 6
  write_one "  ★  Presentation as a code"
  write_one "  ★  High contrast"
  printf "  ★  ${RED}C${CYAN}O${YELLOW}L${BLUE}O${GREEN}R${MY_RED}S ${YELLOW}! ${COLOR_NONE}"
  write_one " ---- but they're hard ◕︵◕"
  write_one "  ★  No transitions"
  write_one "  ★  Unicode emoji \( ﾟ◡ﾟ)/"
  write_one "  ★  Can be dockerized"
  finish_page 16
  n=$(wait_key_press)
}


function page_3() {
  write_one
  center "BACK TO DAEMONS"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 5
  write_one "    In multitasking computer operating systems,"
  write_one "    a daemon (/ˈdiːmən/ or /ˈdeɪmən/) is a computer"
  write_one "    program that runs as a background process,"
  write_one "    rather than being under the direct control of"
  write_one "    an interactive user."
  breaks 3
  printf "${BLUE}"
  center_one "http://en.wikipedia.org/wiki/Daemon_(computing)"
  printf "$COLOR_NONE"
  finish_page 18
  n=$(wait_key_press)
}

function page_4() {
  write_one
  center "BACK TO DAEMONS"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 3
  write_one "    Daemon lifecycle"
  write_one ""
  write_one "    PARENT PROCESS"
  write_one "    |"
  write_one "    (dissociate from TTY)"
  write_one "    |"
  write_one "    |"
  write_one "    +---- FORK (continues running in detached mode)"
  write_one "    |"
  write_one "    |"
  write_one "    (parent exits)"
  breaks 1
  finish_page 19
  n=$(wait_key_press)
}

function page_4a() {
  write_one
  center "WHY BASH? (yಠ,ಠ)y"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 3
  write_one "  ★  We had a problem to solve: Make the most recent locations data"
  write_one "     always available on the filesystem for apps to consume"
  write_one "        1. Check if a new locations data package is available"
  write_one "        2. Download and unpack the new package"
  write_one "        3. Repeat"
  write_one
  write_one "  ★  We have to provide the locations data to a huge number of REA apps"
  write_one "     which are deployed to many different environments"

  finish_page 15
  n=$(wait_key_press)
}
function page_4b() {
  write_one
  center "WHY BASH? (yಠ,ಠ)y"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 3

  write_one "  ★  Minimum set of dependencies (Only standard Linux packages)"
  write_one "  ★  Rock solid technology (Bash + Curl + Tar + Openssl)"
  write_one "  ★  No compilation, language runtime, package managers, libraries"
  write_one "  ★  Will run everywhere: Cloud, Data Centre (even on Lenny), Docker"
  write_one
  write_one "  ★  Not recommended for anything more complex"

  finish_page 13
  n=$(wait_key_press)
}

function page_5() {
  write_one
  center "DETACH IN BASH"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 3
  write_one "$ cat run-detach.sh"
  write_one ""
  write_one "nohup \\              # detach from TTY"
  write_one "  run.sh \\           # the main script (daemon)"
  write_one "  0<&- \\             # close STDIN"
  write_one "  2>err.log \\        # redirect STDERR"
  write_one "  >app.log \\         # redirect STDOUT"
  write_one "  &                  # run in a subshell (fork)"
  write_one
  write_one "echo \$! > app.pid    # store the PID of the main script"
  write_one
  write_one "exit 0               # the parent script exits here"
  finish_page 19
  n=$(wait_key_press)
}

function page_6() {
  write_one
  center "KILLING THE DAEMON"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 3
  write_one "    # /etc/init.d/my_daemon stop"
  write_one "    sends SIGTERM signal to our main script"
  write_one
  write_one "    it's the same as killing the process manually:"
  write_one "    \$ ps ax"
  write_one "    3 s007  S      0:04.13 ./run.sh"
  write_one
  write_one "    \$ kill 3"
  write_one
  write_one "    Will our script cleanly exit?"
  finish_page 17

  n=$(wait_key_press)

}

function page_7() {
  write_one
  center "KILLING THE DAEMON"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 2
  write_one "    \$ cat run.sh"
  write_one "    #!/bin/bash"
  write_one "    while true; do"
  write_one "      echo still alive"
  write_one "      long_running_task"
  write_one "    done"
  write_one
  write_one "    \$ /etc/init.d/my_daemon stop"
  write_one
  write_one "    \$ ps ax"
  write_one "    3 s007  S      0:04.13 long_running_task"
  write_one
  write_one "    ★  The TERM signal is only sent to the main script"
  write_one "    ★  All child processes will continue running"
  finish_page 20

  n=$(wait_key_press)
}

function page_8() {
  write_one
  center "KILLING THE DAEMON AND ALL CHILD PROCESSES"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 2
  write_one "    \$ cat run.sh"
  write_one "    #!/bin/bash"
  write_one "    trap shutdown SIGTERM"
  write_one
  write_one "    function shutdown() {"
  write_one "      pgid=\$(ps -o pgid= \$\$ | grep -o [0-9]*) # Get our process group id"
  write_one "      setsid kill -- -\$pgid  # Kill all processes from the process group"
  write_one "                             # \`setdid\` ensures that the kill will be"
  write_one "                             # executed by the process group leaders"
  write_one "      exit 0"
  write_one "    }"
  write_one
  write_one "    ★  All Bash child processes belong to the same process group"
  write_one "    ★  It is possible to kill all processes with a given process group id"
  finish_page 22

  n=$(wait_key_press)
}

function page_9() {
  write_one
  center "SENDING SIGINT TO THE DAEMON"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 3
  write_one "    $ cat run.sh"
  write_one "    #!/bin/bash"
  write_one "    while true; do"
  write_one "      echo still alive"
  write_one "      sleep 3600"
  write_one "    done"
  write_one
  write_one "    $ kill -INT <pid>    # it's like Ctrl+C"
  write_one
  write_one "    ★  Bash doesn't process SIGINT during sleep"
  write_one "    ★  the daemon will still be running for about 1 hour"
  finish_page 18

  n=$(wait_key_press)
}

function page_10() {
  write_one
  center "SENDING SIGINT TO THE DAEMON"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 3
  write_one "    $ cat run.sh"
  write_one "    #!/bin/bash"
  write_one
  write_one "    while true; do"
  write_one "      echo still alive"
  write_one "      sleep 3600 &     # execute sleep in a subprocess"
  write_one "      wait \$!          # wait for the subprocess to finish"
  write_one "    done"
  finish_page 15

  n=$(wait_key_press)

}

function page_11() {
  write_one
  center "LOGGING FROM THE DAEMON"
  center "‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾"
  breaks 1
  write_one "    $ cat run.sh"
  write_one "    #!/bin/bash"
  write_one
  write_one "    function log() {"
  write_one '      echo -e "[$(date +"%d/%b/%Y:%H:%M:%S %z")]\t$(hostname -f)\t$*"'
  write_one "    }"

  write_one '    log "hello"'
  write_one
  write_one "    $ cat application.log"
  write_one '    [05/May/2015:00:09:54 +1000] VIC009235M.lan  hello'
  write_one
  write_one
  write_one "    ★  Use standard format (i.e. Apache)"
  write_one "    ★  Include timestamp (UTC or with TZ info)"
  finish_page 19

  n=$(wait_key_press)

}

function page_last() {
  write ""
  breaks 8
  center '¯\\_(ツ)_/¯'
  breaks 4
  center 'THE END'
  breaks 3
  finish_page 19
  n=$(wait_key_press)
}

page_1
page_2
page_1a
page_3
page_4
page_4a
page_4b
page_5
page_6
page_7
page_8
page_9
page_10
page_11
page_last