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intro unix/linux 08


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specifying instructions to the shell

specifying instructions to the shell

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  • 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.
  • 4.
    • 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 user’s requests into kernel calls.
    • The login shell is started when a user logs in and exits when the user logs out.
    An Overview of Shell
  • 5. An Overview of Shell
    • A shell is the interface between the user, utilities, the file system, and the kernel.
    • The shell’s 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.
  • 6. 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.
  • 7. 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.
  • 9. 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.
  • 10. 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 “>”, “|”, and “<“ as special characters that control the input and output of a file.
  • 11.
    • 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.
    Communicating with the Shell
  • 12. 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.
  • 13. 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.
  • 14. 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.
  • 15. 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.
  • 16. 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.
  • 17. 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 > 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.
  • 19. Redirecting Input and Output
    • The csh shell can be instructed to overwrite an existing file by placing an exclamation point (!) before the redirection symbol (>).
    • 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.
  • 20. 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 “2>” and a filename to the command-line.
  • 21.
    • The bash, ksh, and sh uses “>” or “1>” to redirect output to a file.
    • The standard error and output can both be redirected to the same file using the “>&” and specifying the filename in the command line.
    Redirecting Input and Output
  • 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.
  • 23. 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.
  • 24. 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.
  • 25. 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.
  • 26. 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.
  • 27. 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.
  • 28. 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.
  • 29. Shell Command-Line Expansion
    • Using shell characters to expand filenames.
    • Creating and using local variables.
    • Passing environment variables to child processes.
  • 30. 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.
  • 31. 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.
  • 32. 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.
  • 33. 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 shell’s memory.
    • In a csh or tcsh shell, the set command is used for declaring a variable and assigning a value to it.
  • 34. 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 shell’s 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.
  • 35. 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.
  • 36. 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.
  • 37. Passing Environment Variables to Child Processes
    • An environmental variable modified by the child process is not reflected in the parent’s 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.
  • 39. 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.
  • 40. 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.
  • 42. 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.
  • 43. 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.
  • 44. 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.
  • 45. 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.
  • 46. 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.
  • 47. 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.
  • 48. 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.
    • 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.