.s1 6. The Shell .es For most users, communication with \*sUNIX\*n is carried on with the aid of a program called the Shell. The Shell is a command line interpreter: it reads lines typed by the user and interprets them as requests to execute other programs. In simplest form, a command line consists of the command name followed by arguments to the command, all separated by spaces: .dc command arg\*t\d1\u\*n arg\*t\d2\u\*n .\|.\|. arg\*t\dn\u\*n .ec The Shell splits up the command name and the arguments into separate strings. Then a file with name \fIcommand\fR is sought; \fIcommand\fR may be a path name including the ``/'' character to specify any file in the system. If \fIcommand\fR is found, it is brought into core and executed. The arguments collected by the Shell are accessible to the command. When the command is finished, the Shell resumes its own execution, and indicates its readiness to accept another command by typing a prompt character. .pg If file \fIcommand\fR cannot be found, the Shell prefixes the string \fI/\|bin/\fR to \fIcommand\fR and attempts again to find the file. Directory \fI/\|bin\fR contains all the commands intended to be generally used. .s2 6.1 Standard I/O .es The discussion of I/O in \(sc3 above seems to imply that every file used by a program must be opened or created by the program in order to get a file descriptor for the file. Programs executed by the Shell, however, start off with two open files which have file descriptors 0 and 1. As such a program begins execution, file 1 is open for writing, and is best understood as the standard output file. Except under circumstances indicated below, this file is the user's typewriter. Thus programs which wish to write informative or diagnostic information ordinarily use file descriptor 1. Conversely, file 0 starts off open for reading, and programs which wish to read messages typed by the user usually read this file. .pg The Shell is able to change the standard assignments of these file descriptors from the user's typewriter printer and keyboard. If one of the arguments to a command is prefixed by ``>'', file descriptor 1 will, for the duration of the command, refer to the file named after the ``>''. For example, .dc ls .ec ordinarily lists, on the typewriter, the names of the files in the current directory. The command .dc ls >there .ec creates a file called \fIthere\fR and places the listing there. Thus the argument ``>there'' means, ``place output on \fIthere\fR.'' On the other hand, .dc ed .ec ordinarily enters the editor, which takes requests from the user via his typewriter. The command .dc ed <script .ec interprets \fIscript\fR as a file of editor commands; thus ``<script'' means, ``take input from \fIscript\fR.'' .pg Although the file name following ``<'' or ``>'' appears to be an argument to the command, in fact it is interpreted completely by the Shell and is not passed to the command at all. Thus no special coding to handle I/O redirection is needed within each command; the command need merely use the standard file descriptors 0 and 1 where appropriate. .s2 6.2 Filters .es An extension of the standard I/O notion is used to direct output from one command to the input of another. A sequence of commands separated by vertical bars causes the Shell to execute all the commands simultaneously and to arrange that the standard output of each command be delivered to the standard input of the next command in the sequence. Thus in the command line .dc ls | pr \(mi2 | opr .ec .it ls lists the names of the files in the current directory; its output is passed to \fIpr\fR, which paginates its input with dated headings. The argument ``\(mi2'' means double column. Likewise the output from \fIpr\fR is input to \fIopr\fR. This command spools its input onto a file for off-line printing. .pg This procedure could have been carried out more clumsily by .dc ls >temp1 .ti 1i pr \(mi2 <temp1 >temp2 .ti 1i opr <temp2 .ec followed by removal of the temporary files. In the absence of the ability to redirect output and input, a still clumsier method would have been to require the .it ls command to accept user requests to paginate its output, to print in multi-column format, and to arrange that its output be delivered off-line. Actually it would be surprising, and in fact unwise for efficiency reasons, to expect authors of commands such as .it ls to provide such a wide variety of output options. .pg A program such as \fIpr\fR which copies its standard input to its standard output (with processing) is called a \fIfilter\fR. Some filters which we have found useful perform character transliteration, sorting of the input, and encryption and decryption. .s2 6.3 Command Separators; Multitasking .es Another feature provided by the Shell is relatively straightforward. Commands need not be on different lines; instead they may be separated by semicolons. .dc ls; ed .ec will first list the contents of the current directory, then enter the editor. .pg A related feature is more interesting. If a command is followed by ``&'', the Shell will not wait for the command to finish before prompting again; instead, it is ready immediately to accept a new command. For example, .dc as source >output & .ec causes \fIsource\fR to be assembled, with diagnostic output going to \fIoutput;\fR no matter how long the assembly takes, the Shell returns immediately. When the Shell does not wait for the completion of a command, the identification of the process running that command is printed. This identification may be used to wait for the completion of the command or to terminate it. The ``&'' may be used several times in a line: .dc as source >output & ls >files & .ec does both the assembly and the listing in the background. In the examples above using ``&'', an output file other than the typewriter was provided; if this had not been done, the outputs of the various commands would have been intermingled. .pg The Shell also allows parentheses in the above operations. For example .dc (\|date; ls\|) >x & .ec prints the current date and time followed by a list of the current directory onto the file \fIx.\fR The Shell also returns immediately for another request. .s2 6.4 The Shell as a Command; Command Files .es The Shell is itself a command, and may be called recursively. Suppose file \fItryout\fR contains the lines .dc as source .ti 1i mv a.out testprog .ti 1i testprog .ec The \fImv\fR command causes the file \fIa.out\fR to be renamed \fItestprog.\fR \fIA.out\fR is the (binary) output of the assembler, ready to be executed. Thus if the three lines above were typed on the console, \fIsource\fR would be assembled, the resulting program renamed \fItestprog\fR, and \fItestprog\fR executed. When the lines are in \fItryout\fR, the command .dc sh <tryout .ec would cause the Shell \fIsh\fR to execute the commands sequentially. .pg The Shell has further capabilities, including the ability to substitute parameters and to construct argument lists from a specified subset of the file names in a directory. It is also possible to execute commands conditionally on character string comparisons or on existence of given files and to perform transfers of control within filed command sequences. .s2 6.5 Implementation of the Shell .es The outline of the operation of the Shell can now be understood. Most of the time, the Shell is waiting for the user to type a command. When the new-line character ending the line is typed, the Shell's \fIread\fR call returns. The Shell analyzes the command line, putting the arguments in a form appropriate for \fIexecute\fR. Then \fIfork\fR is called. The child process, whose code of course is still that of the Shell, attempts to perform an \fIexecute\fR with the appropriate arguments. If successful, this will bring in and start execution of the program whose name was given. Meanwhile, the other process resulting from the \fIfork\fR, which is the parent process, \fIwait\|\fRs for the child process to die. When this happens, the Shell knows the command is finished, so it types its prompt and reads the typewriter to obtain another command. .pg Given this framework, the implementation of background processes is trivial; whenever a command line contains ``&'', the Shell merely refrains from waiting for the process which it created to execute the command. .pg Happily, all of this mechanism meshes very nicely with the notion of standard input and output files. When a process is created by the \fIfork\fR primitive, it inherits not only the core image of its parent but also all the files currently open in its parent, including those with file descriptors 0 and 1. The Shell, of course, uses these files to read command lines and to write its prompts and diagnostics, and in the ordinary case its children_the command programs_inherit them automatically. When an argument with ``<'' or ``>'' is given however, the offspring process, just before it performs \fIexecute,\fR makes the standard I/O file descriptor 0 or 1 respectively refer to the named file. This is easy because, by agreement, the smallest unused file descriptor is assigned when a new file is \fIopen\fR\|ed (or \fIcreate\fR\|d); it is only necessary to close file 0 (or 1) and open the named file. Because the process in which the command program runs simply terminates when it is through, the association between a file specified after ``<'' or ``>'' and file descriptor 0 or 1 is ended automatically when the process dies. Therefore the Shell need not know the actual names of the files which are its own standard input and output, since it need never reopen them. .pg Filters are straightforward extensions of standard I/O redirection with pipes used instead of files. .pg In ordinary circumstances, the main loop of the Shell never terminates. (The main loop includes that branch of the return from \fIfork\fR belonging to the parent process; that is, the branch which does a \fIwait\fR, then reads another command line.) The one thing which causes the Shell to terminate is discovering an end-of-file condition on its input file. Thus, when the Shell is executed as a command with a given input file, as in .dc sh <comfile .ec the commands in \fIcomfile\fR will be executed until the end of \fIcomfile\fR is reached; then the instance of the Shell invoked by \fIsh\fR will terminate. Since this Shell process is the child of another instance of the Shell, the \fIwait\fR executed in the latter will return, and another command may be processed. .s2 6.6 Initialization .es The instances of the Shell to which users type commands are themselves children of another process. The last step in the initialization of \*sUNIX\*n is the creation of a single process and the invocation (via \fIexecute\fR) of a program called \fIinit\fR. The role of \fIinit\fR is to create one process for each typewriter channel which may be dialed up by a user. The various subinstances of \fIinit\fR open the appropriate typewriters for input and output. Since when \fIinit\fR was invoked there were no files open, in each process the typewriter keyboard will receive file descriptor 0 and the printer file descriptor 1. Each process types out a message requesting that the user log in and waits, reading the typewriter, for a reply. At the outset, no one is logged in, so each process simply hangs. Finally someone types his name or other identification. The appropriate instance of \fIinit\fR wakes up, receives the log-in line, and reads a password file. If the user name is found, and if he is able to supply the correct password, \fIinit\fR changes to the user's default current directory, sets the process's user \*sID\*n to that of the person logging in, and performs an \fIexecute\fR of the Shell. At this point the Shell is ready to receive commands and the logging-in protocol is complete. .pg Meanwhile, the mainstream path of \fIinit\fR (the parent of all the subinstances of itself which will later become Shells) does a \fIwait\fR. If one of the child processes terminates, either because a Shell found an end of file or because a user typed an incorrect name or password, this path of \fIinit\fR simply recreates the defunct process, which in turn reopens the appropriate input and output files and types another login message. Thus a user may log out simply by typing the end-of-file sequence in place of a command to the Shell. .s2 6.7 Other programs as Shell .es The Shell as described above is designed to allow users full access to the facilities of the system, since it will invoke the execution of any program with appropriate protection mode. Sometimes, however, a different interface to the system is desirable, and this feature is easily arranged. .pg Recall that after a user has successfully logged in by supplying his name and password, \fIinit\fR ordinarily invokes the Shell to interpret command lines. The user's entry in the password file may contain the name of a program to be invoked after login instead of the Shell. This program is free to interpret the user's messages in any way it wishes. .pg For example, the password file entries for users of a secretarial editing system specify that the editor \fIed\fR is to be used instead of the Shell. Thus when editing system users log in, they are inside the editor and can begin work immediately; also, they can be prevented from invoking \*sUNIX\*n programs not intended for their use. In practice, it has proved desirable to allow a temporary escape from the editor to execute the formatting program and other utilities. .pg Several of the games (e.g., chess, blackjack, 3D tic-tac-toe) available on \*sUNIX\*n illustrate a much more severely restricted environment. For each of these an entry exists in the password file specifying that the appropriate game-playing program is to be invoked instead of the Shell. People who log in as a player of one of the games find themselves limited to the game and unable to investigate the presumably more interesting offerings of \*sUNIX\*n as a whole.