====== Shell ======
Command interpreter plus programming language.

command, executable, binary, Script

Historical Shell
  * Multics Shell, 1965
  * Thompson shell, written by Key Thompson at Bell Labs, Unix 1-6, 1971-1975 

Popular Shells Today
  * sh - Bourne Shell, 1979, Unix 7, written by Steve Bourne at Bell Labs
  * bash - Bourne Again SHell, GNU, enhanced Bourne Shell
  * ksh - Korn shell, written by David G. Korn at Bell Labs
  * csh - C shell, modelled on C language, written by Bill Joy at Berkeley, BSD
  * tcsh - enhanced C shell
  * zsh - Z shell, backward compatible with bash, default shell in MacOS 10.15
  * fish - Friendly Interactive Shell, 2005

Shell Options
  $ shopt  # list all the current shell options
  cmdhist        on
  histappend        on
  promptvars        on
  huponexit        off

===== Variables =====

There are two kinds of variables.
  * Environment variables - global
  * Shell variables - specific to the current execution of the current shell
 
  $ env  # list environment variables
  $ set  # list shell variables, including script functions


  PATH=/home/john/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
  HOME=/home/john
  PWD=/home/john
  PS1=\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$
  SHELL=/bin/bash

===== Alias =====

  $ alias blabber=echo # define an alias
  $ unalias blabber # remove an alias
  $ alias # list all command aliases
  alias egrep='egrep --color=auto'
  alias fgrep='fgrep --color=auto'
  alias grep='grep --color=auto'
  alias l='ls -CF'
  alias la='ls -A'
  alias ll='ls -alF'
  alias ls='ls --color=auto'

===== Nested Shells =====

On login, the shell specified by environment variable SHELL is executed.  Each execution of a shell program creates a separate context for shell variables.f

  $ # on login, you are at level 1
  $ bash   # nested level 2
  $ sh     # nested level 3
  $ bash   # nested level 4
  $ exit   # nested level 3
  $ exit   # nested level 2
  $ exit   # level 1
  $ exit   # logout

When you run a script, you start a shell, which means the script runs at a nested level.
  $ /bin/bash myscript.sh   # level 2

When you call a script from within a script, you go into deeper nested levels.  Shell variables are set in that nested level and disappear when the script execution ends.

To avoid going into nested levels, run the script with the source or dot command.
  $ . ./myscript.sh
  $ source ./myscript.sh  # source is a synonym for . (dot)

The dot command does not start a shell, but instead reads the script file and executes the commands within the current shell.  Therefore, shell variables are set in the current shell and thus remain after completion of the script.  Notice also, it is not necessary to make the script file executable.


env -i
set 

===== Configuration Files =====
Environment variables can be system or user.

Read at startup.

Read at login.

System variables - set by the system during startup

User variables - set by user scripts during login

===== Signals =====

  * SIGHUP - Hang Up, sent to terminal-associated background processes on logout.
  * SIGINT - ?

===== Background Jobs =====
$ myserver &   # run as child process to the current bash session

Normally this background job is still associated with the terminal, and on logout, the system will send the SIGHUP (hang up) signal to the process which will cause it to terminate.  By normally, we mean when shell option huponexit is set on.

  $ shopt | grep hupon  
  huponexit on

Regardless of the setting of huponexit, you can prevent hangup by starting the job with nohup.

  $ nohup <command>  # no hangup signal; continues after logout

  $ kill -SIGHUP <pid>

  echo $!  # print the PID of the forked process, so you can kill it later


fork()
vfork()
exec() - replace the current process with a new one. Control never returns
clone() - creates a child process or thread, with varying degrees of sharing


Things that can be shared:
        Memory space
        Table of file descriptors
        Table of signal handlers


Every child terminates with an integer exit code.


The shell process is the parent.  Programs started are child of the shell.
bash -> bash -> myserver 


service
daemon