self_updating_bash_script.sh

#!/bin/bash

### SELF-UPDATING BASH SCRIPT
### Checks for update online, if not found, runs itself

# CONFIG: SET UPDATE FILE
UPDATE_FILE="update.sh"
UPDATE_URL="http://example.com/${UPDATE_FILE}"

### UPDATING CODE
THIS_FILE=$0
ARGUMENT=$1

function get_update {
  echo "checking for update"
  if [ ! -e $UPDATE_FILE ]; then
    echo "update not found; attempting to download update"
    wget $UPDATE_URL
  fi
  echo "checking again (in case download failed)"
  if [ ! -e $UPDATE_FILE ]; then
    echo "update still not found; copying current file to ${UPDATE_FILE}"
    cp $THIS_FILE $UPDATE_FILE
  fi
}

function execute_update {
  echo "executing updated file"
  chmod +x $UPDATE_FILE
  ./$UPDATE_FILE --run # must invoke with --run
}

function replace_current_file_with_update {
  echo "overwriting current file with update file"
  mv -f $UPDATE_FILE $THIS_FILE
}

function update {
  # if the currently running script is not invoked with --run, 
  # then it is not the updated one
  if [ "${ARGUMENT}" != "--run" ]; then
    get_update
    execute_update
    replace_current_file_with_update
    exit 0 # must exit, or script will recursively call itself for all eternity
  fi
}

update # make the magic happen

### END UP UPDATING CODE: REST OF SCRIPT GOES BELOW THIS LINE

echo "RUNNING ACTUAL CODE NOW"