Shell Features

1
Shell Features

• Chapter 14

2
Overview

• Bourne Shell () Original shell for UNIX
  environment developed for ATT V.2. Does not
  support alias, history or command line editing
  and is used primarily by system admins.
• Korn Shell ()- Superset of the Bourne. Supports
  aliasing, history and command line editing. Most
  widely used.
• C shell () Based on C programming language.
  Supports aliasing and history.
• BASH Bourne-Again Shell () has the feel of
  the Bourne and Korn and incorporates the C and
  Korn.

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Overview

• The Korn shell is the default shell for Solaris
  users and the Bash shell is the default for Linux
  users.
• The shell accepts commands issued by the user,
  interprets these commands, and executes the
  appropriate programs.
• Shells can be command line or graphical, and are
  also interchangeable.
• Several of the shell features include
• Wildcard matching
• Input/output redirection.
• Pipes
• Background processing
• Programming language features are used to create
  shell scripts that perform complex operations.

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Understanding the Shell

• The shell is only loaded when a user logs in at a
  text-mode login or launches a shell
• Shell is like any other program running on Linux.
  It has no special privileges.
• init program displays a login prompt, but doesnt
  load the shell.
• Graphical users are limited to what is available
  on the menu.
• Shells primary purpose is to launch programs.

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Alias

• An alias is
• substitute a short command for a long one alias
  hhistory
• Create a single command for a series of commands
  alias homecdls
• Create alternate forms of existing commands alias
  copycp i
• When an alias is set from the command line, it is
  activated only for the shell in which the alias
  is created.

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Alias

• To display alias
• alias
• To Turn off
• unalias
• To temporarily bypass the alias and use the
  original version of a command
• alias lsls-l
• \ls home/dir1

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History and Repeating Commands

• The history feature in the Korn and Bash shells
  keeps a record of the exact command lines with an
  event number.
• In the Korn shell the history list is stored in
  the user's home directory in a file named
  /.sh_history. By default, the Korn shell keeps a
  record of the last 128 commands entered.
• The Bash shell history list is stored in the
  user's home directory in a file named
  /.bash_history. By default, the Bash shell keeps
  a record of the last 1000 commands entered.
• The number of commands stored in the history list
  file for both the Korn and Bash shells is
  determined by the HISTSIZE variable

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History

• Command Format history options
• - number execute previous (number)
• The r (repeat) command is one of many predefined
  aliases in the Korn shell. This command is
  similar to the F3 key in DOS.
• r number
• The simplest way to repeat commands in the Bash
  shell is to press the up arrow, or CtrlP keys,
  and down arrow, or CtrlN keys. The Bash shell
  offers additional features for repeating previous
  command lines not available in the Korn shell
• !! -Executes the previous command
• ! -repeats all arguments from the previous
  command
• ! -repeats the last argument from the previous
  command
• !number executes command number from the
  history list
• !np - views command n from the history list
  without executing it

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Editing the Command Line

• Both the Korn and Bash shells offer a more
  powerful way to edit previous commands with the
  command line editing feature.
• When enabled from the command line, the command
  line editing feature is activated only for the
  shell in which it was set.
• Command Format set - or o vi or set - or
  o emacs
• When command line editing has been turned on with
  the vi mode, pressing the Esc key activates the
  in line editor
• Example
• more /usr/share/dict/worfs creates error
• set o vi
• more /usr/share/dict/worfs use keystrokes to
  move cursor

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VI

• The Korn shell does not support the use of arrow
  keys. Therefore, the arrow keys cannot be used to
  reposition the cursor during line editing

11
Filename and Command Completion

• Both the Korn and Bash shells contain another
  feature that completes the name of a file or
  command by turning on the command line editor.
• To use the completion feature 
• Type a command, such as ls, cat, rm and so on,
  followed by one or more characters of a file
  name. 
• Press the Esc and backslash keys (Esc \) in
  sequential order, for the Korn shell, or the Tab
  key, for the Bash shell. 
• Another way to use the completion feature is to
  request the shell to display a list of files that
  matches the entered filename. To display this
  list the student uses Esc for the Korn shell or
  Tab Tab, entered twice, in the Bash shell.

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Shell Initialization

• When the shell starts executing, it is
  uninitialized, which means, several parameters
  required for proper operation are not defined.
• Initialization defines parameters.
• Init files are short to provide a complete
  working environment with as little overhead as
  possible.

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Variables Overview

• All the work that is done and processes that are
  executed from the time of log in to the time of
  log out are completed within an environment.
• Within the environment are variables that shell
  programs and processes access for customization.
• Shell variables are names that hold value so
  processes can function properly.
• Many Global Variables are predefined and set
  automatically when the user logs in.

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Variables

• The standard for the Korn and Bash shells is to
  use capital letters for shell variable names.
  Command FormatPS1"MyPrompt
• The unset command removes the variable from the
  current shell and subshells
• To display the value of a single variable, use
  the echo command echo PS1 would return
  "MyPrompt" assuming the PS1 variable was set to
  Myprompt.
• The dollar sign () metacharacter preceding a
  variable tells the shell to use the value of the
  variable, not the name of the variable.
• To display all variables in the local shell and
  the variable's values, type the set command,

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Environmental Variables

• Environment variables are global variables
  available to all shells and subshells, not just
  the local shell.
• To make a variable known to a subshell, it must
  be exported with the export command.
• PS1myprompt
• export PS1
• To have line numbers automatically turned on when
  starting vi, the student could set the ex
  initialization variable (EXINIT) and then export
  the variable
• EXINIT'set number'export EXINIT
• echo EXINIT set number
• Environment variables and their values can be
  displayed
• export or env

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Quoting

• Double quotes are used to print characters
  literally, including spaces
• STRINGMAY DAY
• echo STRING
• MAY DAY
• Single quotes are used to interpret any
  metacharacter (,?,,, and so on) literally, and
  ignore any type of value evaluation
• Back quote is used to assign output from system
  commands to variables
• echo date
• Tue Nov 16 164019 GMT 2004
• Backslash stops the shell from misinterpreting
  the next character if it has special meaning to
  the shell
• echo \

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Variables

• Exporting variables in an initialization file at
  login enables the variables to be used by the
  system, processes, scripts, users, and all
  shells.
• Exporting variables at the command line makes the
  variables available to the current shell and all
  child processes and shells, not the parent shell.

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Custom Prompts

• The default prompt for the Korn and Bash shell is
  the dollar sign (). To customize the shell
  prompt, use shell prompt variable (PS1value).
• The PS1 variable can include a wide range of
  expressions, such as character strings, commands,
  or other variables.
• The variable PS1 is a shell variable. Any change
  in the variable setting remains until the shell
  is exited or until a subshell is opened. 
• Note that prompts all end in a dollar sign (),
  representing the Korn or Bash shell.

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Custom Prompts

• For simple prompt settings in the Bash shell the
  student can also set the PS1 variable to 
• \u for username 
• \d for the date
• \h for the hostname 
• \ for the dollar sign 
• \W for the working directory 
• PS1\u_at_\h\ for example, would result in the
  prompt similar to jamie_at_colorado

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Initialization Files

• Variables are either predefined, usually global,
  and is recognized in any shell.
• Users can customize many of these variables by
  placing them in a user init file.
• Unlike variables, aliases and shell option
  settings cannot be exported or made available to
  all subshells.
• In order to make aliases and shell options
  available in a subshell, they must be placed in a
  secondary initialization file that is read each
  time a subshell is started.

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Initialization Files

• The shell reads these files when the user
• logs in (login script),
• when a subshell is run from the login shell by
  the user or automatically by the system
  (environment script),
• when a user logs out (logout script).
• Only the Bash and C shell programs have logout
  scripts.
• They are two types of init files
• System wide initialization files are maintained
  by a system administrator, stored in the /etc
  directory and are read first.
• The user specific initialization files reside in
  a user's home director are read second.

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/etc/profile file

• The primary system wide initialization file for
  Korn and Bash shell users is the /etc/profile
  file which contains configuration information.
• The user specific initialization files are
  different for the Korn and Bash shells.
• The main function
• Exports environment variables such as LOGNAME
  for login name and PATH for default command path
• Displays contents of /etc/motd file
• Checks for mail
• Other one time only commands such as cal or
  banner "LOGNAME", and so on.

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BASH shell login

• .bash_profile - local global variables.
• .bashrc only interactive shells and user
  scripts read this file. It typically contains
  aliases, turns on shell features, and sets the
  custom prompt.
• If a Bash shell user wants to customize the
  environment, that user needs to create or modify
  the .bash_profile file and then the user must
  export the BASH_ENV variable, as shown here
• BASH_ENV HOME/.bashrcexport BASH_ENV

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BASH shell login

• To have the system reread the .bash_profile or
  .bashrc file after changes have been made the
  user can either log out and log back in or type
  the following from the command line
• . (dot) /.bash_profile or source
  /.bash_profile
• . (dot) /.bashrc or source /.bashrc
• A .bash_logout file is available for Bash shell
  users that allows for customization after the
  user logs out. Clear the screen and print "Bye",
  for instance

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Korn User Init Files

• If a Korn shell user wants to customize the
  environment, that user needs to create or modify
  the .profile file and then the user must export
  the ENV variable, as shown here
• ENV HOME/.kshrcexport ENV
• Korn shell specific commands and features should
  be placed in the .kshrc file. The .kshrc file
  automatically is read each time a new shell, such
  as a new Terminal window, or subshell is started,
  so there is nothing to export.

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Korn Shell

• The contents of the .kshrc file typically include
  the following
• A customized prompt
• Custom variables
• Aliases
• The .profile file is read only once, while the
  .kshrc file is read every time a new Korn shell
  is started. To have the system reread the .kshrc
  or .profile files after changes have been made
  the user either can log out and log back in or
  can type the following from the command line
• . (dot) /.kshrc (or . .kshrc from the home
  directory)
• . (dot) /.profile (or . .profile from the home
  directory)

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