Implementation of a simple shell, xssh(Section 1 version)What is a shell? A process that does command line interpretation Reads a command from standard input (stdin) Executes command corresponding to input line In simple case the shell: Reads a command Forks a child to execute the command Wait for child to complete before reading another command.2What is a shell? A real shell handles: Foreground process groups Background process groups Signals Process pipelines Redirection3UNIX/Linux Shells A shell is a command line interpreter Translates commands typed at a terminal (or in a file) Input is from the terminal (stdin) or file User’s /etc/passwd entry indicates the interactive shell Some Shells Bourne Shell (sh): What some use for shell scripts C-Shell, TC-Shell (csh, tcsh): Typical interactive shell Bash Shell (bash, “GNU Bourne-Again Shell”): Default Linux interactive shell4Shell Scripting Shell Script (Shell Program) A file containing shell commands Used heavily in system administration Scripts in /etc/rc*.d/ are executed during system booting A Tutorial on Scripting: www.ooblick.com/text/sh5Input and Output < File Use File as standard input (stdin) Note: stdin is file descriptor (/dev/fd/0) > File Use File as standard output (stdout, /dev/fd/1) << Word Read from the current file until end of file or an input line containing Word alone  >> File Append stdout to File Command1 | Command2 Create a pipeline Connect stdout of Command1 to stdin of Command26Bash Parameter Expansion ${Parameters} indicates parameter (variable) expansion The result of expansion is the value of the parameter Braces are optional, but … X=foo echo $XX # output is null string echo ${X}X # output is fooX Parameters A Name: A sequence of letters, digits or underscores starting with a letter. Note: these parameters are called variables. A Number: $1 is the first shell or command line argument, $2 is the second, etc. (positional parameters).  Special Parameters: * @ # ? - $ ! 0 _7Essence of Bash Special Parameters $* "$1 $2 ...“ $@ "$1" "$2” ... $# Number of positional parameters $? Status of last executed foreground command $- Options supplied to shell on invocation or by set $$ Process id of this shell $! Process id of last background command $0 Name of shell or script $_ Last argument of previous command man bash for more information8What will our shell do? Execute built-in commands Execute foreground processes Execute background processes Perform variable substitution (expansion) before executing command Perform redirection Ignore comments and blank lines Implement optional features?9Built-in commands echo W1 W2 … W1, W2, ... are words (tokens separated by white space) Compress consecutive white spaces to a single space quit S Exit shell with exit status S, an integer Real shell allows bg processes to continue; xssh shouldn’t !!! wait P Wait for process P to terminate If P is omitted, wait for all background processes to terminate10Built-in commands (cont.) set V Val Create shell variable V if it doesn’t exist Set the value of V to Val All occurrences of ${V} will be replaced by Val until V is changed or removed unset V Remove given variable export V Mark V for export. Details TBA. Special variables:  $$ pid of current process $? status of most recent foreground command  $! pid of most recent background command11Built-in commands (cont.) chdir P (synonymous with cd P) Assume $HOME if P is omitted If P is a valid path for which the user has execute privileges, change PWD to P, a pathname12Commands that aren’t built-in Built-in commands are executed without fork-exec If first word of command is not a built-in command And it is either  An absolute pathname of an executable file, or An executable file in a directory listed in $PATH Execute with fork-exec13Strategy What are the requirements? Are there assumptions? What is most critical? What are the risky parts of the project? What are the main data structures? i.e., Abstract data types (data and function members) What is the overall control flow? What system calls/library functions will be needed? Can you sketch the whole project on a sheet of paper? Assume simple implementations of each feature14Risky parts chdir P Use chdir() Not very risky after all quit S Need to cleanup and wait for background processes15Risky parts wait P Need list/table of background processes Non-builtin commands fork-execvp-wait Variable substitution Should be separable code; i.e., inserted before command processing16Risky parts Environment Details TBA17Observations All system calls and error-prone library functions should be wrapped, e.g.  Put “pid_t Fork(void);” in libxssh.h Put “pid_t Fork(void) {. . .}” in libxssh.cwith a direct call to fork() and appropriate error handling No direct calls to fork() anywhere else18Basic Control Flowmain:Initialize;while (get line not EOF) {Break line into words;if (builtin command) do_builtin;else do_nonbuiltin;}do_builtin:case command { // or directly call function... chdir,echo,quit,wait,set ...if (incorrect #args) { Display msg; break; }}19Basic Control Flowdo_nonbuiltin:if ((pid=Fork( )) error) ... Error ...if (pid == 0) { // child... execvp code ...exit(0);}if (foreground) Wait for child to