After completing this unit, you should be able to:

• Explain the purpose of a shell
• Describe the relationship between parent and child processes
• Monitor the processes
• Start foreground and background processes
• Explain the concept of signals and use them to terminate processes
• Explain the concept of priorities and manage them
• Use crontab files to schedule jobs on a periodic basis
• Use the at command to schedule a job or series of jobs at some time in the future

• The shell is the command line user interface to the kernel.
  • configruable
  • command interpreter
  • program environment
• Available shells:
  • bash
  • ksh
  • csh

• A program is an executable file.
• A process is a program that is being executed.
• Each process has its own environment.

• The unique identifier of a process is PID.
• To see the PID of your current shell process, type:
  $ echo $$

• All processes are started by other processes.
• This is called a parent/child relationship.
  $ ls –l

• Here, bash is the parent, ls is the child.
• A process can be terminated for two reasons.
  • The process terminates itself when done.
  • The process is terminated by a signal from another process.

$ echo $$  # parent shell's pid
$ bash

### in child shell ###
$ echo $$  # child shell's pid
$ date
$ exit
### return to parent shell ###

$ echo $$  # parent shell's pid

• The ps command displays process status information.

• ps supports a large number of options; you typically use :
  ps aux
  a: All processes attached to a terminal
  x: All other processes
  u: Provides more columns

• Another common usage for ps:
  ps -ef

• Foreground processes

  • Foreground processes are invoked by simply typing a command at the command line.
    $ find / -name README

• Background processes

  • Background processes are invoked by putting an ampersand (&) at the end of the command line.
    $ find / -name README &

• Processes can be controlled in two ways.
  • From the shell that started it, using its job number
  • From anywhere on the system, using its PID
• The following actions can be performed on a running process:
  • Terminate
  • Kill
  • Stop/continue
• These actions are performed by sending signals.

jobs: Lists background or suspended jobs

For foreground:
<ctrl-z>: Suspends foreground task
<ctrl-c>: Terminate this task

For background:
fg: Put the task to the foreground

For suspended:
fg: Resumes suspended task in the foreground
bg: Resumes suspended task in the background

For all:
kill: Terminate or kill the task.

Specify a job number for bg, fg, and kill using %job

• Several signals can be sent to a process.
  • Using keyboard interrupts (if foreground process)
  • Using the kill command
    • Synopsis: kill -signal PID
  • Using the killall command to kill all named apps
    • Synopsis: killall -signal application

The nohup command stops a process from being killed if you log off from the system before it completes by intercepting and ignoring the SIGHUP and SIGINTR (hangup and interrupt) signals and redirecting STDOUT and STDERR to a file.

$ nohup find / -name README &
nohup: appending output to `nohup.out´
$ logout

• Processes are scheduled according to priority.

  • Nice value : -20 ~ 19 , default : 0
  • Bigger value means lower priority.
  • Negative values are allowed only for root!

• The nice command is used to start a process with a user-defined priority.
  nice [-n <value>] <original command>

• The renice command is used to change the priority of a currently running process.
  renice <new_value> <PID>

The word daemon refers to a never-ending process, usually a system process, that controls a system resource, such as the printer queue, or performs a network service.

• Scheduling allows you to run commands at a specific moment in the future.
• The crond daemon performs the scheduling for the crontab files.
• The atd daemon is responsible for execution of jobs submitted by the at and batch command.

• Cron is used for repeating tasks (jobs).
• Jobs are configured by adding them to a crontab file. A crontab entry has the following syntax:
  [minute] [hour] [day] [month] [weekday] [command]
  0 8 * * * Once_a_day
0,30 9 * * * Twice_a_day
0,30 8-18 * * * Twenty_Two_times_a_day
*/5 * * * * Every_five_minutes
12 13 1 * * Once_a_month
49 23 16 9 * Once_a_year
0 15 * * 1 Every_monday
1 1 1 1 1 ??? (caveat!)
• Three usage methods:
  crontab -l : List your crontab file
  crontab -r : Remove your crontab file
  crontab -e : Edit your crontab file using $EDITOR

• At runs a command once in the future

  # at 4am
  ps aux
  ^d (CTRL+D)

  # at -f bshfile 16:00 + 3 days
# echo "mail -s report < rep.txt boss" | at now +2min

• To control at jobs, you can also use atq (or at -l) to list all jobs and atrm (or at -r) to delete a given job.

• All processes are started by a parent process (except for init, which is started by the kernel).
• Every process is identified with a process identifier (PID).
• A special process is the shell, which can interpret user commands.
• Processes can terminate by themselves or upon reception of a signal.
• Signals can be sent by the shell, using a keyboard sequence, or by the kill and killall commands.
• Processes are started with equal priority, but this can be changed using the nice and renice commands.
• A daemon is a background process that typically controls a system resource or offers a network service.
• Use crontab files to schedule jobs on a periodic basis
• Use the at command to schedule a job or series of jobs at some time in the future

• Unit 9: Working with processes, Linux Basics and Installation , ERC 7.2, IBM
• Unit 12: Scheduling , Linux System Administratio, ERC 7.2, IBM