Processes, Shell Scripts, Variables, and Command Line Substitution

1
Chapter 9

• Processes, Shell Scripts, Variables, and Command
  Line Substitution

2
Overview

• Will discuss the role of the shell and its
  responsibilities
• The purpose and function of a process will be
  explained

3
Overview

• Will learn commands to look at and interpret, to
  control, and to terminate processes
• Will learn how to run commands in the foreground
  and in the background
• Will learn how to write and execute a shell
  script

4
Overview

• Will learn how to create and delete variables
  and make variables available to child shells
• The effective use of quoting when entering
  commands will be discussed and then will use
  quoting with variables

5
Overview

• Will learn the purpose and function of the type,
  help, dirs, popd, and pushd commands
• The function of command line expansion and the
  order in which the shell processes command line
  expansions will be explained

6
The Shell

• Can issue commands from shell prompt or by
  starting GUI and using startx command

7
The ShellFigure 9.1 The Linux System p. 481
8
The Shell

• Kernel
• Responsible for resource allocation and low
  level hardware interfaces
• Loads first/remains in main memory till computer
  is turned off
• Services requested of other parts of OS or by
  application programs known as system calls

9
The Shell

• Kernel
• Kernel includes
• Interrupt Handler
• Scheduler
• Supervisor
• Manager
• Utilities
• Commands that reside on disk
• Brought into memory when needed

10
The Role of the Shell

• Shell Responsibilities
• Execution of Program
• Internal and External (forking) commands
• Ambiguous file references and variable
  substitution
• Redirection
• Pipes
• Controlling the environment
• Interpreted Programming Language

11
Process Structure

• Process
• An executing program
• OS uses PID number to track every process
• Structure is hierarchical
• init (root)
• Parent to all processes
• Created when initialize system
• All other processes created by fork and exec
  commands
• Referred to as system routines or system calls

12
Process StructureFigure 9.2 fork and exec p. 483
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Process Structure

• Process
• Structure is hierarchical
• init (root)
• getty
• login
• Chosen shell loaded
• Presented with command prompt
• Processes commands
• Log-off
• init starts a new getty

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Process StructureFigure 9.3 The Login Cycle p.
484
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Processes

• A multitasking operating system can run
  processes concurrently or in parallel
• Issuing a command starts a new process
• Consists of program code and private data
• May have other associated resources
• OS must create environment for program to run in

16
Processes

• Processes can assume different states R, S, T,
  and Z
• Launch shell when log in successfully
• Shell can spawn (create) other processes

17
ProcessesFigure 9.4 Shell and Command Execution
p. 485
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Processes

• To see what processes are running use
• psreports on active processes
• pstreeSee what processes are running and what
  processes are child process
• uptimereport current time, amt.. of time logged
  in, of users logged in and system load averages
• topDynamic (combines output of several
  commandsused to sort processes by percentage of
  processor time they use)

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ProcessesTable 9.1 ps Options p. 486
20
ProcessesTable 9.1 ps Options p.486
21
ProcessesTable 9.2 ps top Command Line Options
p. 487
22
ProcessesTable 9.3 Interactive top Options p.
487
23
ProcessesTable 9.3 Interactive top Options p.
487
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Activity Looking at Processes

• KEY CONCEPTS
• Function of ps command
• Kernel manages resource systemall else that
  runs on system is a process
• Shell creates new process each time command is
  entered by forking
• Results of using hyphens before an option
• Purpose and function of parsing
• How wait state (system call) works

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Activity Looking at Processes

• KEY CONCEPTS
• Purpose/function of init
• How pstree command works
• Used uptime to see what load is on system
• Function of load average
• Used M command to sort display by memory usage

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Activity Looking at Processes Table 9.4 ps
Descriptions pp. 489-490
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Activity Looking at Processes Figure 9.5 System
Calls p. 491
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kill and killall commands

• To cause process to end or terminate
• Key in ltCtrlgt C
• All processes sharing terminal are affected
• Use kill command
• Sends signal to terminate a process
• If no signal specified TERM signal is sent
• Process can ignore signal or begin orderly shut
  down process

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kill and killall commands

• To cause process to end or terminate
• Use kill command
• Root user can kill any process
• User can kill processes that user id owns
• Syntax kill option pid
• Use killall command
• Kills a process by name rather than by process
  id number

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kill and killall commandsTable 9.5 kill Options
p. 493
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kill and killall commandsTable 9.6 killall
Options p. 494
32
Activity Using kill and killall Commands with
Processes

• KEY CONCEPTS
• When process killed SIGTERM (15) sent
• The find command used for seeking files
• The ps command displays only processes in
  current terminal window
• The killall command can be used with process name

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The fg, bg, and jobs commands

• Command run in foreground must be completed
  before shell returns to prompt
• Command running in background can continue
  running while other commands are run

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The fg, bg, and jobs commands

• Can place command in background
• by appending (ampersand) to end of command
• that is running in foreground
• Suspend currently running job to regain control
  of terminal (ltCtrlgt Z)
• then use bg command

35
The fg, bg, and jobs commands

• Command placed in background is assigned, by
  the shell, a job number
• Job number is different than a process number
• A job is a command line that was executed

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The fg, bg, and jobs commands

• Can bring background jobs to foreground
• If one background job running use fg
• More than one background running
• fg (most recent)
• fg - (next most recent)
• Another command
• fg command name
• fg job number

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Activity Using the fg, bg, and jobs commands

• KEY CONCEPTS
• The tty command used to print and change
  terminal line settings
• Z displayedpressing ltCtrlgt Z will suspend
  job
• Learned how to bring job in background to
  foreground
• Determined which was the most recent job and the
  last job

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Activity Using the fg, bg, and jobs commands

• KEY CONCEPTS
• Cannot use interrupt key to abort a process
  running in background
• Use kill command to end these processes
• In X Window System environment in GNOME, desktop
  can open multiple terminal windows and run a long
  job in one window and work in another window

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Shell Scripts

• Shell script
• File containing commands that can be executed by
  the shell
• Any command run at command line can be placed in
  a shell script

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Shell Scripts

• Executing a shell script
• Give name on command line
• If shell script created in an editoruse chmod
  to make file executable

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Activity Writing Shell Scripts

• KEY CONCEPTS
• Saw how shell scripts work and placed different
  scripts in background foreground
• Shells first jobdetermine which program to
  execute, then locate program on disk to execute
  it
• Anything that is hidden is preceded by a .
  (period)
• Made shell scripts executable
• When commands are place in background do not
  need to separate commands with a (semi-colon)

42
Activity Writing Shell Scripts

• KEY CONCEPTS
• The following job number - current job
• The - following job number - previous job
• Can group commands with parentheses
• Shell creates copy of itself for each group
• Treats each group as a job
• Creates a new process to execute each of the
  commands
• Can execute a program without it being in search
  path
• Preface command with a period followed by a
  forward slash, then the command

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Command Types

• Order in which commands are executed
• aliases
• keywords
• functions
• built-in commands
• executable programs

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Command Types

• Shell scripts and executable programs
• Stored on disk
• Shell locates them via search path to execute
  them

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Command Types

• Use type command to see what kind of command you
  are using
• Use help command to see purpose of a built-in
  command
• Use alias command to create shortcuts to
  commonly used commands

46
Activity Using alias/type/help

• KEY CONCEPTS
• Can create own aliases
• To capture entire command use quotation marks
• Press ltCtrlgt C to return to command prompt
• Use unalias command followed by alias to remove
  it from list
• Used type with a command
• To retrieve commands quickly use
• Hashing
• Type command with -path option
• Used a man page for a built-in command
• Used help with command to get quick description

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Activity Using alias/type/help

• KEY CONCEPTS
• Use man with command for full tutorial of
  command
• Conditional statements control flow of program
• Decision making based on whether expression
  true/false
• Exit status also called condition or return code
• Shell stores exit status of last command in ?
  variable
• Zero status - command successful
• Not zero statusfalse valuecommand failed
• Aliases kept in .bashrc file from one terminal
  session to another

48
Manipulating the Directory Stack with Built-in
Commands

• Stack
• A list of directories that are used often and
  stored by the user for convenience

49
Manipulating the Directory Stack with Built-in
Commands

• To manipulate a stack use
• pushd command
• Syntax pushd dir N -N
• popd command
• Syntax popd N -N
• dirs command
• Syntax dirs clpv N -N

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Manipulating the Directory Stack with Built-in
CommandsTable 9.7 pushd Options p. 512
51
Manipulating the Directory Stack with Built-in
CommandsTable 9.8 popd Options p. 512
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Manipulating the Directory Stack with Built-in
CommandsTable 9.9 dirs Options p. 512
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Activity Using the Built-in Commands dirs,
pushd, and popd

• KEY CONCEPTS
• Used pushd command with
• directory name
• no options
• n and -n options
• negative numbers
• Used dirs with -l and with -v options
• Used popd with no options and option

54
Variables and the set Command

• Variables
• Contain information used for customizing
• Information required by other processes
• Can contain mix of letters, numbers, or
  underscores
• Never begins with a number

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Variables and the set Command

• Two types of variables
• Local
• Private to shell they are created in
• Namesusually lower case letters
• Environmental
• Passed from parent to child process
• Namesusually upper case letters

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Variables and the set Command

• Shell has number of predefined options that can
  be enabled or disabled

57
Variables and the set CommandTable 9.10 Common
Shell Modes p. 517
58
Activity Using set Command

• KEY CONCEPTS
• noclobber
• Set to offcan overwrite existing file with
  redirection
• Set to oncannot overwrite a file
• Use cp to overwrite file even with noclobber on
• Use man bash to see defined list of commonly
  used options
• When shell (terminal window) exited new shell
  openedoptions return to default value

59
User-created Variables

• Rules for creating variables
• Can be named anything except first character
  cannot be a number
• Assigned values must not precede or follow equal
  sign with blank space or tab mark
• Case sensitive
• Settings remain only for that work session

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User-created Variables

• echo Command
• Can see value of set variable using echo with
  and name of variable
• Syntax variablenamevalue

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Activity Creating/Removing User-created Variables

• KEY CONCEPTS
• Echo command copies argument supplied to
  standard output
• To see value of requested variable name key in
  preceding command
• Use double quotation marks
• Turn off any special meanings of other
  characters
• To preserve any spacing considerations

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Activity Creating/Removing User-created Variables

• KEY CONCEPTS
• Escape sequence used to control display devices
• Using variable as an argument with a command -
  shell replaces name of variable with value of
  variable and passes it to executing program
• Shell parses a command line in a specific order
• Variables created remain until terminal window
  exited
• May remove value of variables at any time during
  a work session

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Activity Creating/Removing User-created
VariablesTable 9.11 Escape Sequences pp. 520-521
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export and declare

• export and declare are built-in commands used to
  create environmental environments

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export and declare

• Environmental variables available
• In shell in which they were created
• Any subshell/process spawned from parent shell
• Environmental variables can be passed
• To any child process started from shell where
  environmental variables created
• To any other shells spawned from that child

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export and declare

• Syntax for export is export variablevalue
• Syntax for declare is
• declare -x variablenamevalue
• Syntax to display all variable names exported in
  shell is export or export -p

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Activity Using export and declare

• KEY CONCEPTS
• To export values use declare -x
• To display current running pid use echo
• Spawned shell by starting second Bash shell
• To pass environmental variable to a child
  process must export or declare it
• Can change value of environmental variable
• Can pass value to a child shell but not to a
  parent

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Command Line Expansion and Quoting

• Command line expansion is what the shell does to
  command line before passing it onto program that
  is being called

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Command Line Expansion and Quoting

• Before executing a command shell
• Parses command into separate parts and
  interprets each part
• Looks for special characters and patterns within
  command line

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Command Line Expansion and Quoting

• Three types of quote characters that the shell
  recognizes and interprets
• Single quote
• Double quote
• Back quote
• Backslash character is the same as
• putting quotes around character

71
The Single Quote, the Double Quote, and the
Backlash

• Whenever shell sees a blank it interprets the
  next item in command line as an option or an
  argument

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The Single Quote, the Double Quote, and the
Backlash

• Single quotes and back quotes stop all types of
  expansions
• Double quotes tells shell to ignore most special
  characters in the quoted statement
• Does not ignore dollar sign, backslashes, back
  quotes

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The Single Quote, the Double Quote, and the
Backlash

• Backslash tells shell not to interpret what
  follows the backslash

74
Activity Single Quotes, Double Quotes, and
Backslash

• KEY CONCEPTS
• Results of pressing ltSpacebargt five times
• Enclose name in quote marks tells shell to treat
  name as one argument
• Created variable to see how quote marks work
• For shell to interpret and treat it as a
  character, use double quotation marks
• Learned two methods of keying in contractions so
  that will be read as one word
• When backslash is last character on a line it
  allows you to continue a line

75
More on Command Line Expansion

• Order in which shell processes a command
• Parses command
• Command broken into separate words
• Examines each word for special characters that
  tells shell to take certain actions
• Performs any command line expansions
• Executes command

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More on Command Line Expansion

• Order shell processes command line expansion
• brace expansion
• tilde expansion
• parameter expansion
• variable substitution
• command substitution
• arithmetic expansion
• word splitting
• pathname expansion

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Activity More on Command Line Expansion

• KEY CONCEPTS
• Curly braces
• means of expansion
• characters that precede curly braces called
  preambles
• that follow curly braces are called postambles
• cd changed directories to last working
  directory
• Can test and modify variables when special
  modifiers are used
• Modifier can check to see if variable is set and
  then assigns a value to variable

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Activity More on Command Line Expansion

• KEY CONCEPTS
• Use command substitution to use standard output
  of command in shell script as well as on command
  line
• All shells use back quotes to perform command
  substitutions
• Remember to use curly braces
• Back quote usually lowercase position on key
• Can perform arithmetic expansions and other
  various mathematical calculation
• Word splittinginterpretation of command line

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Activity More on Command Line ExpansionTable
9.12 Special Modifiers pp. 531-532
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The Initialization Files and Sourcing

• Sourcing causes file settings to become part of
  the current shell and a subshell is not created

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The Initialization Files and Sourcing

• Source (dot) command
• Built-in command
• Takes script name as an argument
• Used to reexecute any of the initialization
  files if they have been modified

82
Activity Altering the .bashrc File

• KEY CONCEPTS
• Altered .bashrc file and added aliases and
  environmental variables
• Items in file preceded by a are comments and
  will not be executed
• Until file is executed none of changes have
  taken effect
• Because set noclobber oncannot use redirection
  to overwrite a file
• Could overwrite file with cp command