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VI. THE SHELL

For most users, communication with the system 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. (The shell is described fully elsewhere, .[ bourne shell bstj %Q This issue .] so this section will discuss only the theory of its operation.) In simplest form, a command line consists of the command name followed by arguments to the command, all separated by spaces:

1 command arg\*s\d1\u\*n arg\*s\d2\u\*n .\|.\|. arg\*s\dn\u\*n

2 The shell splits up the command name and the arguments into separate strings. Then a file with name command is sought; command may be a path name including the ``/'' character to specify any file in the system. If command is found, it is brought into memory 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.

If file command cannot be found, the shell generally prefixes a string such as /\|bin\|/ to command and attempts again to find the file. Directory /\|bin contains commands intended to be generally used. (The sequence of directories to be searched may be changed by user request.)

6.1 Standard I/O

The discussion of I/O in Section III 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 three open files with file descriptors 0, 1, and 2. 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 terminal. Thus programs that wish to write informative information ordinarily use file descriptor 1. Conversely, file 0 starts off open for reading, and programs that wish to read messages typed by the user read this file.

The shell is able to change the standard assignments of these file descriptors from the user's terminal 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:

1 ls

2 ordinarily lists, on the typewriter, the names of the files in the current directory. The command:

1 ls >there

2 creates a file called there and places the listing there. Thus the argument >there means ``place output on there .'' On the other hand:

1 ed

2 ordinarily enters the editor, which takes requests from the user via his keyboard. The command

1 ed <script

2 interprets script as a file of editor commands; thus <script means ``take input from script .''

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.

File descriptor 2 is, like file 1, ordinarily associated with the terminal output stream. When an output-diversion request with ``>'' is specified, file 2 remains attached to the terminal, so that commands may produce diagnostic messages that do not silently end up in the output file.

6.2 Filters

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:

1 ls | pr -2 | opr

2 ls lists the names of the files in the current directory; its output is passed to pr , which paginates its input with dated headings. (The argument ``-2'' requests double-column output.) Likewise, the output from pr is input to opr ; this command spools its input onto a file for off-line printing.

This procedure could have been carried out more clumsily by:

1 ls >temp1 pr -2 <temp1 >temp2 opr <temp2

2 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 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 ls to provide such a wide variety of output options.

A program such as pr which copies its standard input to its standard output (with processing) is called a T filter . Some filters that we have found useful perform character transliteration, selection of lines according to a pattern, sorting of the input, and encryption and decryption.

6.3 Command separators; multitasking

Another feature provided by the shell is relatively straightforward. Commands need not be on different lines; instead they may be separated by semicolons:

1 ls; ed

2 will first list the contents of the current directory, then enter the editor.

A related feature is more interesting. If a command is followed by ``\f3&\f1,'' 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: .bd 3

1 as source >output &

2 causes source to be assembled, with diagnostic output going to output ; 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 number 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 ``\f3&\f1'' may be used several times in a line:

1 as source >output & ls >files &

2 does both the assembly and the listing in the background. In these examples, an output file other than the terminal was provided; if this had not been done, the outputs of the various commands would have been intermingled.

The shell also allows parentheses in the above operations. For example:

1 (\|date; ls\|) >x &

2 writes the current date and time followed by a list of the current directory onto the file x . The shell also returns immediately for another request.

6.4 The shell as a command; command files

The shell is itself a command, and may be called recursively. Suppose file tryout contains the lines:

1 as source mv a.out testprog testprog

2 The mv command causes the file a.out to be renamed testprog. a.out is the (binary) output of the assembler, ready to be executed. Thus if the three lines above were typed on the keyboard, source would be assembled, the resulting program renamed testprog , and testprog executed. When the lines are in tryout , the command:

1 sh <tryout

2 would cause the shell sh to execute the commands sequentially.

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 also provides general conditional and looping constructions.

6.5 Implementation of the shell

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 newline character ending the line is typed, the shell's read call returns. The shell analyzes the command line, putting the arguments in a form appropriate for execute . Then fork is called. The child process, whose code of course is still that of the shell, attempts to perform an execute 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 fork , which is the parent process, wait s for the child process to die. When this happens, the shell knows the command is finished, so it types its prompt and reads the keyboard to obtain another command.

Given this framework, the implementation of background processes is trivial; whenever a command line contains ``\f3&\f1,'' the shell merely refrains from waiting for the process that it created to execute the command.

Happily, all of this mechanism meshes very nicely with the notion of standard input and output files. When a process is created by the fork primitive, it inherits not only the memory image of its parent but also all the files currently open in its parent, including those with file descriptors 0, 1, and 2. 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\(emthe command programs\(eminherit them automatically. When an argument with ``<'' or ``>'' is given, however, the offspring process, just before it performs execute, 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 open ed (or create 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 that are its own standard input and output, because it need never reopen them.

Filters are straightforward extensions of standard I/O redirection with pipes used instead of files.

In ordinary circumstances, the main loop of the shell never terminates. (The main loop includes the branch of the return from fork belonging to the parent process; that is, the branch that does a wait , then reads another command line.) The one thing that 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:

1 sh <comfile

2 the commands in comfile will be executed until the end of comfile is reached; then the instance of the shell invoked by sh will terminate. Because this shell process is the child of another instance of the shell, the wait executed in the latter will return, and another command may then be processed.

6.6 Initialization

The instances of the shell to which users type commands are themselves children of another process. The last step in the initialization of the system is the creation of a single process and the invocation (via execute ) of a program called init . The role of init is to create one process for each terminal channel. The various subinstances of init open the appropriate terminals for input and output on files 0, 1, and 2, waiting, if necessary, for carrier to be established on dial-up lines. Then a message is typed out requesting that the user log in. When the user types a name or other identification, the appropriate instance of init wakes up, receives the log-in line, and reads a password file. If the user's name is found, and if he is able to supply the correct password, init 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 execute of the shell. At this point, the shell is ready to receive commands and the logging-in protocol is complete.

Meanwhile, the mainstream path of init (the parent of all the subinstances of itself that will later become shells) does a wait . 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 init simply recreates the defunct process, which in turn reopens the appropriate input and output files and types another log-in message. Thus a user may log out simply by typing the end-of-file sequence to the shell.

6.7 Other programs as shell

The shell as described above is designed to allow users full access to the facilities of the system, because 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 for.

Recall that after a user has successfully logged in by supplying a name and password, init 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 log-in instead of the shell. This program is free to interpret the user's messages in any way it wishes.

For example, the password file entries for users of a secretarial editing system might specify that the editor ed is to be used instead of the shell. Thus when users of the editing system log in, they are inside the editor and can begin work immediately; also, they can be prevented from invoking 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.

Several of the games (e.g., chess, blackjack, 3D tic-tac-toe) available on the system 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 these games find themselves limited to the game and unable to investigate the (presumably more interesting) offerings of the X system as a whole.