Lesson 8-Specifying Instructions to the Shell

1
Lesson 8-Specifying Instructions to the Shell
2
Overview

• An overview of shell.
• Execution of commands in a shell.
• Shell command-line expansion.
• Customizing the functioning of the shell.
• Employing advanced user features.

3
An Overview of Shell

• A shell interprets and executes the syntax of the
  command-lines in a specific way.
• The kernel is the core program of UNIX/Linux,
  which schedules processes, allocates memory, and
  handles input/output and other peripherals.
• User cannot directly communicate with the shell.

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An Overview of Shell

• The shell interacts with the kernel to execute a
  request.
• The shell is the middleman between the user and
  the kernel.
• A shell translates a users requests into kernel
  calls.
• The login shell is started when a user logs in
  and exits when the user logs out.

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An Overview of Shell

• A shell is the interface between the user,
  utilities, the file system, and the kernel.
• The shells primary function is to read the
  command-line, examine its component, and
  interpret it according to its rules.
• The shell performs the given task and returns the
  prompt for further requests.

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Execution of Commands in a Shell

• Interacting with the shell.
• Communicating with the shell.
• Identifying utilities for output redirection.
• Identifying utilities in pipelines.
• Starting processes to run utilities.
• Redirecting input and output.

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Execution of Commands in a Shell

• Passing arguments to processes.
• Identifying tokens on the command-line.
• The exit code status after a utility execution.
• Using the model to interpret command-lines.
• Changing the behavior of a command execution.

8
Interacting with the Shell

• Entering a command from the keyboard is the basic
  way of communicating with the shell.
• For each utility requested by the user, the shell
  starts a new child process to execute the code of
  that utility.
• The child process inherits the environment
  variables like pid, user, etc.
• The ps utility can be used for obtaining the
  process identification numbers.

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Communicating with the Shell

• The shell proceeds through a series of specific
  steps after a user issues commands.
• The complete command-line is first interpreted by
  the shell.
• The shell interprets the ENTER key as the
  completion of a command.
• The shell interprets \ as an instruction not to
  interpret the special meaning of the single
  character that immediately follows it.

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Communicating with the Shell

• The commands entered at the shell prompt usually
  include several words or tokens.
• The shell interprets some tokens as utilities and
  others as filenames.
• The command line interprets the gt, , and lt
  as special characters that control the input and
  output of a file.

11
Communicating with the Shell

• The shell uses white space to identify the words
  or tokens of a command-line.
• The sign is recognized by the shell as the
  start of a new variable.

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Identifying Utilities for Output Redirection

• The shell interprets the first word in the
  command-line of the shell as a utility.
• In a C shell, when assigning value to variables,
  it should not include any spaces around the
  sign.
• The value of a set variable can be displayed by
  adding a sign ahead of the variable name.
• The shell interprets the token following the pipe
  as a utility and the token following the
  redirection operator as a file.

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Identifying Utilities in Pipelines

• A pipeline is a set of one or more utilities that
  handle data independently.
• The semicolon can be used to indicate the end
  of one pipeline.
• A shell can run one pipeline after another on a
  single command-line by separating them with
  semicolons.
• The first token after the semicolon begins a new
  pipeline, and hence must be a utility.

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Identifying Utilities in Pipelines

• The logical AND () operator can instruct the
  pipeline to run the next utility based on the
  success or failure of the preceding pipeline.
• The command-line is successful only if both are
  executed.
• A token following the operator is interpreted
  as a utility.
• The logical OR () operator executes only one of
  the two utilities in the command-line.

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Identifying Utilities in Pipelines

• The shell can easily interpret a variable in all
  tokens since a sign precedes them.
• The x option tells the shell to explain how it
  interprets the command-line before executing it.

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Starting Processes to Run Utilities

• The shell is an active process and runs in the
  foreground.
• The resources allocated to a running process are
  called process space or process image.
• The shell makes an exact copy of the process
  space, including environment variables, when
  running a utility.

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Starting Processes to Run Utilities

• A new child process space is an exact copy of the
  shell.
• The child process inherits the input, output,
  error destination, and variable information from
  the parent.

18
Redirecting Input and Output

• The shell interprets the gt as an instruction to
  redirect the output from the screen to a file.
• When redirecting output to a file, a new file is
  created, depending on the shell and the noclobber
  variable set.
• Existing files can be protected by setting the
  noclobber variable.

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Redirecting Input and Output

• The csh shell can be instructed to overwrite an
  existing file by placing an exclamation point (!)
  before the redirection symbol (gt).
• In bash and ksh shells, the pipe following the
  redirect is an instruction to overwrite even if
  noclobber is set off.
• The i option, when used with the mv and cp
  utilities, protects files from accidental
  removal.
• A utility, uses the keyboard as default input.

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Redirecting Input and Output

• The input, the output and the error files all are
  connected to the default output, the monitor.
• An error message is displayed on the screen if a
  command is not able to execute.
• An error message can be redirected to a file by
  using the 2gt and a filename to the command-line.

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Redirecting Input and Output

• The bash, ksh, and sh uses gt or 1gt to
  redirect output to a file.
• The standard error and output can both be
  redirected to the same file using the gt and
  specifying the filename in the command line.

22
Passing Arguments to Processes

• The ls utility interprets an argument as a file
  if it is not preceded by a minus sign.
• Any tokens left over on the command-line when the
  shell has completed the interpretation are passed
  as an argument to the associated utility.
• The shell also interprets command options as
  arguments.
• Two or more option flags can be specified on the
  command-line as one argument.

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Identifying Tokens on the Command-Line

• The wild card character instructs the shell
  to include all the filenames.
• The C shell variable path and the family variable
  PATH contain a list of directories that the
  respective shells search to locate the code for
  each requested utility.
• The /bin directory usually contains all the
  executables.

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Identifying Tokens on the Command-Line

• The strings utility ignores all machine code and
  outputs only the strings of ASCII characters that
  it finds.
• The shell does not search the path to locate the
  utility if the absolute path of the utility is
  specified.

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The Exit Code Status After a Utility Execution

• The shell interprets the variable ? as the exit
  code of the last process.
• Exit codes other than zero are error codes.
• Every time a process completes its execution and
  exits, it informs its parent about the status of
  the exit code.

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Using the Model to Interpret Command-Lines

• When redirecting output to a file and if the
  noclobber variable is set to off, no error
  message is displayed when the file is being
  overwritten.
• A utility that requires a filename as an
  argument will start reading from the input if the
  filename is not specified.
• A dash (-) argument instructs the sort utility to
  read from the input.

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Changing the Behavior of a Command Execution

• When the shell executes a child process in the
  foreground, it waits for a child process to
  complete execution and then displays the prompt.
• A command consisting of utilities, arguments, and
  redirection terminated by ENTER is called a job.
• A job can be placed in the background by
  appending it with the sign.

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Changing the Behavior of a Command Execution

• The ps command can be used for listing all
  current processes.
• A current process in the foreground can be
  suspended by pressing CTRL-Z.
• The csh, tcsh, bash, and ksh shells allow a user
  to suspend a job midstream and return to it
  later.
• The fg command allows a job to be brought back to
  the foreground.

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Shell Command-Line Expansion

• Using shell characters to expand filenames.
• Creating and using local variables.
• Passing environment variables to child processes.

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Using Shell Characters to Expand Filenames

• Some characters are interpreted by the shell as
  wildcard characters, while others can be used for
  specifying a range of characters.
• The filename expansion of the filename-matching
  feature allows the selection of many filenames
  while entering only one name with special
  characters embedded.
• The and ? are interpreted by the shell as
  special characters.

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Using Shell Characters to Expand Filenames

• The asterisk () character can be used for
  matching any number of characters, while the
  question mark (?) is used only for matching a
  single character.
• Shell variable names and values are stored in the
  memory and are hence available regardless of the
  directory location.

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Using Shell Characters to Expand Filenames

• The shell also allows a range of letters or
  characters to be specified with the help of
  square brackets.
• The curly brace characters, and , are also
  used by the bash shell and modern ksh shells for
  matching and creating multiple filenames from one
  pattern.
• The curly braces match existing filenames if each
  match is specified in the braces, but does not
  expand ranges.

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Creating and Using Local Variables

• Local and environmental are the two different
  kinds of variables identified by the shell.
• The set or env command lists the variables
  that are set in the shells memory.
• In a csh or tcsh shell, the set command is used
  for declaring a variable and assigning a value to
  it.

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Creating and Using Local Variables

• In ksh, bash, or sh shell, a variable is directly
  defined and assigned a value without the set
  command.
• The shell interprets the character as an
  instruction to locate in the shells memory a
  variable that has the name of the character
  string that follows the .
• The variable must be enclosed in single quotes if
  it includes any spaces.

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Passing Environment Variables to Child Processes

• There are two types of variables - the local
  variable and the global variable.
• The local variables of a shell are not passed to
  a child process.
• In a C shell, the setenv command is used for
  setting an environmental variable.
• The set, env, or printenv commands can be used
  for listing the environmental variables.

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Passing Environment Variables to Child Processes

• The unset command can be used for removing a
  local variable.
• An environmental variable can be removed with the
  help of the unsetenv command.
• The export command is used for making a local
  variable available to a child process.

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Passing Environment Variables to Child Processes

• An environmental variable modified by the child
  process is not reflected in the parents
  environmental variables.
• The shell also allows a variable to be created
  and exported at the same time.
• The variables set in a child process are lost
  once the child process exits.
• The child shell takes the memory of the variable
  when it exits.

38
Customizing the Functioning of the Shell

• Using and modifying the search path.
• Creating personalized shell prompts.

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Using and Modifying the Search Path

• The path or PATH variable is searched when a
  user requests for a utility.
• The path is a local variable and is usually
  assigned a value in the startup script.
• The C shell also maintains a PATH environmental
  variable, which also holds the path, and passes
  it to child processes.
• In the C shell, two variables are intertwined
  where change in one is automatically reflected in
  another.

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Using and Modifying the Search Path

• A single dot is used for denoting the current
  directory and can be set in the path variable.
• A colon () at the beginning or the end of a path
  string is interpreted by the sh family of shells
  as an instruction to search the current
  directory.
• An empty field using two colons () can
  explicitly request the current directory,
  anywhere in the path, using a dot.

41
Creating Personalized Shell Prompts

• In a tcsh or csh shell, the prompt variable is
  used for modifying the display prompt.
• The man and info pages describe the
  collection of variables that can be used in
  constructing a prompt.
• The ksh and the bash shells use the value in the
  variable PS1 as its prompt.

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Employing Advanced User Features

• Completing filenames
• The variable filec in a tcsh shell, when set in
  the environment, instructs the shell to search
  for matching filenames.
• When a shell cannot distinguish between two
  existing files, it either displays all matching
  files or simply flashes or produces beeps.
• Filename completion can also be used for files,
  directories, and executables.

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Employing Advanced User Features

• Completing filenames (continued)
• The filename-completion variable can be set in
  the Korn shell by executing either the set o
  vi or the set o vi-tabcomplete command.
• The set o vi or set o posix commands can be
  used for turning on the file-completion feature
  if it is not working.
• Many C shells include filename completion, but
  use the ESC key to trigger completion of
  filenames.

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Employing Advanced User Features

• Evaluating shell variables
• The bash and ksh shells also provide built-in
  variables that are useful in interacting with the
  shell.
• The SECOND shell variable can be used for
  determining the number of seconds since the shell
  was started.
• In a bash shell, the PROMPT_COMMAND variable
  allows a user to execute any command just before
  it displays the prompt.
• A dot file is a run-control file for a specific
  utility or shell.

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Employing Advanced User Features

• Customizing shell startup files
• The csh shell can be customized with the help of
  the .cshrc file in the /etc directory since it is
  always read at startup.
• The bash shell reads the file .bashrc whenever it
  starts.

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Employing Advanced User Features

• Customizing shell startup files (continued)
• A system setup to start a ksh file reads the
  .kshrc file at startup.
• The .kshrc file is not read if the ENV
  environmental variable is not set.
• The ksh shell is programmed to read at startup
  whatever file is the value of the ENV variable.

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Summary

• All shells accomplish the primary task of
  interpreting the commands issued by a user.
• A shell process executes code that resides in a
  file in a system directory.
• A child process started by the shell for each
  utility execution inherits the input, output, and
  error destinations, as well as environmental
  variables.

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Summary

• Redirecting input and output from the default
  destination to files and other utilities is one
  of the functions of a shell.
• A command-line inside back quotes is interpreted
  and executed as a complete command line.
• Local variables are not passed to child
  processes, while environmental variables are
  passed to child processes.

49
Summary

• The most recent shells include file completion,
  which allows us to type part of a file,
  directory, or utility name.
• A user can place instructions in the startup
  files to tailor how the shell functions.