====== Shell Programming ======

sed
  $ sed “s/day/night/” <old >new
  $ sed “s/day/night/; 14 q” <old >new    # chain commands, quit after line 14
  $ sed -E "s/\{s:([0-9]*)[[:space:],]*l:([0-9]*)[[:space:] ,]*n:([0-9]*).*/\1\t\2\t\3/g" <test.js

awk

==== Run program in background ====

  $ tar -czf home.tar.gz . &  # append & to run command in background
or
  $ tar -czf home.tar.gz .
  Ctrl+Z      # stop the job
  $ bg   # then continue the job in the background


  $ jobs # list jobs running in background
  $ fg  # put a background job back into the foreground
  $ disown -h %1  # separate job from shell 
  $ nohup tar -czf home.tar.gz . &  # run in background and separate it from shell
  $ firefox </dev/null &>/dev/null &

  $ kill %2  # kill job number 2
  $ fg %2  # put job 2 in the foreground
  $ bg %2  # put job 2 in the background 

  $ export PS1='$ '    # set prompt
  $ . activate # dot command, run in parent context from within script
  $ source activate # same as dot command

==== Directory Stack ====
  $ pushd $PWD # push current directory onto the stack
  $ cd /home/whatever
  $ popd # return to pushed directory

==== Redirection ====
  $ cat >newfile.txt                 # redirect stdout, replace file
  $ cat >>newfile.txt                # redirect stdout, append to file
  $ cat &newfile.txt                 # redirect stderr, replace file
  $ cat &>newfile.txt                # redirect stdout and stderr, replace file
  $ cat &>>newfile.txt               # redirect stdout and stderr, append to file
  $ cat <myfile.txt                  # redirect stdin
  $ cat <oldfile.txt >newfile.txt    # copy a file

==== Pipe ====
  $ ps -e | grep ping                   # redirect stdout of first command to stdin of second command
  $ ps -e | grep ping | less            # chain pipes

==== chmod ====
  $ chmod +x ~/.local/bin/myscript.sh   # make a script executable

==== chown ====
  $ chown root myfile.txt    # change owner of myfile to root

==== How to stop a program ====
Signaling

Kill

==== How to start a program ====
  $ <command>
  $ bash <command>
  $ source <command>
  $ . <command>
  $ exec <command>
  $ <command> &
  $ nohup <command>
  $ systemd

  $ nice <command>

==== foreground, background, daemon ====

run in background
run as background job
redirect stdout and stderr

  $ disown %1    # suppress SIGHUP and remove from jobs list
  $ disown -h %1 # suppress SIGHUP 

  $ nohup <command> # suppress SIGHUP when starting command

==== glossary ====
  * session
  * state Ss
  * terminal session
  * one foreground process
  * multiple background processes
  * multiple background jobs