Mastering Shell Commands: A Comprehensive Guide

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. 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 user’s requests into kernel calls. • The login shell is started when a user logs in and exits when the user logs out.

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. 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.

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. Redirecting Input and Output • 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.

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. 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.