ITSC 1329 Module 6

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ITSC 1329 Module 6

• Using Commands Within the Shell

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Module 6 Objectives

• Use shell metacharacters
• Describe the Korn shell variables
• Display the command history
• Redirect commands
• Describe the command-line interpreter
• Work with user initialization files

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Three Types of Shell Metacharacters

• Korn shell metacharacters are specific
  characters, usually symbols, that have special
  meaning to the shell
• There are three types
• Path Name
• File Name Substitution
• Redirection
• Note Never use any of the shell metacharacters
  when naming files and directories

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Pathname Metacharacters

• Tilde Character ()
• Represents home directory of current user
• Tilde Character with a Username (user1)
• Represents home directory of specified user
• The tilde is not available in the Bourne shell
• Dash Character (-)
• Represents the previous working directory

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File Name Substitution Metacharacters

• File name substitution metacharacters are also
  known as Wildcards
• Metacharacters allow the user to specify parts of
  filenames and let the shell find the rest
• The metacharacters we will cover
• Asterisk
• Question Mark ?
• Square Brackets
• Exclamation Point !

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The Asterisk Character

• Used to represent zero or more characters (except
  the leading period . of a hidden file)
• Used with various commands to represent all files
  
• cp dir2 Copies all files from pwd to dir2
• ls f Lists all files that begin with f
• ls f1 Lists all files that begin with f and
  end with 1
• DOS used . to represent all files

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The Question Mark ? Character

• Used to represent any single character (except
  the leading period . of a hidden file)
• ls dir?
• ls empty?
• ls ?a
• ls j???a
• ls .
• ls dir2/b
• ls /etc/d

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The Square Bracket Characters

• The square bracket characters represent a set or
  range of characters for a single character
  position
• ls ejt
• ls a-f
• ls A-Z0-9
• ls e0-9
• ls j???ac14
• ls ?ua0-9

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The Exclamation Point ! Character

• The exclamation point (also called bang) is used
  to exclude the pattern. It means not
• ls e!0-9
• ls !A-Z
• ls !e
• ls junk!0-9
• ls j???!ac14
• ls !A-Z0-9
• ls ?!m1

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Introducing Korn Shell Variables

• A variable refers to a temporary storage area in
  memory. Variables contain
• Information needed for customizing the shell
• Information needed by other processes to function
  properly
• Korn shell programming uses two types of
  variables
• Environment variables
• Variables that are passed to child processes
• Local variables
• Variables that are not passed to child processes

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Korn Shell Commands for Variables
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Displaying Variables

• To list all shell variables with their current
  values, use set
• set
• Use echo or print to display the value that is
  stored inside a variable
• print PS1
• echo PWD
• echo PWD
• The sign tells the shell that what follows is a
  variable

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Setting and Unsetting Variables

• Syntax for setting variables
• varvalue or VARvalue
• rockdir/export/home/user1/Music/Types/Rock
• set
• cd rockdir
• Syntax for unsetting variables
• unset var or unset VAR
• unset rockdir
• set

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Exporting Variables to Sub-Shells

• All shells have built-in commands, such at the cd
  and ls commands
• Any process in UNIX that is not a built-in
  command (like a shell script) is forked into a
  child process (sub-shell) and runs independently
  of the parent shell
• Local variables are not used by child processes
• Environmental variables are used by all processes

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Exporting Variables Example

• VAR1first_variable Sets the variable
• set Shows set variables
• echo VAR1 Shows value of VAR1
• sh Changes to Bourne Shell
• echo VAR1 Doesnt work
• exit Exits child shell
• export VAR1 Exports VAR1 to child processes
• sh
• echo VAR1 Success!

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Default Korn Shell Variables
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More Default Variables
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Ignoring Metacharacters

• There are several ways to turn off metacharacters
• The most reliable is the backslash \
• echo p
• echo p
• echo p\
• echo SHELL
• echo \SHELL
• echo SHELL Uses forward single quote

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Calling Other Commands

• You may need to execute a command within another
  command
• echo Today's date is date.
• echo Today's date is date.
• Uses backtics to enclose command
• Syntax command command
• echo Today's date is (date).
• Uses ( ) to enclose command
• Syntax command (command)

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Using the History Command

• Syntax for history command history option
• By default, the history command displays the last
  16 commands
• history
• Your history file (.sh_history) contains up to
  999 commands
• cat .sh_history
• Specify how many commands to display, not
  counting the history command itself
• history -10
• Actually displays last 11 commands

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Displaying Previous Commands

• Finding a previous command
• history ls
• history cat ls
• Using SSH
• ltctrlgt p
• Using the r command
• r cat
• r ls
• r c
• r history

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Understanding Command Redirection

• Typically, the shell receives or reads command
  input from the keyboard and displays or writes
  the command output to the screen
• There are two redirection metacharacters
• Greater than gt
• Less than lt
• Both of these can be used with the pipe

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The File Descriptors
File descriptors determine where the input to the
command originates and where the output and error
messages are sent
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Redirection Examples

• cat
• Takes you to a blank line
• Hi!
• Input received from stdin and sent to stdout
• cat lt dante
• Tells cat command to receive input from dante
  file instead of keyboard
• ls l gt lsfile
• Redirects stdout from screen to a file
• echo This is a long list gtgt lsfile cat lsfile
• Appends to lsfile instead of overwriting

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Redirecting Error Messages

• You can redirect error message using the stderr
  file descriptor number (2) and the greater than
  symbol (gt)
• ls lR /etc
• There are a few error messages due to file
  permissions
• ls lR /etc 2gt etc-errors
• The command still runs, but no error messages are
  displayed
• cat etc-errors
• The file containing our redirected error messages

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The Pipe Character

• The pipe command redirects the stdout of one
  command to the stdin of another
• ls lR more
• cat dante wc w

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User Initialization Files
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Korn Shell Initialization Files

• Initialization files are used to customize your
  working environment
• The /.profile file is run by the login process
• The /.kshrc file contains shell variables and
  aliases. To use it, the ENV variable must be
  defined in the .profile file
• Changes to .profile and .kshrc take effect upon
  next login.
• Run . /.profile or . /.kshrc to update
  immediately

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Module 6 Objectives Review

• Use shell metacharacters
• Describe the Korn shell variables
• Display the command history
• Redirect commands
• Describe the command-line interpreter
• Work with user initialization files